Thèses sur le sujet « Star cluster formation and evolution »

We report the large effort that is producing comprehensive high-level young star cluster (YSC) catalogs for a significant fraction of galaxies observed with the Legacy ExtraGalactic UV Survey (LEGUS) Hubble treasury program. We present the methodology developed to extract cluster positions, verify their genuine nature, produce multiband photometry (from NUV to NIR), and derive their physical properties via spectral energy distribution fitting analyses. We use the nearby spiral galaxy NGC 628 as a test case for demonstrating the impact that LEGUS will have on our understanding of the formation and evolution of YSCs and compact stellar associations within their host galaxy. Our analysis of the cluster luminosity function from the UV to the NIR finds a steepening at the bright end and at all wavelengths suggesting a dearth of luminous clusters. The cluster mass function of NGC 628 is consistent with a power-law distribution of slopes similar to-2 and a truncation of a few times 10(5) M-circle dot. After their formation, YSCs and compact associations follow different evolutionary paths. YSCs survive for a longer time frame, confirming their being potentially bound systems. Associations disappear on timescales comparable to hierarchically organized star-forming regions, suggesting that they are expanding systems. We find massindependent cluster disruption in the inner region of NGC 628, while in the outer part of the galaxy there is little or no disruption. We observe faster disruption rates for low mass (<= 10(4) M-circle dot) clusters, suggesting that a massdependent component is necessary to fully describe the YSC disruption process in NGC 628.

Recently acquired WFC3 UV (F275W and F336W) imaging mosaics under the Legacy Extragalactic UV Survey (LEGUS), combined with archival ACS data of M51, are used to study the young star cluster (YSC) population of this interacting system. Our newly extracted source catalogue contains 2834 cluster candidates, morphologically classified to be compact and uniform in colour, for which ages, masses and extinction are derived. In this first work we study the main properties of the YSC population of the whole galaxy, considering a mass-limited sample. Both luminosity and mass functions follow a power-law shape with slope -2, but at high luminosities and masses a dearth of sources is observed. The analysis of the mass function suggests that it is best fitted by a Schechter function with slope -2 and a truncation mass at 1.00 +/- 0.12 x 10(5) M-circle dot . Through Monte Carlo simulations, we confirm this result and link the shape of the luminosity function to the presence of a truncation in the mass function. A mass limited age function analysis, between 10 and 200 Myr, suggests that the cluster population is undergoing only moderate disruption. We observe little variation in the shape of the mass function at masses above 1 x 10(4) M-circle dot over this age range. The fraction of star formation happening in the form of bound clusters in M51 is similar to 20 per cent in the age range 10-100 Myr and little variation is observed over the whole range from 1 to 200 Myr.

[Special characters cannot be displayed. Please see the pdf version of the Abstract for an accurate reproduction.] We studied the circumstellar discs, the initial mass function (IMF), mass distribution, binarity and the fundamental properties of the [special character] 9 Myr-old pre-main sequence (PMS) [special character] Chamaeleontis cluster. Using JHKL colour-colour and colour-excess diagrams, we found the circumstellar disc fraction to be [special character] 0.60 among the late-type members. Four stars with [special character] (K - L) > 0.4 were identified as experiencing ongoing accretion which was later confirmed by high-resolution spectroscopic study. Quantitative analysis of the H[special character] profiles found accretion in these four stars at rates comparable to that of two members of the similarly-aged TW Hydrae Association (TWA); rates 1 - 3 orders of magnitude lower than in younger classical T Tauri stars. Together these results suggest that, while the mass accretion rate decreases with age, PMS stars can retain their inner discs for [special character] 10 Myr. An optical photometric survey spanning 1.3 ?? 1.3 pc added two low-mass stars to the cluster inventory. Together with other recent surveys the population is likely to be significantly complete for primaries with masses M > 0.15M[special character]. The cluster now consists of 18 primaries and 9 confirmed and candidate secondaries, with [special character] 2-4 times higher multiplicity than seen in field dwarfs. The cluster IMF is consistent with that of rich young clusters and field stars. By extending the IMF to lower masses, we predict 20-29 low-mass stars and brown dwarfs may remain undiscovered. From study of the cluster???s spatial and mass distribution, we find the [special character] Cha cluster has significant mass segregation, with > 50 per cent of the stellar mass residing within the central 0.17 pc. Lastly we classified members of the cluster with low-resolution spectra, providing information about the fundamental properties of the PMS stars by comparison to standard dwarfs. Broadband VRI colours and pseudocontinuum indices derived for the cluster stars are indistinguishable from dwarfs at visual and red wavelengths. This suggests the temperature sequence for the PMS [special character] Cha cluster is similar to that of the dwarf sequence. Narrow-band spectral indices for the [special character] Cha cluster possibly indicate higher metallicity and strongly indicate lower surface gravity than the dwarf indices.

Les jeunes amas d'étoiles sont sous-structurés et évoluent dynamiquement pour former des amas sphériques à l'équilibre. Je présente une nouvelle méthode pour générer des conditions initiales réalistes pour simuler ce processus: la fragmentation de Hubble-Lemaitre. Je laisse le système développer spontanément des surdensités au cours d'une expansion du système. Le modèle résultant se compare bien aux simulations hydrodynamiques de formation stellaire et aux observations des jeunes amas. Le modèle fragmenté s'effondre de manière plus douce qu'un modèle uniforme. L'injection d'une population d'étoile binaire avant l'effondrement a montré qu'un système sous-structuré détruisait bien plus de binaires qu'un système à l'équilibre. Des binaires particulièrement larges ou serrées, jusqu’à 0.01 AU, ont également été détectées dans ces modèles. Cette méthode est très prometteuse, un exemple d'application est la génération d'observations synthétiques de régions de formation stellaire
Young star clusters are substructured and undergo a dynamical evolution erasing this substructure to form relaxed spherical clusters. I present a new method to generate realistic initial conditions to perform N-body simulations of this process: the Hubble-Lemaitre fragmentation. By expanding an initially uniform sphere, I allow spontaneous overdensities to grow, creating a realistic model for young clumpy stellar systems. This method is validated by analysing the distribution and content of the clumps and comparing them to hydrodynamical simulations of star formation as well as observations of star forming regions. These systems undergo a softer collapse than uniform ones. I injected binary stars in the fragmented models and found they were heavily processed when substructure was present. I also found extreme short and tight binaries, down to 0.01 AU, to formin the models. The method has a lot of potential, such as the generation of mock observations of star-forming regions

We present an extremely deep CO(1-0) observation of a confirmed z = 1.62 galaxy cluster. We detect two spectroscopically confirmed cluster members in CO(1-0) with signal-to-noise ratio >5. Both galaxies have log (M-star/M-circle dot) > 11 and are gas rich, with M-mol/(M-star + M-mol) similar to 0.17-0.45. One of these galaxies lies on the star formation rate (SFR)-M-star sequence, while the other lies an order of magnitude below. We compare the cluster galaxies to other SFR-selected galaxies with CO measurements and find that they have CO luminosities consistent with expectations given their infrared luminosities. We also find that they have gas fractions and star formation efficiencies (SFE) comparable to what is expected from published field galaxy scaling relations. The galaxies are compact in their stellar light distribution, at the extreme end for all high-redshift star-forming galaxies. However, their SFE is consistent with other field galaxies at comparable compactness. This is similar to two other sources selected in a blind CO survey of the HDF-N. Despite living in a highly quenched protocluster core, the molecular gas properties of these two galaxies, one of which may be in the process of quenching, appear entirely consistent with field scaling relations between the molecular gas content, stellar mass, star formation rate, and redshift. We speculate that these cluster galaxies cannot have any further substantive gas accretion if they are to become members of the dominant passive population in z < 1 clusters.

This thesis focuses on how a galaxy's environment affects its star formation, from the galactic environment of the most luminous IR galaxies in the universe to groups and massive clusters of galaxies. Initially, we studied a class of high-redshift galaxies with extremely red optical-to-mid-IR colors. We used Spitzer spectra and photometry to identify whether the IR outputs of these objects are dominated by AGNs or star formation. In accordance with the expectation that the AGN contribution should increase with IR luminosity, we find most of our very red IR-luminous galaxies to be dominated by an AGN, though a few appear to be star-formation dominated. We then observed how the density of the extraglactic environment plays a role in galaxy evolution. We begin with Spitzer and HST observations of intermediate-redshift groups. Although the environment has clearly changed some properties of its members, group galaxies at a given mass and morphology have comparable amounts of star formation as field galaxies. We conclude the main difference between the two environments is the higher fraction of massive early-type galaxies in groups. Clusters show even more distinct trends. Using three different star-formation indicators, we found the mass--SFR relation for cluster galaxies can look similar to the field (A2029) or have a population of low-star-forming galaxies in addition to the field-like galaxies (Coma). We contribute this to differing merger histories: recently-accreted galaxies would not have time for their star formation to be quenched by the cluster environment (A2029), while an accretion event in the past few Gyr would give galaxies enough time to have their star formation suppressed by the cluster environment. Since these two main quenching mechanisms depend on the density of the intracluster gas, we turn to a group of X-ray under luminous clusters to study how star-forming galaxies have been affected in clusters with lower than expected X-ray emission. We find the distribution of star-forming galaxies with respect to stellar mass varies from cluster to cluster, echoing what we found for Coma and A2029. In other words, while some preprocessing occurs in groups, the cluster environment still contributes to the quenching of star formation.

