Dissertations / Theses on the topic 'Cluster of galaxie'

This thesis is focused on design and implementation of procedural generator of galaxy for games or simulations. Second goal is implementation of demonstration application which is showing possible usage of generator. Generator is able to create galaxy by predefined types or by user created map.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2004.
Includes bibliographical references (p. 85).
Clusters of galaxies have become an important cosmological tool, yet we do not un-erstand many aspects of their formation and development. In this thesis, I pursue two projects aimed at using clusters to constrain cosmology and better understanding cluster evolution. First, I examine the Chandra observation of MS1054-0321. MS1054-0321 is the highest redshift cluster in the Einstein Medium Sensitivity Survey (EMSS), and it was one of the first high-redshift clusters observed with Chandra. I confirm that this cluster is hot and massive, although its temperature is slightly lower than inferred previously. I also detected an iron line in this cluster, one of the first detections of iron in a cluster at these redshifts, with an abundance consistent with early enrichment of the ICM. MS1054-0321 exhibits significant substructure, which I study in detail for the first time. In X-rays, it appears to be a nearly equal mass double cluster in the process of merging. Both the cluster galaxies and mass associated with the western subclump are offset from the X-ray peak, possibly indicating that the gas in the subclump is being stripped off as it falls into the cluster. Despite the lower temperature, I find that the detection of this cluster in the EMSS constrains Qm to be less than one.
(cont.) In the second project, I investigate the evolution of cluster substructure with redshift, quantifying for the first time cluster structure out to z [approx.] 1. My sample includes 40 X-ray selected, luminous clusters from the Chandra archive, and I quantify cluster morphology using the power ratio method (Buote & Tsai 1995). I find that, as expected qualitatively from hierarchical models of structure formation, high-redshift clusters have more substructure and are dynamically more active than low-redshift clusters. Specifically, the clusters with z > 0.5 have significantly higher average third and fourth order power ratios than the lower redshift clusters. This observation of structure evolution indicates that dynamical state may be an important systematic effect in cluster studies seeking to constrain cosmology, and when calibrated against numerical simulations, structure evolution will itself provide interesting bounds on cosmological models.
by Tesla Erin Jeltema.
Ph.D.

This thesis is concerned with the role the rich cluster environment plays in the evolution of its galaxy population. We approach this issue from two angles, first we use deep wide-field imaging to investigate the effect of the cluster environment on the spatial and luminosity distribution of galaxies. Secondly, we focus on one particularly interesting class of galaxy, the enigmatic E+A galaxies, using a combination of state-of-the-art telescopes and novel instrumentation to elucidate the physical mechanisms and environmental influences causing the rapid change in star-formation activity in these galaxies. We present results from a deep photometric study of the rich galaxy cluster Abell 2218 (z=0.18) based on Hubble Space Telescope images. These have been used to derive the cluster luminosity function to extremely faint limits. We find the faint-end slope of the luminosity function to vary with environment within the cluster - in the sense that the ratio of `dwarf' galaxies to `giant' galaxies increases in the lower-density outskirt regions. Using imaging obtained with the Isaac Newton Telescope (INT) we confirm the presence of luminosity segregation in Abell 2218. However, luminosity segregation in clusters does not appear to be ubiquitous, with two other clusters studied with the INT (A119 at z=0.04 and A2443 at z=0.11) showing no sign of luminosity segregation of their galaxy populations. We use integral field spectroscopy of a sample of E+A galaxies in intermediate redshift clusters, obtained with the FLAMES system on the European Southern Observatory's VLT and the GMOS instrument on Gemini-North, to determine the radial variation in the strength of Hdelta absorption in these galaxies, and hence map out the distribution of the recently formed stellar population. We find a diversity of behaviour amongst these galaxies in terms of the radial variation in Hdelta absorption: with gradients that are either negative, flat, or positive. By comparing with numerical simulations we suggest that the first of these different types of radial behaviour provides evidence for a merger/interaction origin, whereas the latter two types of behaviour are more consistent with the truncation of star formation in normal disk galaxies.