The aim of this work is to study the formation and evolution of double compact-object binaries (DCOBs, i.e. black hole-black hole, black hole-neutron star and neutron star-neutron star binaries) in young (< 100 Myr) dense (>~ 10^3 star/pc^3) star clusters (YSCs). DCOBs, when merging, are expected to be powerful sources of gravitational waves (GWs) observable by Virgo and LIGO detectors. Best merger candidates (BMCs), i.e. those sources with a chance to be observed, have a coalescence timescale shorter than one Hubble time (t_H) and produce a signal strong enough (strain >~ 10^-21) to be visible from Earth. This project is particularly timely because the second generation Virgo and LIGO detectors are expected to start operating in 2016. The importance of choosing YSCs as environment for this study has two motivations.First, YSCs are the place where >~ 80% of the stars form, in particular the most massive ones. The remnants of these massive stars will dominate the dynamics of the cluster and will form the kind of binaries we are looking for. This makes YSCs the best environment where to look for DCOBs. Second, YSCs are collisional environments (2-body relaxation timescale t_relax ~ 10 Myr (M_tot / 3500 Msun)^1/2 (r_hm / 1 pc)^3/2,where M_tot and r_hm are the total mass and the half mass radius of the YSC, respectively). Close encounters between single stars and binaries may result in the binary getting closer or even in an exchange, i.e. the single star taking the place of one of the binary members. In the field (i.e. the galactic disk), instead, a binary exists only if the two stars were born already bound, can shrink only because of stellar and binary evolution (GW emission, common envelope, ...) and does not undergo exchanges. Thus, dynamical processes have a fundamental role in shaping the DCOB population in YSCs. Moreover, YSCs have a short lifetime: they are thought to dissolve into the galactic disk in O(10^2) Myr, releasing their DCOB content into the disk. Thus, the estimate of the population of GW source candidates in the field has to take into account the population of DCOBs in YSCs. In order to study the population of DCOBs in YSCs, I performed and analyzed >~ 10^3 direct N-body simulations coupled with stellar evolution recipes of YSCs. The simulations were run with the software environment Starlab (Portegies Zwart et al., 2001) modified to include up-to-date metallicity dependent stellar evolution recipes (Mapelli and Bressan 2013). These recipes take into account metallicity dependent stellar winds and the possibility that a massive star collapses directly to a black hole (BH), without supernova (SN) explosion. This BH formation process, called "direct collapse" or "failed SN", allows the formation of more massive BHs. In addition, I developed sltools, a suite of programs to help the production and management of simulations. They provide tools to automate most of the steps needed to obtain clean datasets ready for the analysis, including an automated quality control and error management. In my analysis, I traced the life of compact-object binaries and I investigated the impact of dynamical interactions, metallicity and structural properties of the host YSCs on the population of BMCs. I focused on how many DCOBs per YSC are produced (~4 stable BH-BH binaries, ~1 unstable BH-BH binaries, ~0.1 NS-NS and ~0.1 BH-NS per YSC along the entire simulation) and how this quantity changes with time: hard BH-BH binaries grows monotonically in time from 0 to _SC(t=100 Myr) ~ 0.2-0.4 while soft BH-BH binaries show a peak (after the core collapse, which occurs at different times for different densities) and then decrease to ~0.05. I found that >~90% of BH-BH binaries form from exchanges. This result indicates that BHs are extremely efficient in finding companions through dynamical exchanges. Moreover, low metallicity, thanks to the higher masses allowed for the remnants, favors the early formation of these heavy and stable BH-BH binaries. I also found that NS-NS binaries are, at least, ten times less numerous than BH-BH ones, despite the initial mass function. My analysis showed that BH-BH formation is favored also by high density (3x10^3 Msun pc^-3) and high concentration (dimensionless central potential W_0 >~ 3), while it is not very sensitive to primordial binary fraction. However, it is worth noting that only 23% of BH-BH BMCs come from exchanges, while the rest are primordial binaries. On the other hand, dynamical encounters are important also in primordial binaries, because they are responsible for the shrinking of BH-BH binary semi-major axis (SMA) a. Then, BH-BH binaries are able to reach values of the SMA short enough that the binary evolution is dominated by GW emission. Without dynamics, this process would have taken much more time. As to NS-NS binaries, I found that only 6% of NS-NS BMCs come from exchanges. The fact that the vast majority of NS-NS binaries are primordial is consistent with our expectations, because it is unlikely for a NS to acquire a NS companion if the dynamical interactions are dominated by BHs. Thus, it is interesting that we found even also some NS-NS binaries (6%) formed through exchanges. I also analyzed DCOB properties, i.e. masses and chirp masses (m_chirp = (m_1m_2)^3/5 / (m_1+m_2)^1/5, where m_1 and m_2 are the masses of the two members of the binary), SMAs, and eccentricities. In my simulations, BHs are heavier at low metallicity (maximum BH mass ~80 Msun at Z=0.01 Zsun) because of the stellar evolution and failed SN recipes I adopted. In addition, even more massive BHs can form because of mergers with stellar companion. Thus, the maximum BH mass I find in BH-BH binaries is ~125 Msun. This trend is reflected by the chirp mass values which are up to ~80 Msun. However, the maximum chirp mass for a BMC binary is quite lower (40 Msun) and the rest of BMCs chirp masses are below 20 Msun. Semi-major axis distributions show that, while NS-NS binaries are much less numerous than BH-BH, their SMA are much shorter (minimum SMA for NS-NS a_min_NS-NS ~ 10^-3 AU compared to a_min_BH-BH ~ 10^-1 AU). This is a selection effect: the NS-NS binaries I found come from binaries close enough to survive two SN explosions and dynamical encounters. This result is reflected by the coalescence timescale (time a binary needs to merge only because GWs emission, t_GW \propto (a^4(1-e^2)^7/2) / (m_1m_2m_tot)), where G is the gravitational constant, m_1 are m_2 the masses of the two members of the binary, a is the semi-major axis and e is the eccentricity): NS-NS binaries have shorter coalescence timescales (t_GW_min_NS-NS ~ 10^-5 Gyr compared to t_GW_min_BH-BH ~ 10^-1 Gyr for BH-BH). In fact, I found that 76% of NS-NS BMC binaries merge during the simulations (36% of all the NS-NS binaries), while none of BH-BH binaries does. While there is not observational evidence of BH-BH binaries in our Galaxy, we observe 10 NS-NS binaries (Lorimer, 2008). I compared the observed NS-NS binaries properties (period, eccentricity and coalescence timescale) to the ones from my simulations. The agreement is very good. The only differences can be found at the shortest and longest periods. The differences are due to selection effects: at very short periods (<~2 hours) NS-NS binaries merge very fast and it is hard to observe them in this state, while the longest periods (>~10^3 days) are too long to be observed since now. Finally, I derived the expected merger rate from my simulations, and I investigated whether it depends on YSC properties (mass, density, concentration primordial binary fractions, metallicity). I found no significant dependence of BH-BH merger rates on the structural properties of YSCs, within the considered ranges. However, uncertainties are still quite large. The global merger rate for BH-BH binaries derived from my simulations is R_merger_BH-BH = 0.0019+/-0.0007 Mpc^-3 Myr^-1. The final BH-BH detection rate shows a dependence (though not very significant because of the large uncertainties) on the density and concentration of the host YSC: they are higher for more dense and concentrated clusters, in agreement with the average number of BH-BH binaries produced during the cluster life. Moreover, the BH-BH detection rate anti-correlates with the primordial binary fraction. This result needs further investigations. The global detection rate for BH-BH binaries is R_detection_BH-BH= 0.8+/-0.2 yr^-1. Merger and detection rates for NS-NS and BH-NS are R_merger_NS-NS} = 0.258+/-0.005 Mpc^-3 Myr^-1, R_merger_BH-NS} = 0.0009+/-0.0002 Mpc^-3 Myr^-1, R_detection_NS-NS} = 0.65+/-0.01 yr^-1, R_detection_BH-NS} = 0.0107+/-0.0006 yr^-1 for NS-NS and BH-NS, respectively. The merger and detection rates of BH-BH and NS-NS binaries are consistent with the pessimistic rates provided by Virgo and LIGO collaboration (Abadie et al., 2010). The BH-NS merger and detection rate are even lower than the most pessimistic prediction in literature because BH-NS mergers are disfavored by dynamical processes that favor BH-BH production at the expense of BH-NS ones.
L'obiettivo di questo lavoro e` studiare la fomazione ed evoluzione di binarie di oggetti compatti (DCOBs, ovvero buchi neri binari, stelle di neutroni binarie e binarie buco nero-stella di neutroni) in ammassi stellari (YSCs) giovani ( < 100 Myr) e densi ( >~ 10^3 stelle/pc ^3} ). La teoria prevede che i DCOBs, coalescendo, diventino potenti sorgenti di onde gravitazionali (GWs) osservabili dai rivelatori Virgo and LIGO. I migliori candidati per l'osservazione (BMCs), hanno un tempo scala di coalescenza minore di un tempo di Hubble ( t_H} ) e producono un segnale sufficientemente forte (strain h>~10^-21} ) da essere osservabile da Terra. Questo è proprio il momento giusto per svolgere un progetto del genere in quanto la seconda generazione dei rivelatori Virgo e LIGO inizierà le osservazioni nel 2016. La scelta degli YSCs come ambiente per lo studio dei DCOBs è particolarmente importante per due motivazioni. Innanzitutto, gli YSCs sono il luogo in cui >~ 80% delle stelle si forma, in particolare le più massive. Gli oggetti compatti che si formano alla morte di queste stelle massive dominano la dinamica del cluster e formano il tipo di binarie che vogliamo studiare. Questo rende gli YSC il migliore ambiente dove cercare DCOBs. Secondo, gli YSCs sono collisionali (tempo scala di rilassamento a due corpi t_relax ~ 10 Myr (M_tot / 3500 Msun)^1/2 (r_hm / 1 pc)^3/2, dove M_tot e r_hm sono la massa totale e il raggio di metà massa dello YSC, rispettivamente). Incontri ravvicinati tra singole stelle e binarie possono rendere la binaria più stretta o perfino portare la stella singola a prendere il posto di uno dei componenti della binaria. Nel campo (disco galattico), invece, una binaria esiste solo se le due stelle che la compongono si sono formate già legate, può stringersi solo a causa di effetti legati all'evoluzione stellare o in binaria (emissione di GW, common envelope, ...) e non può essere oggetto di scambi dinamici. Per queste ragioni, i processi dinamici hanno un ruolo fondamentale nel dare forma alla popolazione di DCOBs negli YSCs. Inoltre, gli YSCs hanno un tempo di vita breve: essi tendono a dissolversi nel disco galattico in O(10^2) Myr, rilasciando il loro contenuto di DCOBs nel disco. Questo implica che le stime sulla popolazione di DCOBs nel disco galattico devono tenere conto della popolazione di DCOBs negli YSCs. Allo scopo di studiare la popolazione di DCOBs negli YSCs, ho effettuato e analizzato >~ 10^3 simulazioni dirette a N-corpi di YSCs accoppiate ad un programma di evoluzione stellare, Le simulazioni sono state prodotte con l'ambiente software Starlab (Portegies Zwart et al., 2001), modificato per includere algoritmi aggiornati di evoluzione stellare in funzione della metallicità (Mapelli and Bressan, 2013). Questi algoritmi comprendono venti stellari in funzione della metallicità e la possibilità che una stella massiva collassi direttamente in un buco nero (BH), senza esplosione di supernova (SN). Questo processo di formazione dei BH, chiamato "collasso diretto" o "SN fallita", permette la formazione di BHs più massivi. In aggiunta, ho sviluppato sltools, una suite di programmi che facilitano la produzione e gestione delle simulazioni. Questi provvedono strumenti per automatizzare la maggior parte dei passaggi necessari per ottenere dati puliti e pronti per essere analizzati, inclusi un controllo della qualità automatico e la gestione degli errori. Nella mia analisi ho seguito la vita delle binarie di oggetti compatti e ho investigato l'impatto delle interazioni dinamiche, della metallicità e delle proprietà strutturali degli YSCs ospiti sulla popolazione di BMCs. Mi sono focalizzato su quanti DCOBs vengono prodotti in media per YSCs ( ~ 4 binarie BH-BH stabili, ~ 1 binarie BH-BH instabili, ~ 0.1 NS-NS e ~ 0.1 BH-NS per YSC durante tutta la simulazione) e su come questa quantità cambia nel tempo: se considero solo le binarie BH-BH stabili, trovo che il loro numero cresce monotonicamente nel tempo da 0 a ~ 0.4 , mentre le binarie BH-BH instabili mostrano un picco dopo il collasso del core e poi una decrescita fino a ~ 0.05 . Ho trovato che >~ 90% delle binarie BH-BH si formano da scambi. I risultati indicano che i BHs sono estremamente efficienti nell'acquisire compagni attraverso scambi dinamici. Inoltre, una metallicità bassa, grazie al fatto che i BH possono avere masse maggiori, favorisce la formazione di binarie BH-BH massicce e stabili in tempi più brevi. Ho anche trovato che le binarie NS-NS sono, almeno, dieci volte meno numerose delle binarie BH-BH, nonostante la funzione di massa iniziale. La mia analisi ha mostrato che la formazione di BH-BH è anche favorita da alta densità ( ~ 3 x 10^3 Msun pc^-3) e alta concentrazione (potenziale centrale adimensionale W_0 >~ 3 ), mentre non è molto sensibile alla frazione di binarie primordiali. Vale comunque la pena notare che solo il 23% dei BMCs tra le binarie BH-BH viene da scambi, mentre il resto e` costituito da binarie primordiali. D'altra parte, gli incontri dinamici sono importanti anche per le binarie primordiali, in quanti sono responsabili per la diminuzione del semiasse maggiore a della binarie BH-BH (SMA). Le binarie BH-BH sono in grado di raggiungere valori dello SMA sufficientemente bassi che l'evoluzione della binaria è dominata dall'emissione di GWs. Senza la dinamica, questo processo avrebbe impiegato un tempo molto maggiore. Ho trovato che solo 6% dei BMCs NS-NS si sono formati attraverso scambi. Il fatto che la maggior parte delle binarie NS-NS sia primordiale è consistente con le nostre aspettative percheé è poco probabile che una NS acquisisca una compagna NS se le interazioni dinamiche sono dominate dai BHs. Per questa ragione è interessante che io abbia trovato alcune binarie NS-NS (6%) formate attraverso scambi. Ho anche analizzato le proprietà dei DCOBs: masse, masse chirp ( m_chirp = (m_1m_2)^3/5 / (m_1+m_2)^1/5, dove m_1 e m_2 sono le masse dei due membri della binaria), SMAs e eccentricità. Nelle mie simulazioni i BHs sono più massivi a metallicità minori (massa massima di un BH ~ 80 Msun a Z=0.01 Zsun ) grazie agli algoritmi di evoluzione stellare e di collasso diretto adottati. In aggiunta, BHs ancora più massivi si possono formare grazie a coalescenza con compagni stellari. Di conseguenza, la massa massima che trovo per i BH è ~ 125 Msun . Questo andamento si riflette nelle masse chirp, che raggiungono valori di ~ 80 Msun . Tuttavia, la massa chirp per una binaria BMC è più bassa ( ~ 40 Msun ) e il resto delle masse dei BMCs sono inferiori a 20 Msun . La distribuzione dei SMA mostra che, sebbene le binarie NS-NS siano molto meno numerose delle binarie BH-BH, i loro SMA sono molto minori (SMA minimo per le binarie NS-NS a_min_NS-NS ~ 10^-3 AU in confronto a a_min_BH-BH ~ 10^-1 AU). Questo è un effetto di selezione: le binarie NS-NS che trovo provengono da binarie sufficientemente strette da sopravvivere a due esplosioni di SN e agli incontri dinamici. Questo risultato si ritrova nei tempi scala di coalescenza (tempo necessario perch\'e una binaria coalesca solo per effetto dell'emissione di GWs, t_GW \propto (a^4(1-e^2)^7/2) / (m_1m_2m_tot)), dove G è la costante gravitazionale, m_1 e m_2 sono le masse dei due membri della binaria, a è il semiasse maggiore e e è l'eccentricità): le binarie NS-NS hanno tempi scala più corti ( t_GW_min_NS-NS ~ 10^-5 Gyr in confronto a t_GW_min_BH-BH ~ 10^-1 Gyr per i BH-BH). Infatti, trovo che il 76% delle binarie NS-NS coalesce durante le simulazioni (36% di tutte le binarie NS-NS), mentre nessuna delle binarie BH-BH coalesce. Mentre non esistono evidenze osservative delle binarie BH-BH, nella nostra galassia sono state osservate 10 binarie NS-NS (Lorimer, 2008). Ho confrontato le proprietà delle binarie NS-NS osservate (periodo, eccentricità e tempo scala di coalescenza) con quelle delle binarie NS-NS nelle mie simulazioni e ho trovato un accordo molto buono. Le uniche differenze si possono trovare ai periodi più corti e più lunghi. Queste differenze sono dovute a effetti di selezione: per periodi molto corti ( <~ 2 hours) le binarie NS-NS coalescono in tempi molto brevi ed è difficile osservarle in questo stato. Periodi molto lunghi ( >~ 10^3 days) sono troppo lunghi per essere osservati fino ad ora. Infine, ho derivato il tasso di coalescenza atteso nelle mie simulazioni e ho investigato se questo tasso dipende dalle proprietà dello YSC (massa, densità, concentrazione, frazione di binarie primordiali e metallicità). Non ho trovato alcuna dipendenza significativa del rate di coalescenza delle binarie BH-BH dalle proprietà strutturali degli YSCs all'interno dei valori considerati. Le incertezze, comunque, sono abbastanza grandi. Il tasso di coalescenza globale per le binarie BH-BH derivato dalle mie simulazioni è R_merger_BH-BH = 0.0019+/-0.0007 Mpc^-3 Myr^-1 . Il tasso di detezioni mostra una dipendenza (sebbene non molto significativa, a causa delle incertezze) dalla densità e dalla concentrazione dello YSC ospite: il tasso di detezioni è più alto tanto più l'ammasso è denso e concentrato, in accordo con quanto trovato per il numero medio di binarie BH-BH prodotto durante la vita dell'ammasso. Inoltre, il tasso di detezioni per le binarie BH-BH anticorrela con la frazione di binarie primordiali. Questo risultato necessita di maggiori approfondimenti. Il tasso globale di osservazione per le binarie BH-BH è R_detection_BH-BH = 0.8+/-0.2 yr^-1. I tassi di coalescenza e osservazioni attesi per le binarie NS-NS and BH-NS sono R_merger_NS-NS} = 0.258+/-0.005 Mpc^-3 Myr^-1 , R_merger_BH-NS = 0.0009+/-0.0002 Mpc^-3 Myr^-1 , R_detection_NS-NS = 0.65+/-0.01 yr ^-1 , R_detection_BH-NS = 0.0107+/-0.0006 yr ^-1 per binarie NS-NS e BH-NS, rispettivamente. I tassi di coalescenza e osservazione di binarie BH-BH e NS-NS sono consistenti con le previsioni pessimistiche fornite dalla collaborazione Virgo/LIGO (Abadie et al., 2010). I tassi di coalescenza e osservazione di binarie BH-NS sono minori della previsione più pessimistica in letteratura dal momento che la formazione di binarie BH-NS è sfavorita dai processi dinamici che favoriscono la produzione di binarie BH-BH a discapito delle binarie BH-NS.

In this thesis, we aim to further elucidate the phenomenon of galaxy evolution in the environment of galaxy clusters using the methodology of numerical simulations. For that, we have developed hydrodynamic models in which idealized gas-rich galaxies move within the ICM of idealized galaxy clusters, allowing us to probe in a detailed and controlled manner their evolution in this extreme environment. The main code used in our simulations is RAMSES, and our results concern the changes in gas composition, star formation rate, luminosity and color of infalling galaxies. Additionally to processes taking place inside the galaxies themselves, we have also described the dynamics of the gas that is stripped from those galaxies with unprecedented resolution for simulations of this nature (122 pc in a box including an entire 1e14 Msun cluster), finding that clumps of molecular gas are formed within the tails of ram pressure stripped galaxies, which proceed to live in isolation within the ICM of a galaxy cluster for up to 300 Myr. Those molecular clumps possibly represent a new class of objects; similar objects have been observed in both galaxy clusters and groups, but no comprehensive description of them has been given until now. We additionally create a hydrodynamic model for the A901/2 multi-cluster system, and correlate the gas conditions in this model to the locations of a sample of candidate jellyfish galaxies in the system; this has allowed us to infer a possible mechanism for the generation of jellyfish morphologies in galaxy cluster collisions in general.
Nesta tese, nós visamos a contribuir para o entendimento do fenômeno da evolução de galáxias no ambiente de aglomerados de galáxias usando a metodologia de simulações numéricas. Para isso, desenvolvemos modelos hidrodinâmicos nos quais galáxias idealizadas ricas em gás movem-se em meio ao gás difuso de aglomerados de galáxias idealizados, permitindo um estudo detalhado e controlado da evolução destas galáxias neste ambiente extremo. O principal código usado em nossas simulações é o RAMSES, e nossos resultados tratam das mudanças em composição do gás, taxa de formação estelar, luminosidade e cor de galáxias caindo em aglomerados. Adicionalmente a processos acontecendo dentro das próprias galáxias, nós também descrevemos a dinâmica do gás que é varrido dessas galáxias com resolução sem precedentes para simulações dessa natureza (122 pc em uma caixa incluindo um aglomerado de 1e14 Msun inteiro), encontrando que aglomerados de gás molecular são formados nas caudas de galáxias que passaram por varrimento de gás por pressão de arraste, aglomerados estes que procedem a viver em isolamento em meio ao gás difuso de um aglomerado de galáxias por até 300 Myr. Esses aglomerados moleculares possivelmente representam uma nova classe de objetos; objetos similares foram previamente observados tanto em aglomerados quanto em grupos de galáxias, mas um tratamento compreensivo deles não foi apresentado até agora. Nós adicionalmente criamos um modelo hidrodinâmico para o sistema multi-aglomerado A901/2, e correlacionamos as condições do gás nesse modelo com a localização de uma amostra de galáxias jellyfish nesse sistema; isso nos permitiu inferir um possível mecanismo para a geração de morfologias jellyfish em colisões de aglomerados de galáxias em geral.

My Ph.D. has been composed of work involving the use of far–IR and submm observations of AGN. During this time it has focused on the in- terplay between AGN and their host galaxies and cluster environments. Understanding the role of AGN, and how they affect the evolution of both their host galaxies and surrounding environments, is a pressing concern in cosmological models of the universe, affecting as they do the chemical makeup, star formation rate, and morphology of their host galaxies. In Chapter 2, we focus on attempting to determine whether there is an inherent physical difference between Broad Absorption Line Quasars and non–BAL QSOs using Herschel observations taken at 250, 350 and 500 μm as part of the H–ATLAS (Eales et al. 2010) survey. BAL QSOs have been considered the most visible form of AGN feedback, and therefore are a prime starting point for understanding how galaxy evolution may be affected by the presence of an AGN. By using matched samples of 50 BAL and 329 non–BAL QSOs, we create weighted stacks at each wavelength, finding similar far–IR flux–densities for each sample within the errors. By SED modelling using a simple modified black body (Hildebrand 1983) fit to Mrk 231 and IZw1, we derive likely upper and lower limits for the BAL and non–BAL QSOs in each wavelength, again finding they are consistent within the errors. A bevy of statistical tests run on either population similarly finds no evidence to reject the null hypothesis they are drawn from the same parent population. These results would imply that HiBAL QSOs can be unified with ordinary QSOs within a simple orientation dependent scheme. We cannot make the same distinction for LoBALs or FeLoBALs, which the literature suggests may well be a separate evolutionary phase. In Chapter 3, we determine whether the presence of an AGN correlates to an overdensity of star–forming galaxies in the FIR, as has been found at shorter wavelengths (Falder et al. 2010). For the SHAGs study, 171 AGN were observed and selected at z∼1. By using observations at 250 μm, we are able to trace close to the peak of the grey–body SED created by reprocessing by dust of radiation from young O and B stars. Following data reduction, we determine number counts and correct for completeness within a 1Mpc radius of the central AGN. We find an overdensity on the order of around 0.4 sources per AGN, implying a degree of activity already significantly lower than at higher redshifts. This overdensity appears to be somewhat different between RL AGN and RQQ within 1Mpc. A cor- relation is found between radio luminosity and star formation overdensity, consistent with a stronger dependence found by Falder et al. (2010) at 3.6 μm, and there also appears to be a correlation between stellar mass and star formation overdensity for radio–loud QSOs. The galaxies in the environs of the AGN have LIRG–level luminosities, and are likely the pro- genitors of modern day S0 galaxies, whose population increases steadily from z∼1 to the present day (Postman et al. 2005; Smith et al. 2005). Our work with SCUBA–2, presented in Chapter 4, follows on from a prior sample of X–ray–absorbed QSOs (Stevens et al. 2005). This new sample is composed of more highly–absorbed X–ray QSOs and covers a larger area than the initial sample, so is ideal for an analysis of source counts around AGN at high–redshift. Data from the JCMT have been reduced, and completeness corrections and flux corrections applied to catalogues to determine the number counts around AGN. A comparison background, created using data from the Cosmology Legacy Survey has been used to derive comparison counts. The AGN have been investigated, yet none are detected above 3 at 850 μm, in contrast to the original sample. This may suggest that star formation in their host galaxies has been suppressed. Upon stacking in redshift and BAL classification, no difference in flux– density is apparent and the sources studied here have a similar stacked submm output to an unabsorbed QSO sample created for the original X– ray absorbed QSOs. However, over half of the sources here are BAL QSOs in contrast to the original absorbed QSO sample which contained only 1 BAL QSO. From the work in Chapter 2, one might expect BAL and non–BAL QSOs to have similar flux–densities. We argue that the sources studied in this thesis have likely undergone rapid evolution owing to a strong outflow, and as such star formation has been suppressed sufficiently that the submm emission is below the confusion noise. BAL winds may still be present, but essentially, the show is already over. A similar mechanism may already have occurred in unabsorbed QSOs if all QSOs pass through an X–ray–absorbed phase. With regard to source counts, we find that there is tentative evidence for an overdensity of sources around these AGN. The SFRs of the companion sources have been calculated using several greybody analogues, all of which imply a high degree of activity, suggesting these fields will evolve to become some of the most massive regions at the present epoch, in keeping with current theories of SMGs and high–redshift clusters.

We directly detect dust emission in an optically detected, multiply imaged galaxy lensed by the Frontier Fields cluster MACSJ0717.5+3745. We detect two images of the same galaxy at 1.1 mm with the AzTEC camera on the Large Millimeter Telescope leaving no ambiguity in the counterpart identification. This galaxy, MACS0717_Az9, is at z > 4 and the strong lensing model (mu = 7.5) allows us to calculate an intrinsic IR luminosity of 9.7 x 10(10) L-circle dot and an obscured star formation rate of 14.6 +/- 4.5 M-circle dot yr(-1). The unobscured star formation rate from the UV is only 4.1 +/- 0.3 M-circle dot yr(-1), which means the total star formation rate (18.7 +/- 4.5 M-circle dot yr(-1)) is dominated (75%-80%) by the obscured component. With an intrinsic stellar mass of only 6.9 x 10(9) M circle dot, MACS0717_Az9 is one of only a handful of z. >. 4 galaxies at these lower masses that is detected in dust emission. This galaxy lies close to the estimated star formation sequence at this epoch. However, it does not lie on the dust obscuration relation (IRX-beta) for local starburst galaxies and is instead consistent with the Small Magellanic Cloud attenuation law. This remarkable lower mass galaxy, showing signs of both low metallicity and high dust content, may challenge our picture of dust production in the early universe.

Globular clusters are tracers of the formation and evolution of their host galaxies. Kinematics, chemical abundances, age and position of the clusters allows tracing interactions between Milky Way and surrounding galaxies and outlines their chemical enrichment history. In this thesis we analyse mid-resolution spectra of about 800 red giant stars in 51 Galactic globular clusters. It is the first time that [Fe/H] and [Mg/Fe] derived in a consistent way are published for such a huge sample of globular clusters, almost 1/3 of the total number of catalogued clusters. Our metallicities are showed to be more precise than previous works based on mid-resolution spectroscopy. A turnover at [Fe/H] ~ -1.0 is found in the plot [Fe/H] vs. [Mg/Fe] for bulge and halo, although bulge seems to have a more metal-rich turnover, i.e, bulge has more efficient formation than the halo. Comparing the abundances with age the timescale for SNIa to start to become important is 1Gyr. [Fe/H] vs. age corroborates the different star formation efficiency of bulge and halo while [Mg/Fe] does not follow that. Halo was formed in mini halos or dwarf galaxies, and two multiple population clusters had their origin analysed to check it. M 22 seems to have been formed in the Milky Way while NGC 5824 possibly was originated in a dwarf galaxy, although our results are inconclusive for NGC 5824. The Galactic bulge seems to have been formed fast i.e., probably the oldest globular cluster is there. In fact HP 1 has a bluer horizontal branch than expected for its metallicity and we interpret that as an age effect. We determine its distance using light curves of variable stars in order to constrain future age determinations via colour-magnitude diagram. Finally, we investigate interaction between Milky Way and its neighbour galaxy SMC. We find that some star clusters are being stripped out of the SMC main body, which is consistent with tidal stripping scenario for the interaction between the galaxies, instead of ram pressure that would only affect gas.
Aglomerados globulares são traçadores da formação e evolução de suas galáxias. Cinemática, abundâncias químicas, idades e posições dos aglomerados permitem traçar interações entre Via Láctea e galáxias vizinhas e suas histórias de enriquecimento químico. Nesta tese analisamos espectros de média resolução de mais de 800 estrelas gigantes vermelhas em 51 aglomerados globulares Galácticos. É a primeira vez que [Fe/H] and [Mg/Fe] determinados de modo consistente são publicados para uma amostra desse porte, ~1/3 dos objetos catalogados. Nossas metalicidades são mais precisas que trabalhos anteriores similares. Uma quebra em [Fe/H] ~ -1.0 é encontrada no gráfico [Fe/H] vs. [Mg/Fe] para o bojo e halo, embora bojo parece ter uma quebra em [Fe/H] maior, i.e, bojo tem formaçãao mais eficiente que o halo. Comparando abundâncias com idade, a escala de tempo para SNIa ficar importante é 1Gano. [Fe/H] vs. idade corrobora diferentes eficiências de formação do bojo e halo, mas [Mg/Fe] vs. idade não mostra isso. O halo foi formado em mini halos ou galáxias anãs, e dois aglomerados com dispersão em [Fe/H] tiveram suas origens analisadas. M 22 parece ter sido formado na Via Láctea e NGC 5824 possivelmente foi originado em uma galáxia anã, embora os resultados são inconclusivos para NGC 5824. O bojo parece ter sido formado rapidamente e deve possuir o aglomerado mais velho. De fato, HP 1 tem um ramo horizontal mais azul que o esperado para sua metalicidade e vemos isso como um efeito da idade. Determinamos sua distância usando curvas de luz de RR Lyrae de maneira a restringir futuras determinações de idade via diagrama cor-magnitude. Finalmente, investigamos a interação entre Via Láctea e sua galáxia vizinha SMC. Encontramos aglomerados sendo removidos do corpo central da SMC, consistente com cenário de remoção por força de maré para a interação entre as galáxias, em vez de ``ram pressure\'\' que afeta só gás.

We present a detailed, multi-wavelength study of star formation (SF) and active galactic nucleus (AGN) activity in 11 near-infrared (IR) selected, spectroscopically confirmed massive (greater than or similar to 10(14)M(circle dot)) galaxy clusters at 1 < z < 1.75. Using new deep Herschel/PACS imaging, we characterize the optical to far-IR spectral energy distributions (SEDs) for IR-luminous cluster galaxies, finding that they can, on average, be well described by field galaxy templates. Identification and decomposition of AGNs through SED fittings allows us to include the contribution to cluster SF from AGN host galaxies. We quantify the star-forming fraction, dust-obscured SF rates (SFRs) and specific SFRs for cluster galaxies as a function of cluster-centric radius and redshift. In good agreement with previous studies, we find that SF in cluster galaxies at z greater than or similar to 1.4 is largely consistent with field galaxies at similar epochs, indicating an era before significant quenching in the cluster cores (r < 0.5 Mpc). This is followed by a transition to lower SF activity as environmental quenching dominates by z similar to 1. Enhanced SFRs are found in lower mass (10.1< logM(kappa)/M-circle dot < 10.8) cluster galaxies. We find significant variation in SF from cluster to cluster within our uniformly selected sample, indicating that caution should be taken when evaluating individual clusters. We examine AGNs in clusters from z = 0.5-2, finding an excess AGN fraction at z greater than or similar to 1, suggesting environmental triggering of AGNs during this epoch. We argue that our results-a transition from field-like to quenched SF, enhanced SF in lower mass galaxies in the cluster cores, and excess AGNs-are consistent with a co-evolution between SF and AGNs in clusters and an increased merger rate in massive halos at high redshift.

This thesis explores the properties of galaxies that reside in high density regions in the local Universe. The driving motivation is to shed light on how galaxies stop forming stars and the role played by the environment in this process. A wide variety of environmentallyrelated quenching mechanisms have been proposed and the smoking gun proving that these are actually transforming galaxies from being star forming to passive is the existence of a population of galaxies in transition with intermediate features. In this Thesis I will mainly focus on the properties of two population of galaxies: those that are forming stars at a reduced level compared to other galaxies at similar mass, and those that have recently and abruptly interrupted their star formation. The data exploited in this Thesis come from surveys which focus on different environments in the local Universe: the WIde-field Nearby Galaxy-cluster Survey (WINGS, Fasano et al. 2006) with its recent extension OmegaWINGS (Gullieuszik et al., 2015a, Moretti et al. submitted) and the Padova Millennium Galaxy and Group catalogue (PM2GC, Calvi et al. 2011). The Thesis starts with a characterization of the WINGS survey and then focuses on the new OmegaWINGS data, illustrating the most important aspects of the photometric and spectroscopic observations. I will describe in detail my personal contribution to the survey, from the data analysis to the redshift and velocity dispersion measurements, to the membership assignments. I will also present the complete galaxy catalog built from the combination of the two cluster surveys considered. While halo mass estimates are quite easily obtained for clusters, at the group regime it is less straightforward to derive robust measurements. I will then present a procedure I developed to compute halo masses from observable quantities for the PM2GC. My approach exploits mock galaxy catalogs extracted from the De Lucia & Blaizot (2007) semi-analytic model, run on the Millennium Simulation (Springel et al. 2005). Subsequently, I will discuss the methods adopted to compute the galaxy stellar properties, exploiting both spectroscopic and photometric information. Using the data from the WINGS+OmegaWINGS surveys, I will present an analysis of the ongoing Star Formation Rate (SFR) and the Specific Star Formation Rate (SSFR) - stellar mass (M∗) relations in field and cluster mass limited samples. I contrast trends in the different environments and, in clusters, at different clustercentric distances. The main result is that in clusters a population of galaxies with a reduced SFR is detected, named transition galaxies, which is much rarer in the field. The spatial location of these galaxies, together with the analysis of the star formation histories, colors and average ages, suggest that transition galaxies have had a reduced SFR for the past 2-5 Gyr. This is compatible with a strangulation scenario, even if other processes like ram pressure stripping can not be excluded. I characterize the properties of post starburst (PSB) galaxies, which are galaxies that abruptly interrupted their star formation sometimes during the past < 1.5 Gyr and present recognizable features in their spectra (no emission and Hδ in absorption). Exploiting an observed magnitude limited sample drawn from WINGS+OmegaWINGS, I will present the first complete characterization of PSBs in clusters and contrast their properties to those of passive (PAS) and emission line (EML) galaxies. The main finding is that the incidence of PSBs increases from the outskirts toward the cluster centers and, more gently, from the least toward the most luminous and massive clusters. PSBs have stellar masses, magnitudes, colors and morphologies intermediate between PAS and EML galaxies, typical of a population that has recently become passive. The phase-space analysis and the velocity dispersion profiles also suggest that PSBs represent a combination of galaxies with different accretion histories. In particular, PSBs with the strongest Hδ are consistent with being recently accreted. This analysis suggests that as a galaxy is accreted onto a cluster and approaches the cluster virialized region, ram pressure stripping (or other interactions) induces either a burst of the star formation with a subsequent fast quenching, or simply a fast quenching and this is more efficient in more massive clusters. I then characterize the PSB population also in smaller systems. Combining the WINGS+OmegaWINGS and PM2GC data, I will show how the fraction of PSB galaxies and the quenching efficiency depend on the halo mass, and increase from single galaxies, to binaries, groups and clusters. In the different environments, different physical mechanisms are probably responsible for the production of PSB galaxies, but all of them produce a truncation of star formation on short time-scales. If ram pressure stripping is the most likely candidate in clusters, gravitational interaction between galaxies could be more efficient in lower density regions. To conclude, the role of environment is undisputed in driving the galaxy evolution. In this Thesis I identified two possible evolutionary pathways related to the end of star formation in clusters: one requires a gradual reduction of star formation (long time-scale quenching t>2Gyr), the other a rapid truncation of the star formation activity (t∼0.1Gyr). Comparing the fraction of PSBs to the fraction of galaxies in transition, it appears that the short timescale star-formation quenching channel contributes two times more than the long timescale one to the growth of the passive population in clusters. The emerging picture is that ram pressure stripping is probably the most successful process affecting star formation in clusters, followed by strangulation. Other mechanisms might play a role, even though they most likely take place in lower density environments.
L’obiettivo del lavoro presentato in questa tesi è la caratterizzazione osservativa delle proprietà delle galassie, per vincolarne gli scenari di formazione ed evoluzione. In particolare, mi sono concentrata nel determinare come l’ambiente in cui evolvono le galassie influisca sul processo di formazione stellare nelle regioni ad alta densità nell’Universo locale. Tra i diversi meccanismi proposti come responsabili del quenching, ovvero la tendenza delle galassie a smettere di produrre stelle, quelli relativi all’ambiente sembrano svolgere un ruolo cruciale. La prova che confermerebbe l’azione primaria di questi meccanismi dovrebbe essere l’esistenza, in ammassi e gruppi di galassie, di una popolazione di galassie in transizione con caratteristiche intermedie. Con lo scopo dunque di identificare questa nuova popolazione, mi concentrerò sullo studio delle proprietà di due particolari classi di galassie: quelle che stanno formando stelle ad un livello ridotto rispetto ad altre galassie di massa simile, e quelle che hanno recentemente, e molto probabilmente bruscamente, interrotto la loro formazione stellare. Utilizzerò i dati provenienti da diverse campagne osservative: la WIde-field Nearby Galaxy-cluster Survey (WINGS, Fasano et al. 2006) con la sua recente estensione OmegaWINGS (Gullieuszik et al., 2015a, Moretti et al. 2017), e il Padova Millennium Galaxy and Group catalogue (PM2GC, Calvi et al. 2011). La prima parte della tesi è riservata alla descrizione delle survey WINGS e OmegaWINGS. Particolare attenzione sarà dedicata alla presentazione degli aspetti più importanti delle osservazioni fotometriche e spettroscopiche di OmegaWINGS, in cui sono stata personalmente coinvolta. Descriverò in dettaglio il mio contributo alla survey, che va dall’analisi dei dati alle misurazioni di redshift e dispersione di velocità, all’individuazione delle galassie appartenenti agli ammassi. Presenterò dunque il catalogo completo costruito dalla combinazione delle due survey considerate. Mentre è relativamente facile ricavare una stima della massa di alone per gli ammassi, e più complicato ottenere misurazioni affidabili per i gruppi. Ho quindi sviluppato una procedura per derivare le masse di alone per i dati PM2GC da quantità osservabili, che descriverò in dettaglio. Tale procedura utilizza cataloghi di galassie estratti dai modelli semi analitici di De Lucia & Blaizot (2007), applicati alla Millennium Simulation (Springel et al. 2005). Successivamente, descriverò i metodi adottati per il calcolo delle propriet`a delle galassie, sfruttando le informazioni provenienti da osservazioni spettroscopiche e fotometriche. Utilizzando i dati WINGS+OmegaWINGS, discuterò l’analisi delle relazioni tra il tasso di formazione stellare (SFR), il tasso specifico di formazione stellare (SSFR) e la massa stellare di una galassia (M∗), nel campo e negli ammassi, in campioni limitati in massa. Confronterò gli andamenti nei diversi ambienti e, negli ammassi, a diverse distanze dal centro. Il risultato principale è l’individuazione di una popolazione di galassie in ammasso con SFR ridotta, denominate galassie in transizione, che è molto più rara nel campo. La distribuzione spaziale di queste galassie, insieme all’analisi della storia di formazione stellare, dei colori ed età medie, suggeriscono che le galassie in transizione abbiano avuto un SFR ridotta per circa 2-5 Gyr. Questo è compatibile con uno scenario di “strangulation”, anche se altri processi come la ram pressure stripping non possono essere esclusi. Successivamente caratterizzerò le proprietà delle galassie cosiddette post starburst (PSB), cioè galassie che hanno bruscamente interrotto la loro formazione stellare all’incirca 1 miliardo di anni fa e che presentano caratteristiche ben riconoscibili nei loro spettri (nessuna emissione e Hδ in assorbimento). Sfruttando un campione limitato in magnitudine apparente estratto dai dati WINGS + OmegaWINGS, presenterò la prima caratterizzazione completa di galassie PSB in ammasso e confronterò le loro proprietà con quelle di galassie passive (PAS) e con righe di emissione (EML). Il principale risultato riguarda il numero relativo di galassie PSB, che aumenta leggermente dalla periferia verso il centro degli ammassi e dall’ammasso meno luminoso/massiccio a quello più luminoso/massiccio. Le galassie PSB hanno proprietà, quali masse stellari, magnitudini, colori e morfologie, intermedie tra le PAS e EML, tipiche di una popolazione che è recentemente diventata passiva. L’analisi dello spazio delle fasi e dei profili di dispersione di velocità indicano anche che le PSB rappresentano una combinazione di galassie con diverse storie di accrescimento. In particolare, PSB con forte Hδ sono consistenti con l’essere state recentemente accresciute. Questa analisi suggerisce che, nel processo di accrescimento di una galassia su un ammasso, all’avvicinarsi alla regione virializzata, per effetto della ram pressure stripping (o di altre interazioni) viene indotto un rapido quenching, preceduto o meno da un forte episodio di formazione stellare; inoltre quest’effetto è più forte in ammassi più massicci. Descriverò successivamente la popolazione di PSB in sistemi più piccoli. Combinando i dati WINGS+OmegaWINGS ai dati PM2GC, mostrerò come la frazione di galassie PSB e l’efficienza del quenching dipendano dalla massa dell’alone e aumentino andando da galassie singole, a sistemi binari, gruppi e ammassi. Nei diversi ambienti, diversi meccanismi fisici sono probabilmente responsabili per la produzione di galassie PSB, ma tutti producono un troncamento della formazione stellare su brevi scale temporali. Mentre negli ammassi la ram presssure stripping sembra essere il candidato più probabile, l’interazione gravitazionale tra galassie potrebbe essere più efficiente nelle regioni a bassa densità. Dal confronto tra la frazione di PSB e di galassie in transizione, si può dedurre che il canale di quenching più rapido constribuisce circa due volte di più alla crescita della popolazione di galassie passive rispetto al canale di quenching più lento. Il quadro che emerge è che la ram pressure stripping è probabilmente il processo che maggiormente incide sul quenching della formazione stellare negli ammasi, seguito dalla strangulation. Altri meccanismi potrebbero influire, anche se probabilmente con effetto maggiore in ambienti a minore densità.

We report the detection of diffuse Ly alpha emission, or Lya halos (LAHs), around star-forming galaxies at z approximate to 3.78 and 2.66 in the NOAO Deep Wide-Field Survey Bootes field. Our samples consist of a total of similar to 1400 galaxies, within two separate regions containing spectroscopically confirmed galaxy overdensities. They provide a unique opportunity to investigate how the LAH characteristics vary with host galaxy large-scale environment and physical properties. We stack Ly alpha images of different samples defined by these properties and measure their median LAH sizes by decomposing the stacked Ly alpha radial profile into a compact galaxy-like and an extended halo-like component. We find that the exponential scale-length of LAHs depends on UV continuum and Ly alpha luminosities, but not on Ly alpha equivalent widths or galaxy overdensity parameters. The full samples, which are dominated by low UV-continuum luminosity Lya emitters (M-UV greater than or similar to -21), exhibit LAH sizes of 5-6 kpc. However, the most UV- or Ly alpha-luminous galaxies have more extended halos with scale-lengths of 7-9 kpc. The stacked Ly alpha radial profiles decline more steeply than recent theoretical predictions that include the contributions from gravitational cooling of infalling gas and from low-level star formation in satellites. However, the LAH extent matches what one would expect for photons produced in the galaxy and then resonantly scattered by gas in an outflowing envelope. The observed trends of LAH sizes with host galaxy properties suggest that the physical conditions of the circumgalactic medium (covering fraction, H I column density, and outflow velocity) change with halo mass and/or star formation rates.

We present a study of the relation between galaxy stellar age and mass for 14 members of the z = 1.62 protocluster IRC 0218, using multiband imaging and HST G102 and G141 grism spectroscopy. Using UVJ colors to separate galaxies into star-forming and quiescent populations, we find that, at stellar masses M*>= 10(10.85)M circle dot the quiescent fraction in the protocluster is f(Q) = 1.0(-0.37)(+0.00), consistent with a similar to 2x enhancement relative to the field value, f(Q) = 0.45(-0.03)(+0.03). At masses 10(10.2)M circle dot <= M* <= 10(10.85)M circle dot, f(Q) in the cluster is f(Q) = 0.40(-0.18)(+0.20), consistent with the field value of f(Q) = 0.28(-0.02)(+0.02). Using galaxy D-n(4000) values derived from the G102 spectroscopy, we find no relation between galaxy stellar age and mass. These results may reflect the impact of merger- driven mass redistribution-which is plausible, as this cluster is known to host many dry mergers. Alternately, they may imply that the trend in f(Q) in IRC 0218 was imprinted over a short timescale in the protocluster's assembly history. Comparing our results with those of other high- redshift studies and studies of clusters at z similar to 1, we determine that our observed relation between f(Q) and stellar mass only mildly evolves between z similar to 1.6 and z similar to 1, and only at stellar masses M* <= 10(10.85) M circle dot Both the z similar to 1 and z similar to 1.6 results are in agreement that the red sequence in dense environments was already populated at high redshift, z greater than or similar to 3, placing constraints on the mechanism(s) responsible for quenching in dense environments at z >= 1.5.

Massive stars emit strong fluxes of ionising radiation and their dynamical impact on their natal clusters is expected to be severe. The outflows generated expel residual gas from the cluster and can potentially gravitationally disrupt it. The loss of its reserves of molecular gas also prevents the cluster forming more stars. Star-formation and star cluster evolution cannot be fully understood without a proper treatment of feedback. I present a novel technique I have developed to allow the inclusion of the effects of ionising radiation in smoothed particle hydrodynamics (SPH) simulations of star clusters. The new algorithm is able to reproduce the results of simple analytical models and also gives results in good agreement with a more sophisticated Monte Carlo radiative transfer code when tested under highly anisotropic conditions. I simulate the effects of ionising radiation in globular clusters and compare my results with one-dimensional calculations with which I find good agreement. I investigate three cases in which different quantities of gas are distributed in my model cluster such that the as becomes fully ionised either during the HII region’s formation phase, or during its expansion phase, or such that the HII region is trapped inside the cluster core. I find gas expulsion to be quite efficient in the calculations in which the HII region escapes the core. I observe an instability in the second calculation which causes the shocked shell driven by the ionisation front to fragment as the HII region exits the core. The instability produces new structure from the smooth gas in the system, but this structure is rapidly destroyed by the radiation field and the effect of the instability on the evolution of the system is minimal. I also simulate feedback in the context of young embedded clusters, a highly inhomogeneous and anisotropic environment. I find that, again, photoionisation is able to produce novel structure in the ambient gas, causing it to fragment into filaments and beads. This fragmentation of the neutral gas, together with compression by hot ionised gas, which decreases the Jeans mass, lead me to conclude that feedback promotes star formation.

This thesis presents one of the first census of the properties of galaxies in X-ray selected groups and clusters at intermediate redshift, with the aim of assessing the role of envi- ronment on the galaxy stellar mass assembly, star formation activity and observed stellar population properties. My project is framed in the XXL Survey (Pierre et al. 2016), the largest XMM-Newton programme approved to date, covering two extragalactic regions in the sky of 25 deg2 each one. Extended X-ray sources identified as groups and clusters are spectroscopically confirmed and their main properties are characterised either via direct measurements or by means of scaling relations. Among them, inferred X-ray luminosities and temperatures, virial masses and radii are of fundamental importance for the development of this thesis. The great advantage of XXL is that the XXL-North field (XXL-N) is fully covered by photometric and spectroscopic observations coming from the most recent extragalactic surveys of galaxies. The availability of such a treasure trove of information motivates the development of my research on galaxy populations at 0.1≤z≤0.6 in XXL-N, exploring the most diverse environments ranging from the field, to groups, clusters and superclusters. The first task of my work consists in the creation of a homogeneous spectrophotometric sample of galaxies, released in Guglielmo et al. (2017), suitable for scientific purposes. The catalogue contains spectroscopic redshifts, membership information on groups and clus- ters, spectroscopic completeness weights as a function of position in the sky and observed magnitude, stellar masses and absolute magnitudes computed by means of a spectral en- ergy distribution (SED) technique. The catalogue is fundamental for all XXL studies that aims at relating optical properties derived from galaxies with X-ray information and is widely used in the whole XXL collaboration. The released spectrophotometric catalogue enables the first scientific achievement of this thesis regarding the study of the galaxy stellar mass function (GSMF). The goal of this analysis is to unveil whether the mass assembly of galaxies depends on global environment, i.e. field vs groups and clusters and, among groups and clusters, on X-ray luminosity, used as a proxy for the halo mass. I performed the analysis in four redshift bins in the range 0.1≤z≤0.6, finding overall that environment does not affect the GSMF, at least in the mass range probed. The result is further confirmed by the invariance of the mean mass of member galaxies on X-ray luminosity. I also looked into the evolution of the mass assembly from z=0.6 down to z=0.1, finding that the high mass end is already in place at the oldest epoch and does not evolve and detecting an increase in the low-mass galaxy population in the same redshift range. This study is one of the first systematic studies on the GSMF conducted for X-ray extended sources ranging from the group to the cluster environment, and is published in the second part of Guglielmo et al. (2017). Having assessed the independence of the mass distributions on the global environment, I proceed investigating whether and to what extent the environment affects the star formation activity and the observed properties of the galaxy stellar populations. I started this analysis from the richest supercluster identified in XXL-N, XLSSsC N01, located at redshift z∼0.3 and composed of 14 groups and clusters. This work has been submitted in Guglielmo et al. (2018a). With focus on the region surrounding XLSSsC N01, I divided galaxies in different environments, ranging from the virial regions of groups and clusters to the field, using a combination of global and local environment parametri- sations. The main results of this study are that, in the supercluster environment, while the star forming fractions and quenching efficiency strongly depend on environment, the SFR-mass relation does not. The star forming fraction progressively declines from the field to filaments to the virialised regions of groups and clusters, with an interesting en- hancement in the outer regions of the X-ray structures. Moreover, while the average luminosity weighted (LW)-age-mass relation is independent of the environment, a clear signature for recent star formation quenching is found in the stellar ages of passive galaxies in the virialised regions of X-ray structures. Finally, I extend the analysis of this peculiar supercluster to the whole XXL-N field. This work will be enclosed in two articles in preparation (Guglielmo et al. 2018 b,c in prep.). Thanks to the higher statistics of the entire sample, I investigated the properties of galaxies and their evolution at 0.1≤z≤0.5 in different environments, with the goal of characterising the changing in the stellar population properties and the build up of the passive population via environmental quenching. Besides distinguishing among galaxies in the field, and in groups and clusters (virial regions and outskirts), I also focused on galaxies located in structures of different X-ray luminosity and in galaxies located within superclusters. Simultaneously, I also investigated the properties of galaxies located at different projected local densities (LD). In particular, I characterised the fraction of star forming/blue galaxies and of the SFR- mass relation, as a function of both global and local environment. The fraction of star forming and blue galaxies is strictly related to the environment, having the lowest value in the virial regions of groups and clusters, and the highest in the field. In outer members, the same fraction is similar to that in the field at z≥0.2, and assumes intermediate values with respect to virial members and the field at 0.1≤z<0.2. The SFR-mass relation is also environment dependent, and in particular the number of virial member galaxies having reduced SFR (galaxies in transition) nearly doubles that of field galaxies. Again, outer members show intermediate properties: the fraction of galaxies in transition is similar to the virial population at z>0.3, when it is found to be associated to the supercluster environment, and then reduces to values typical of field galaxies at 0.1≤z<0.3. The star forming and blue fractions also decrease with increasing LD at all redshifts. On the contrary, the fraction of galaxies in transition does not vary in the same LD range. These significant differences emerging among the global and local environments are intrinsically related to the different physical meaning of the two parametrisations, thus to the different physical mechanisms acting on galaxies when bound in the potential well of a dark matter halo (according to the global definition) or when exposed to interactions with other galaxies in over dense and highly populated regions (according to the local definition). During the first stages of my PhD, I also completed the analysis of my master thesis, and I report the full text of the published paper in the Appendix of the thesis (Guglielmo et al. 2015). The results are closely related to the scientific questions tackled in my PhD project, addressed through a complementary approach that reconstructed the star formation history of low- redshift galaxies in clusters and in the field to study the dependence on global environment, stellar mass and observed morphology.
Questa tesi presenta uno dei primi censimenti delle proprietà delle galassie in gruppi ed ammassi selezionati nei raggi X a redshift intermedio, con lo scopo di valutare il ruolo dell'ambiente sull'assemblamento della massa stellare delle galassie, l'attività di formazione stellare e le proprietà delle popolazioni stellari osservate. Il mio progetto è inquadrato nella Survey XXL (Pierre et al., 2016), il più grande programma XMM-Newton approvato fino ad oggi, che copre due regioni extra-galattiche di 25 gradi quadrati ciascuna. Le sorgenti di raggi X estese ed identificate come gruppi ed ammassi sono state confermate spettroscopicamente e le loro proprietà principali sono caratterizzate o tramite misurazioni dirette o mediante relazioni di scala. Tra queste, le luminosità e le temperature X, le masse virali ed i raggi viriali sono di fondamentale importanza per lo sviluppo di questa tesi. Il grande vantaggio di XXL è che il campo XXL-Nord (XXL-N) è completamente coperto da osservazioni fotometriche e spettroscopiche provenienti dalle più recenti survey extra-galattiche di galassie. La disponibilità di una tale miniera di informazioni motiva lo sviluppo della mia ricerca sulle popolazioni di galassie nell'intervallo di redshift 0.1≤z≤0.6 nel campo XXL-N, esplorando gli ambienti più diversi dalle regioni di campo, ai gruppi, agli ammassi e ai superammassi. Il primo compito del mio lavoro consiste nella creazione di un campione spettrofotometrico omogeneo di galassie, pubblicato in Guglielmo et al. (2017), e adatto ad essere utilizzato in un'analisi scientifica. Il catalogo contiene redshift spettroscopici, informazioni sull'appartenenza a gruppi e ammassi, pesi di completezza spettroscopica in funzione della posizione nel cielo e magnitudine osservata, le masse stellari e le magnitudini assolute calcolate mediante una tecnica di spectral energy distribution (SED). Il catalogo è fondamentale per tutti gli studi all'interno della collaborazione XXL che mirano a correlare le proprietà ottiche derivate dalle galassie con informazioni sulle strutture X ed è ampiamente utilizzato nell'intera collaborazione XXL. Il catalogo spettrofotometrico pubblicato consente il primo risultato scientifico di questa tesi riguardante lo studio della funzione di massa stellare delle galassie (GSMF). L'obiettivo di questa analisi è di svelare se il processo di assemblamento della massa delle galassie dipende dall'ambiente cosiddetto globale, cioè dall'appartenenza al campo o a gruppi ed ammassi e, tra quest'ultimi, dalla luminosità X, utilizzata come proxy per la massa di alone. Ho eseguito l'analisi in quattro intervalli di redshift nel range 0.1≤z≤0.6, trovando nel complesso che l'ambiente non influenza la GSMF, almeno nell'intervallo di massa campionato. Il risultato è ulteriormente confermato dall'invarianza della massa media delle galassie membre rispetto alla luminosità X. Ho anche esaminato l'evoluzione della GSMF da z = 0.6 fino a z = 0.1, trovando che l'estremità della funzione a masse elevate è già formata nell'epoca più antica e non evolve, e rilevando invece un aumento del numero di galassie a bassa massa nello stesso intervallo di redshift. Questo studio è uno dei primi studi sistematici della GSMF condotto per sorgenti estese a raggi X che vanno da masse di alone tipiche di gruppi fino agli ammassi, ed è pubblicato nella seconda parte di Guglielmo et al. (2017). Avendo valutato l'indipendenza delle distribuzioni di massa rispetto all'ambiente globale, procedo a indagare se e in che misura l'ambiente influisce sull'attività di formazione stellare e sulle proprietà osservate delle popolazioni stellari delle galassie. Ho iniziato questa analisi dal superammasso più ricco identificato in XXL-N, XLSSsC N01, situato a redshift z~0.3 e composto da 14 gruppi ed ammassi. Questo lavoro è stato sottomesso in Guglielmo et al. (2018a). Concentrandosi sulla regione che circonda XLSSsC N01, ho diviso le galassie in diversi ambienti, che vanno dalle regioni virali di gruppi ed ammassi fino al campo, utilizzando una combinazione di parametrizzazioni ambientali globali e locali. I risultati principali di questo studio sono che, nell'ambiente del superammasso, mentre la frazione di galassie che formano stelle e l'efficienza di arresto dell'attività di formazione stellare dipendono fortemente dall'ambiente, lo stesso non vale per la relazione fra massa e tasso di formazione stellare (SFR). La frazione di galassie che formano stelle declina progressivamente dal campo ai filamenti fino alle regioni virializzate di gruppi ed ammassi, con un interessante aumento della stessa nelle regioni esterne delle strutture X. Inoltre, mentre la relazione media fra l'età delle galassie pesata per la luminosità (LW-age) e la massa stellare delle stesse è indipendente dall'ambiente, una chiara evidenza di una recente estinzione della formazione stellare si rileva nelle galassie passive situate nelle regioni virializzate delle strutture X. Infine, si estende l'analisi di questo particolare superammasso all'intero campo XXL-N. Questo lavoro sarà racchiuso in due articoli in preparazione (Guglielmo et al., 2018 b, c in preparazione). Grazie alla statistica più elevata dell'intero campione, ho studiato le proprietà delle galassie e la loro evoluzione nell'intervallo 0.1≤z≤0.5 in diversi ambienti, con l'obiettivo di caratterizzare il cambiamento nelle proprietà delle popolazioni stellari e la formazione della popolazione passiva al variare dell'ambiente. Oltre a distinguere tra le galassie di campo, e in gruppi ed ammassi (regioni virali e periferiche), mi sono concentrata anche su galassie situate in strutture di diversa luminosità X e sulle galassie situate all'interno di superammassi. Contemporaneamente, ho anche studiato le proprietà delle galassie situate a diverse densità locali proiettate (LD). In particolare, ho caratterizzato la frazione galassie con formazione stellare attiva / galassie blu e ho studiato la relazione fra massa e SFR, in funzione sia dell'ambiente globale che di quello locale. La frazione di galassie con formazione stellare attiva e di galassie blu è strettamente correlata all'ambiente, con il valore più basso nelle regioni virali di gruppi ed ammassi e il più alto nel campo. Nei membri esterni, la stessa frazione è simile a quella nel campo a z≥0.2 e assume valori intermedi rispetto ai membri virali e il campo a 0.1≤z<0.2. La relazione fra SFR e massa è anch'essa dipendente dall'ambiente, e in particolare il numero di galassie dei membri virali che hanno una ridotta SFR (galassie in transizione) risulta essere quasi il doppio di quella di galassie di campo. Ancora una volta, i membri esterni mostrano proprietà intermedie: la frazione di galassie in transizione è simile alla popolazione virale a z>0,3, ed in particolare risulta associata all'ambiente dei superammassi, e si riduce successivamente a valori tipici delle galassie di campo a 0.1≤z<0.3. Inoltre, la frazione di galassie attive nella formazione stellare e quella di galassie blu diminuiscono con l'aumentare della LD a tutti i redshift. Al contrario, la frazione di galassie in transizione non varia nello stesso intervallo LD. Queste differenze significative emergenti tra gli ambienti globali e locali sono intrinsecamente correlate al diverso significato fisico delle due parametrizzazioni, quindi ai diversi meccanismi fisici che agiscono sulle galassie quando sono legate al potenziale gravitazionale di un alone di materia oscura (secondo la definizione globale) o quando esposto ad interazioni con altre galassie in regioni densamente popolate (secondo la definizione locale). Durante le prime fasi del mio dottorato di ricerca, ho anche completato l'analisi della mia tesi magistrale, e riporto il testo completo del documento pubblicato in Appendice alla tesi (Guglielmo et al., 2015). I risultati sono strettamente correlati alle domande scientifiche trattate nel mio progetto di dottorato, ma vengono affrontate attraverso un approccio complementare, che mira alla ricostruzione della storia della formazione stellare delle galassie a basso redshift in ammassi e nel campo per studiare la dipendenza dall'ambiente globale, dalla massa stellare e dalla morfologia osservata.

This thesis is concerned with the dynamical evolution of globular star clusters modelled as the classical gravitational N-body problem. The models in this thesis are idealised in order to allow the detailed study of particular dynamical aspects of the cluster evolution. Examples of properties which tend to be omitted are stellar evolution, primordial binaries and the effect of an external tidal gravitational field. The methods used in this thesis are gas models, N-body models and physical arguments. One of the main topics in this thesis is gravothermal oscillations in multicomponent star clusters. The evolution of one-component globular clusters, systems with equal particle masses, is quite well understood. However, the evolution of more realistic globular clusters, with a range of particle masses, is a much more complicated matter. The condition for the on-set of gravothermal oscillations in a one-component system is simply that the number of stars is greater than a certain number ( ≈7000). In a multi-component system the relationship between the number of stars at which the gravothermal oscillations first appear and the stellar mass distribution of a cluster is a complex one. In order to investigate this phenomenon two different types of multi-component systems were studied: two-component systems (the simplest approximation of a mass spectrum, Chapter 2) and ten-component systems (which were realisations of continuous power law IMFs, Chapter 3). In both cases the critical number of stars at which gravothermal oscillations first appear are found empirically for a range of stellar mass distributions. The nature of the oscillations themselves are investigated and it is shown that the oscillations can be understood by focusing on the behaviour of the heavier stars within the cluster. A parameter Nef (de nined Mtot/mmax where Mtot is the total mass and mmax is the maximum stellar mass) acts as an approximate stability boundary for multicomponent systems.The stability boundary was found to be at Nef ~- 12000. In this Chapter 4, globular star clusters which contain a sub-system of stellar-mass black holes (BH) are investigated. This is done by considering two-component models, as these are the simplest approximation of more realistic multi-mass systems, where one component represents the BH population and the other represents all the other stars. These systems are found to undergo a long phase of evolution where the centre of the system is dominated by a BH sub-system. After mass segregation has driven most of the BH into a compact sub-system, the evolution of the BH sub-system is found to be in uenced by the cluster in which it is contained. The BH sub-system evolves in such a way as to satisfy the energy demands of the whole cluster, just as the core of a one component system must satisfies the energy demands of the whole cluster. The BH sub-system is found to exist for a significant amount of time. It takes approximately 10trh;i, where trh;i is the initial half-mass relaxation time, from the formation of the compact BH sub-system up until the time when 90% of the sub-system total mass is lost (which is of order 103 times the half-mass relaxation time of the BH sub-system at its time of formation). Based on theoretical arguments the rate of mass loss from the BH sub-system (M2) is predicted to be (βζM)/(αtrh): where M is the total mass, trh is the half-mass relaxation time, and α, β, ζ are three dimensionless parameters. (see Section 4.3 for details). An interesting consequence of this is that the rate of mass loss from the BH sub-system is approximately independent of the stellar mass ratio (m2/m1) and the total mass ratio (M2/M1) (in the range m2/m1 ≥ 10 and M2/M1 ≈ 10-2, where m1, m2 are the masses of individual low-mass and high-mass particles respectively, and M1, M2 are the corresponding total mass). The theory is found to be in reasonable agreement with most of the results of a series of N-body simulations, and all of the models if the value of ζ is suitable adjusted. Predictions based on theoretical arguments are also made about the structure of BH sub-systems. Other aspects of the evolution are also considered such as the conditions for the onset of gravothermal oscillation. The final chapter (Chapter 5) of the thesis contains some concluding comments as well as a discussion on some possible future projects, for which the results in this thesis would be useful.

Nouvelle methode pour la synthese de populations stellaires dans les noyaux de galaxies utilisant une bibliotheque de spectres integres d'amas d'etoiles. L'avantage de cette methode porte sur deux parametres: l'age et la metallicite. Une attention speciale a ete donnee aux differentes sources de rougissement interstellaire affectant les amas d'etoiles et les galaxies. Cette approche permet une estimation directe de l'evolution chimique a partir du spectre en absorption des noyaux de galaxies

Nous étudions la formation d’étoiles et d’amas d’étoiles dans les galaxies dominées par le gaz. Ce terme réfère en premier lieu aux galaxies de l’époque du pic de formation d’étoiles dans l’histoire de l’Univers, qui s’est déroulé vers z ~ 2, mais aussi à leurs analogues locaux, les galaxies naines de marées. En premier lieu, en utilisant des simulations numériques, nous montrons que les galaxies massives typiques de z=2, avec une fraction de gaz d’environ 50%, forment des structures gazeuses massives (10**7-8 masses solaires) et liées gravitationnellement, appelées grumeaux dans la suite. Ces grumeaux ne se forment dans des galaxies avec une fraction de gaz inférieure à 25%. Nous présentons ensuite une étude observationnelle d’un analogue local de grumeaux de galaxies à z=2, la galaxie naine de marée NGC 5291N. Une analyse des raies d’émission de cette galaxie montre la présence de chocs sur les pourtours de l’objet. La photométrie des amas d’étoiles de cette galaxie montre que les amas les plus jeunes (< 10 millions d’années) sont significativement moins massifs que les amas plus âgés. Ceci peut être le signe de fusions progressives d’amas et/ou d’une forte activité de formation stellaire dans ce système il y a environ 500 millions d’années.Dans un second lieu nous étudions comment la fraction de gaz influe sur la formation d’étoiles et d’amas stellaires dans des fusions de galaxies à z=2. En utilisant des simulations numériques nous montrons que ces fusions n’augmentent que relativement peu le taux de formation d’étoiles et d’amas stellaires comparativement aux fusions de galaxies locales, à faible fraction de gaz. Nous montrons que ceci est due à une saturation de plusieurs facteurs physiques, qui sont déjà présents naturellement dans les galaxies isolées à z=2 et sont donc comparativement peu accentués par les fusions. Il s’agit de la turbulence du gaz, des zones de champ de marée compressif et des flux de matières vers le noyau de la galaxie. Nous montrons aussi que les structures stellaires formées au sein des grumeaux de gaz sont préservées par la fusion : elles sont éjectées des disques et orbitent dans le halo de la galaxie résultante de la fusion, où elles peuvent devenir les progéniteurs de certains amas globulaires
We study the formation of stars and stellar clusters in gas-dominated galaxies. This term primarily refers to galaxies from the epoch of the peak of the cosmic star formation history, which occurred at z ~ 2, but also to their local analogues, the tidal dwarf galaxies.Firstly, using numerical simulations, we show that the massive galaxies at z = 2, which have a gas fraction of about 50%, form massive (10**7-8 solar masses) and gravitationally bound structures, which we call clumps thereafter. These clumps do not form in galaxies with a gas fraction below 25%. We then present an observational study of a local analogue of a z = 2 galactic clump, which is the tidal dwarf galaxy NGC 5291N. The analysis of emission lines show the presence of shocks on the outskirts of the object. Photometry of this galaxy’s stellar clusters show that the youngest clusters (< 10 million years) are significantly less massive than older clusters. This could be the sign of ongoing cluster mergers and/or of a strong star formation activity in this system about 500 million years ago).Secondly, we study how the gas fraction impacts the formation of stars and stellar clusters in galaxy mergers at z = 2. Using numerical simulations we show that these mergers only slightly increase the star and stellar cluster formation rate, compared to local galaxy mergers, which have a lower gas fraction. We show that this is due to the saturation of several physical quantities, which are already strong in isolated z=2 galaxies and are thus less enhanced by the merger. These factors are gas turbulence, compressive tides and nuclear gas inflows, We also show that the stellar structures formed in the gaseous clumps are preserved by the fusion: they are ejected from the disk and orbit in the halo of the remnant galaxy, where they may become the progenitors of some globular clusters

We present new deep photometry of the rich globular cluster (GC) systems around the Brightest Cluster Galaxies UGC 9799 (Abell 2052) and UGC 10143 (Abell 2147), obtained with the Hubble Space Telescope (HST) ACS and WFC3 cameras. For comparison, we also present new reductions of similar HST/ACS data for the Coma supergiants NGC 4874 and 4889. All four of these galaxies have huge cluster populations (to the radial limits of our data, comprising from 12,000 to 23,000 clusters per galaxy). The metallicity distribution functions (MDFs) of the GCs can still be matched by a bimodal-Gaussian form where the metal-rich and metal-poor modes are separated by similar or equal to 0.8 dex, but the internal dispersions of each mode are so large that the total MDF becomes very broad and nearly continuous from [Fe/H] similar or equal to-2.4 to solar. There are, however, significant differences between galaxies in the relative numbers of metal-rich clusters, suggesting that they underwent significantly different histories of mergers with massive gas-rich halos. Last, the proportion of metal-poor GCs rises especially rapidly outside projected radii R >= 4 R-eff, suggesting the importance of accreted dwarf satellites in the outer halo. Comprehensive models for the formation of GCs as part of the hierarchical formation of their parent galaxies will be needed to trace the systematic change in structure of the MDF with galaxy mass, from the distinctly bimodal form in smaller galaxies up to the broad continuum that we see in the very largest systems.

This thesis is about an investigation into the formation of spheroidal type galaxies. The investigation began with modelling studies of early-type galaxies and spiral bulges (SBs). From galaxy formation modelling studies led by experiments with a sample galaxy, some results were obtained; nonsolar abundance ratios in Elliptical galaxies (Es) achieved better fits between model and data than solar abundance ratios. For both early-type and late-type galaxies, best fits with non-solar abundance ratios were more constrained than in the solar abundance ratio case. A strong link between star formation histories and the supernova Ia rate for the early and late-type galaxies was shown. The model code itself was tested by way of pseudo galaxy experiments, and shown to reliably reproduce model parameters. In the topic area of galaxy formation, regions of spectra particularly sensitive to a galaxy's age and metallicity were measured as equivalent widths and then calibrated to the common scale of the Lick Indices. The Lick Indices were used in deriving all key results throughout the thesis. The modelled sample of galaxies from Proctor & Sansom (2002) lacked data on low velocity dispersion (a) galaxies for line strengths versus kinematics correlations. In regards to low a galaxies, Low Luminosity Es (LLE5) were considered to be likely candidates. Long-slit spectra of a sample of 12 LLEs, taken at the European Southern Observatory New Technology Telescope, were sub-selected for their low velocity dispersions. The spectra of 10 of these LLEs were successfully reduced. Line strengths and kinematics were measured. The Lick Indices of these LLEs were correlated with velocity dispersion (a), alongside the previously modelled companion data set. Ages and metallicities of the LLEs were estimated. From these results, the LLEs were found to have significant correlations of line strength versus a with SBs. However, the LLEs do not appear to be younger than SBs, but younger than Es. The LLEs seem to consist of a low metallicity group (possibly misclassified dwarf spheroidal galaxies) and a high metallicity group. Future possible work that may uncover which models of galaxy formation for high and low metallicity LLEs these results support is suggested.

Although there has been much progress in understanding how galaxies evolve, we still do not understand how and when they stop forming stars and become quiescent. We address this by applying our galaxy spectral energy distribution models, which incorporate physically motivated star formation histories (SFHs) from cosmological simulations, to a sample of quiescent galaxies at 0.2 < z < 2.1. A total of 845 quiescent galaxies with multi-band photometry spanning rest-frame ultraviolet through near-infrared wavelengths are selected from the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS) data set. We compute median SFHs of these galaxies in bins of stellar mass and redshift. At all redshifts and stellar masses, the median SFHs rise, reach a peak, and then decline to reach quiescence. At high redshift, we find that the rise and decline are fast, as expected, because the universe is young. At low redshift, the duration of these phases depends strongly on stellar mass. Low-mass galaxies (log(M*/M-circle dot) similar to 9.5) grow on average slowly, take a long time to reach their peak of star formation (greater than or similar to 4 Gyr), and then the declining phase is fast (less than or similar to 2 Gyr). Conversely, high-mass galaxies (log(M*/M-circle dot) similar to 11) grow on average fast (less than or similar to 2 Gyr), and, after reaching their peak, decrease the star formation slowly (greater than or similar to 3). These findings are consistent with galaxy stellar mass being a driving factor in determining how evolved galaxies are, with high-mass galaxies being the most evolved at any time (i.e., downsizing). The different durations we observe in the declining phases also suggest that low- and high-mass galaxies experience different quenching mechanisms, which operate on different timescales.

In this thesis, I explore the star formation histories of both spiral and elliptical galaxies. In Part 1,1 present an in-depth study of the star formation histories of spiral galaxies with a wide range of properties. Optical and near-infrared colours are used in conjunction with up-to-date stellar population synthesis models to constrain the ages and metallicities of my sample galaxies. I find that age and metallicity gradients are common in spiral galaxies of all types. The age of a spiral galaxy correlates mainly with its surface brightness, and its metallicity correlates strongly with both its surface brightness and absolute magnitude. Using simple models, I demonstrate that the correlations observed in this thesis show that the star formation history of a region within a galaxy depends primarily on its surface density, and possibly on the dynamical time. Metal- enriched outflow from low mass galaxies seems to be required to reproduce a reasonably strong metallicity-magnitude correlation. These variations in star formation history are a continuous function of the physical parameters: in particular, I find no evidence for a bimodal spiral galaxy surface brightness distribution. In Part 2, I present a short study on the formation epoch of early-type galaxies. I developed a photometric redshift estimator optimised for redshifts z ~ 1. The redshift estimator provides redshifts accurate to ~ 10 per cent. This redshift estimator is then applied to a sample of morphologically-selected early-type galaxies in the northern Hubble Deep Field. Comparison of their colour-magnitude relation with a passively evolved Coma cluster colour-magnitude relation indicates that over half of the sample must form at redshifts greater than two.

In this thesis we investigate the evolution of galaxies since z = 3. There are several methods to measure the star formation rate (SFR) of galaxies, they all however have drawbacks. Several studies have investigated the SFR at high redshifts using SFR trac­ers that suffer from uncertainties, either from the tracer used, or from the uncertainties correcting for the effects of dust. We have new measurements of the Ha emission line for a sample of galaxies at =~1; Ha is a more accurate SFR-tracer than other com­monly used tracers, but until now had been technically difficult to measure at : > 0.85. We investigate methods to correct these observations for dust and we use these mea­surements to investigate the relation between SFR, stellar mass and colour. We find that there is a drop in the fraction of massive (M, > 1011 M.) star-forming galaxies at = < 0.9 and that the fraction of all galaxies that are star-forming drops steadily and significantly with redder (U — B) colours. We find that the M„-SFR (galaxy main sequence, GMS) is flatter than previously measured and that for the most massive galaxies, star formation shuts off abruptly at =~1.

We have measured central line strengths and line-strength gradients for a complete sample of early-type galaxies in the Fornax cluster, comprising 11 elliptical and 11 lenticular galaxies, more luminous than M(_B) = -17. We find that the centres of Fornax ellipticals follow the locus of galaxies of fixed age in Worthey's models and have metallicities varying roughly from half solar to 2.5 times solar. Line-strength gradients indicate that elliptical galaxies do not show age gradients with radius but do exhibit a decrease of ~ 0.4 dex in [Fe/H] between the centre and one effective radius. The centres of lenticular galaxies however exhibit a substantial spread to younger luminosity weighted ages indicating a more extended star formation history. Metallicity gradients are generally shallower than for ellipticals. Five of the faint S0s have experienced a central starburst on top of an underlying older stellar population. Our conclusions are based on several age/metallicity diagnostic diagrams in the Lick/IDS system comprising established indices such as [MgFe] and Hβ as well as new and more sensitive indices such as C4668, Fe3 and Hγ(_A). The inferred difference in the age distribution between lenticular and elliptical galaxies is a robust conclusion as the models generate consistent relative ages using different age and metallicity indicators even though the absolute ages remain uncertain. The absolute age uncertainty is caused by the effects of non-solar abundance ratios which are not accounted for in the stellar population models. We find that Es are generally overabundant in magnesium where the most luminous galaxies show stronger overabundances. The luminosity weighted stellar populations of young S0s are consistent with solar abundance ratios, however the bulges of the two large S0s in our sample have [Mg/Fe] > 0. We have analysed in detail the sources of scatter in the Mg-crg relation by investigating the effects of age, metallicity and [Mg/Fe] variations. We find that young stellar populations are responsible for most of the scatter towards weak Mg-absorption. However, for the roughly coeval ellipticals the scatter at a given ctq is correlated with [Mg/Fe variations; metallicity and age effects seem to be less important. The young luminosity weighted ages of the faint S0s in the Fornax cluster are consistent with the recent discovery that the fraction of S0 galaxies in intermediate redshift clusters is a factor of 2-3 lower than found locally, and suggests that a fraction of the cluster spiral galaxy population has evolved into these faint S0s in the 5 Gyrs interval from z = 0.5 to the present. One of the proposed transformation mechanisms is the galaxy harassment picture. The properties of young S0s with large bulge to disk ratios in our sample are remarkably similar to the proposed end-products of galaxy harassment. However, we note that there are young disky S0s which are unlikely to be the result of harassment. Two of the faintest lenticular galaxies in our sample have blue continua and extremely strong Balmer-line absorption suggesting starbursts < 2 Gyrs ago.

My research has focused on locating and measuring star formation and AGN activity in different environments with interferometric and single-dish radio observations. As my first PhD project, I studied the complex interaction between an intermediate redshift (z 0.3) starburst galaxy and a nearby ( 7 kpc separation) QSO using sub-arcsecond VLA observations. I found new evidence for jet-induced star formation activity in the companion galaxy, making the system a strong candidate for this rare, and potentially important process in the early Universe. In my second paper, I investigated the infrared-radio correlation (IRRC) of spheroid- and disc-dominated galaxies in the COSMOS field out to z 1.5. With 1.4 GHz data and Herschel photometry I found that the redshift evolution reported in recent works is due to an increasing radio excess emission associated with spheroid-dominated galaxies, compared to disc-dominated ones, i.e. the ‘purest' star-forming systems in our sample. I theorize that the extra radio power in spheroid-dominated systems is due to low-level AGN activity, even though these sources were not identified by most commonly-used diagnostics as AGN hosts. This finding will significantly increase the accuracy of future high-redshift radio surveys measuring star formation. In my third project I assembled and analysed the largest-to-date low-z IRRC sample of galaxies. I demonstrated the importance of selection effects influencing IRRC statistics, and carried out an improved IRRC analysis that yielded more accurate measures of the correlation's properties. With rich ancillary data it will provide insight into the physical processes that give rise to the IRRC. Finally, I adopted an MCMC-based model optimization to fit a radiative transfer model to ammonia line spectra of a binary molecular cloud core. I determined the physical structures and the masses of the cores and found they are gravitationally unbound.

Observations of galaxy environments have revealed numerous correlations associated with their intrinsic properties. It is therefore clear that if we are to understand the processes by which galaxies form and evolve, we have to consider the role of their immediate environment and how these trends change across cosmic time. In this thesis, I investigate the relationship between the environmental densities of galaxies and their associated properties by developing and implementing a novel approach to measuring galaxy environments on individual galaxy scales with Voronoi tessellations. Using optical spectroscopy and photometry from GAMA and SDSS, with 250μm far-infrared observations from the Herschel-ATLAS SDP and Phase-One fields, the environmental and star formation properties of far-infrared detected and non–far-infrared detected galaxies are compared out to z ∼ 0.5. Applying statistical analyses to colour, magnitude and redshift-matched samples, I show there to be significant differences between the normalised density distributions of the optical and far-infrared selected samples, at the 3.5σ level for the SDP increasing to > 5σ when combined with the Phase-One data. This is such that infrared emission (a tracer of star formation activity) favours underdense regions, in agreement with previous studies that have proposed such a correlation. I then apply my method to synthetic light cones generated from semianalytic models (SAMs), finding that over the whole redshift distribution the same correlations between star-formation rate and environmental density are found. However, as the SAMs restrict the role of ram-pressure stripping, the fact that we find the same qualitative results may preclude ram-pressure as a key mechanism in truncating star formation. I also find significant correlations between isothermal dust temperature and environment, such that the coldest sources reside in the densest regions at the 3.9σ level, indicating that the observed far-infrared emission in these densest regions is the product of ISM heating by the older stellar populations. I then extend my analysis to a deeper sample of galaxies out to z ∼ 2.2, combining near-infrared and optical photometry from the VIDEO and CFHTLS-D1 observations, cross-matched in colour, magnitude and redshift against 1.4 GHz VLA radio observations. Across the entire radio sample, galaxies with radio detected emission are found to reside in more overdense environments at a 4.0σ significance level. I then divide my radio sample to investigate environmental dependence on both radio detected star-forming galaxies and radio detected AGN individually, based upon a luminosity selection defined as L = 1023 W Hz−1. The same trends with environment are shown by my Radio-AGN sample (L > 1023 W Hz−1) which favour overdense regions at the 4.5σ level, suggestive of the interaction processes (i.e. major mergers) that are believed to trigger accretion, in agreement with earlier work that has suggested such a relationship. At lower radio luminosities, my Radio-SF sample (L < 1023 W Hz−1) also display a significant trend towards overdense regions in comparison to my nonradio detected sample, at the less significant level of 2.7σ. This is suggestive of the low overall bolometric luminosity of radio emission in star forming galaxies, leading to only the brightest radio emitting star forming galaxies being observed and a bias towards overdense regions. This is in addition to the fact that the luminosity selection used to separate AGN from star forming galaxies is not a perfect selection and open to AGN contamination in the low-luminosity sample. I conclude that the next generation of deep radio surveys, which are expected to reach many orders of magnitude deeper than current observations, will remove radio-loud AGN contamination and allow for the detection of low-luminosity star forming galaxies via radio emission out to high redshifts. This work has allowed for the environments of galaxies to be probed on smallerscales and across both wider and deeper samples than previous studies. With significant environmental correlations being returned, this indicates that the established processes responsible for such trends must have influence on the most local of scales.

This thesis investigates the evolution of massive galaxies throughout the last 11 billion years using measured stellar masses and star formation rates. Firstly, we present a study of the resolved star-forming properties of a sample of distant massive (M > 10{11}) galaxies in the GOODS NICMOS Survey (GNS) within the redshift range 1.5 < z < 3 in order to measure the spatial location of ongoing star formation (SF). We find that the SFRs present in different regions of a galaxy reflect the already existent stellar mass density, i.e. high density regions have higher SFRs than lower density regions, on average. We find that these massive galaxies fall into three broad classifications of SF distributions. These different SF distributions increase the effective radii to z=0, by ~16 plus-minus 5 % , with little change in the Sersic index (n), with an average delta n = -0.9 plus-minus 0.9, after evolution. These results are not in agreement with the observed change in the effective radius and n between z ~2.5 and z ~0. We conclude that SF and stellar migration alone cannot account for the observed change in structural parameters for this galaxy population, implying that other mechanisms must additionally be at work to produce the evolution, such as merging. In Chapter 2, we present a study of the stellar mass growth of the progenitors of local massive galaxies at number densities of n < or = 1x10{-4} Mpc{-3} in the redshift range 0.31.5 SF is the dominant form of stellar mass growth, while at z<1.5 mergers become the dominant form with minor mergers the dominant form of growth at z<1.0. We also explore the implication of these results on other galaxy formation processes such as the cold gas accretion rate of the progenitors of most massive galaxies over the same redshift range. We find that the gas accretion rate decreases with redshift with an average gas accretion rate of ~65 M yr{-1} over the redshift range of 1.51.7 and transitioning to high n objects at z<1.7. Splitting the high and low $n$ objects into SFing and passive samples. We find that 41 plus-minus 4 % of the sample at z>2.5 are passive low n systems, possibly implying that local massive galaxies were passive disk-like systems at early cosmic times.

Stars form via the gravitational collapse of molecular clouds during which time the protostellar object contracts by over seven orders of magnitude. If all the angular momentum present in the natal cloud was conserved during collapse, stars would approach rotational velocities rapid enough to tear themselves apart within just a few Myr. In contrast to this, observations of pre-main sequence rotation rates are relatively slow (∼ 1 − 15 days) indicating that significant quantities of angular momentum must be removed from the star. I use observations of fully convective pre-main sequence stars in two well-studied, nearby regions of star formation (namely the Orion Nebula Cluster and Taurus-Auriga) to determine the removal rate of stellar angular momentum. I find the accretion disc-hosting stars to be rotating at a slower rate and contain less specific angular momentum than the disc-less stars. I interpret this as indicating a period of accretion disc-regulated angular momentum evolution followed by near-constant rotational evolution following disc dispersal. Furthermore, assuming that the age spread inferred from the Hertzsprung-Russell diagram constructed for the star forming region is real, I find that the removal rate of angular momentum during the accretion-disc hosting phase to be more rapid than that expected from simple disc-locking theory whereby contraction occurs at a fixed rotation period. This indicates a more efficient process of angular momentum removal must operate, most likely in the form of an accretion-driven stellar wind or outflow emanating from the star-disc interaction. The initial circumstellar envelope that surrounds a protostellar object during the earliest stages of star formation is rotationally flattened into a disc as the star contracts. An effective viscosity, present within the disc, enables the disc to evolve: mass accretes inwards through the disc and onto the star while momentum migrates outwards, forcing the outer regions of the disc to expand. I used spatially resolved submillimetre detections of the dust and gas components of protoplanetary discs, gathered from the literature, to measure the radial extent of discs around low-mass pre-main sequence stars of ∼ 1−10 Myr and probe their viscous evolution. I find no clear observational evidence for the radial expansion of the dust component. However, I find tentative evidence for the expansion ofthe gas component. This suggests that the evolution of the gas and dust components of protoplanetary discs are likely governed by different astrophysical processes. Observations of jets and outflows emanating from protostars and pre-main sequence stars highlight that it may also be possible to remove angular momentum from the circumstellar material. Using the sample of spatially resolved protoplanetary discs, I find no evidence for angular momentum removal during disc evolution. I also use the spatially resolved debris discs from the Submillimetre Common-User Bolometer Array-2 Observations of Nearby Stars survey to constrain the amount of angular momentum retained within planetary systems. This sample is compared to the protoplanetary disc angular momenta and to the angular momentum contained within pre-stellar cores. I find that significant quantities of angular momentum must be removed during disc formation and disc dispersal. This likely occurs via magnetic braking during the formation of the disc, via the launching of a disc or photo-evaporative wind, and/or via ejection of planetary material following dynamical interactions.

Theory of cosmic structure formation outlines how stars, galaxies, clusters of galaxies, and large-scale structures formed out of primordial density fluctuations. It presents us a picture of cosmic mass assembly, and places strong constraints on cosmological model. Both observations and theories suggest that structures formation follows a "bottom up" process, in which small, low-mass component form first, and gradually develop into larger, more massive systems. This dissertation focuses on three crucial stages of cosmic structure formation: first generation stars, quasar host galaxies and the large-scale galaxy overdensities. In Chapter 1, I present an overview of structure formation, acquainting readers with a general picture from first object in the Universe to large-scale structures at later epochs. In Chapter 2 and Chapter 3, I derive strong constraints to the star formation rates (SFRs) of very massive Population III (Pop III) stars in two high redshift galaxies at z = 7. By probing the He II emission lines for both galaxies, I conclude that the contributions of very massive Pop III stars to total the SFRs are less than 3%. In Chapter 4, I move to more massive systems, quasar host galaxies at z ~ 3. Using damped Lyman alpha absorption systems as natural coronagraphs, I report that rest-frame far-UV emission of quasar host galaxy correlates strongly with quasar luminosity. This result suggests a co-evolution of supermassive black holes and their host galaxies. In Chapter 5, I develop a novel method for searching the most massive protoclusters at z = 2-3, by utilizing intergalactic Lyman alpha absorption. My investigations suggest that large intergalactic Lyman alpha absorption systems effectively trace the most overdense regions at large scale of ~ 15 h⁻¹ Mpc. In Chapter 6, I present our imaging observations of an extreme galaxy overdensity (protocluster) BOSS1441+4000, which is discovered using the techniques developed in Chapter 5. Furthermore, I report an intergalactic-scale Lyman alpha nebula detected at the density peak of BOSS1441+4000. This discovery, together with previously discovered Slug nebula, provide us a first look of intergalactic medium in emission in the early Universe. In the Chapter 7, I give a summary of this dissertation and discuss several future prospects.

The main aims of this thesis are: (a) an investigation into the evolutionary histories of nearby galaxies, via the use of ultraviolet-infrared colour-mass diagrams, to uncover the dominant mechanism driving nearby galaxy evolution; (b) the determination of the impact of feedback from active galactic nuclei on star formation; and (c) a study of chemical evolution of star-forming galaxies in different environments. The analysis confirms recent observations of a population of late-type galaxies with reduced levels of star formation. I demonstrate that feedback from active nuclei is unlikely to be the dominant mechanism quenching star formation and driving the evolution of thee late-type galaxies. In fact, galaxies with quenched star formation are typically gas-deficient cluster galaxies, suggesting that environmental effects are responsible for removing the gas required to fuel star formation. A fraction of quenched late-type galaxies are, however, not gas deficient, and form a more heterogeneous class of objects with more complex evolutionary histories. On the other hand, the chemical evolution of star-forming galaxies, as traced by the stellar mass-metallicity relation, is shown to be invariant with environment, suggesting that internal processes drive chemical evolution. The clear observational evidence presented here indicates that, in the concordance model of galaxy formation and evolution, environmental effects must be taken into account in order to gain a better understanding of galaxy evolution in the local universe.

Neutral hydrogen surveys in the last decade have revealed a new class of galaxies where the gas component can be dominant compared to the stellar one (Mhi/Lb < 1). These objects may be either young (< 1 Gyr) or in a way "retarded", having evolved at a very slow rate without efficiently converting their large amount of gas into stars within a Hubble time. Here we analyse three such galaxies in the Centaurus A group at a distance of about 4.5 Mpc (discovered with the HIPASS survey), which have no analogues in the Local Group (Banks et al. 1999). From their optical morphology they appear to be dwarf spheroidals and low surface brightness, yet they are gas-rich (Mhi/Lb > 1) with gas-mass-to-stellar light ratios larger than typical dwarf irregular galaxies. These systems should be favoured hosts for starburst, yet a faint star formation region has been detected in only one object. Are these galaxies truly young Or rather, what inhibits the conversion of gas into stars slowing down their evolution Is there a connection with the environment they are evolving in the attempt to answer some of the above questions we have analysed data in Hi, Ha, V and / bands (taken with the Hubble Space Telescope) and the optical spectrum of the Hll region of one dwarf (HIPASS J1337-39). In particular we have constructed /, (V - I) Colour Magnitude Diagrams (CMDs) and we have compared the data-sets with theoretical models, using isochrones of various metallicities (Bertelli et al. 1994) and simulated CMDs (Harris & Zaritsky 2001). All three galaxies have well determined Red Giant Branches (RGBs) which put them in the Centaurus A group at distances between 4.5 and 5 Mpc. The well populated RGBs suggest that these systems cannot . be younger than 2 Gyr. The evidence of asymptotic giant branch (AGB) stars in two out of three galaxies suggest that they contain populations with ages of up to 10 Gyr. Older Horizontal Branch (HB) and RR Lyrae stars would be two magnitudes fainter than the photometric limit reached with the HST at this distance, therefore we cannot infer the presence of an old (> 10 Gyr) stellar population. The age-metallicity degeneracy has been broken in one case (HIPASS J1337-39) where we measured a low metal abundance (Z 1/30 solar) from the analysis of the Hll region. The remaining two galaxies do also show properties which are at least consistent with similarly low metallicities. From the comparison of the observed and model CMD's we infer that all three galaxies are gas-rich because their Star Formation Rates have been very low during their long lives (< 10--3 A/ yr_1). We argue that in such systems, star formation (SF) may have been sporadic and local, although one object (HIPASS J1321-31) has a peculiar red plume in its CMD suggesting that most of its stars were formed in a "miniburst" 300--500 Myr ago. Neither their low Hi surface densities, nor their low metal content favour high star formation rates or starbursts, although we can not exclude connections with the local environment. Two out of three galaxies seem to be located in the outskirts of the group, being at about or less than 1 Mpc from the more massive galaxies of the group. The low metal abundances may be the consequence of the ejection of enriched material via supernovae-driven winds during the SF episodes, but it is likely they were not violent enough to blow away the majority of gas (Mac Low k Ferrara 1999, Ferrara & Tolstoy 2000). The question of why there are no similar dwarf galaxies in the Local Group remains open and the study of similar objects in the nearby groups is necessary to widen the sample of galaxies with such properties. In the Appendix A we present the first part of a still ongoing project with the aim of studying the stellar population of "retarded" galaxies at the other end of the mass scale---spirals with Mhi > 10io A/ and Mhi/Lb 1---The optical, near IR and 21cm observations are briefly presented and discussed.