In this thesis, I explore how the environments of both galaxy and cluster-scale strong gravitational lenses affect studies of cosmology and the properties of the earliest galaxies.Galaxy-scale lenses with measured time delays can be used to determine the Hubble constant, given an accurate lens model. However, perturbations from structures along the line of sight can introduce errors into the measurement. I use data from a survey towards known lenses in group environments to calculate the external shear in these systems, which is typically marginalized over in standard lens analyses. In three of six systems where I compare the independently-calculated environment shear to lens model shears, the quantities disagree at greater than 95% confidence. We explore possible sources of this disagreement. Using these data, I generate fiducial lines of sight and insert mock lenses with assumed input physical and cosmological parameters and find that those parameters can be recovered with∼ 5-10% scatter when uncertainties in my characterization of the environment are applied. The lenses in groups have larger bias and scatter. I predict how well new time delay lenses from LSST will constrain H_0 and find that an ensemble of 500 quad lenses will recover H_0 with∼ 2% bias with∼ 0.3% precision.On larger scales, galaxy cluster lenses can magnify the earliest galaxies into detectability. While past studies have focused on single massive clusters, I investigate the properties of lines of sight, or ``beams", containing multiple cluster-scale halos in projection. Even for beams of similar total mass, those with multiple halos have higher lensing cross sections on average. The optimal configurations for maximizing the cross section are also those that maximize faint z∼ 10 detections. I present a new selection technique to identify beams in wide-area photometric surveys that contain high total masses and often multiple clusters in projection as traced by luminous red galaxies. I apply this technique to the Sloan Digital Sky Survey and present the 200 most promising beams. Several are confirmed spectroscopically to be among the highest mass beams known with some containing multiple clusters. These are among the best fields to search for faint high-redshift galaxies.

We present a multi-wavelength perspective of star-forming galaxies within high-redshift galaxy clusters. The clusters derive from the Red-sequence Cluster Survey (RCS) and the Spitzer Adaptation of the Red-sequence Cluster Survey (SpARCS), and possess ample spectroscopic coverage, yielding numerous confirmed cluster members. This thesis consists of a collection of distinct but related works, focusing on environmental effects within the dense regions of clusters---some of the rarest structures in the Universe. We exploit the high sensitivities of cutting-edge infrared and submillimeter telescopes to glean the wealth of information encoded within the thermal portion of the spectral energy distribution, including infrared luminosities and dust temperatures. This allows us to uncover various trends within the star-forming population as a function of environment. Moreover, we develop a novel definition of environment, based on the phase space of radius and velocity, to account for the various accretion histories of galaxies onto clusters; it thereby probes the time-averaged density that each galaxy population has experienced. Using this tracer of environment, we find a significant depression in the star formation rate per unit stellar mass for star-forming galaxies within cluster cores at z~0.9 and z~1.2, in contrast to the flat trend that results from conventional definitions of environment. We also discover a population of galaxies that have lower dust temperatures compared to both infalling galaxies and those that were accreted at the earliest stages of the formation of the cluster. Taken together, these trends in star formation rate and dust temperature can help elucidate which, if any, quenching mechanisms are active within cluster environments. Finally, we report the serendipitous detection of an overdensity of submillimeter-bright galaxies located behind a merging z~0.9 supercluster, which could signify a highly star-forming protocluster at z~3.
Nous présentons une analyse multi-fréquentielle de galaxies à formation d'étoiles situées à l'intérieur d'amas de galaxies fortement décalés vers le rouge. Ces amas en provenance du sondage Red-sequence Cluster Survey (RCS) et de son adaptation par Spitzer (SpARCS) possèdent une large couverture spectroscopique et contiennent plusieurs membres confirmés. Cette thèse représente une collection de travaux distincts mais reliés, qui se concentrent sur les effets environnementaux présents à l'intérieur des régions denses d'amas de galaxies, des structures parmi les plus rares de l'Univers. Nous exploitons la sensibilité élevée des télescopes infrarouges et submillimétriques de pointe pour collecter l'information encodée dans la portion thermale de la distribution spectrale d'énergie, ce qui inclue des mesures de luminosité infrarouge et des températures de poussière. Cette méthode nous permet de dévoiler plusieurs tendances en cours à l'intérieur de la population de galaxies à formation d'étoiles en fonction de l'environnement où se trouvent ces galaxies. De plus, nous développons une nouvelle définition de ce qui constitue l'environnement d'une galaxie basée sur l'espace de phase du rayon et de la vitesse. Cette définition tient compte des nombreux épisodes d'accrétion de galaxies par les amas de galaxies, traçant ainsi la densité moyenne connue par chaque population de galaxies. En utilisant ce nouveau traceur d'environnement, nous trouvons un manque important dans le taux de formation d'étoiles par unité de masse stellaire pour les galaxies à formation d'étoiles situées au coeur d'amas de galaxies entre z~0.9 et z~1.2, ce qui contraste avec le taux constant résultant d'une définition conventionnelle de l'environnement. Nous avons aussi découvert une population de galaxies ayant des températures de poussière plus basse que celles associées à des galaxies qui tombent ou qui ont été accrétées au début de la formation de l'amas de galaxies. Considérées ensemble, ces tendances caractérisant le taux de formation d'étoiles et la température de poussière peuvent aider à mettre en lumière les mécanismes de relaxation actifs à l'intérieur des amas de galaxies. Pour terminer, nous rapportons la détection fortuite d'une surdensité de galaxies submillimétriques situées derrière un superamas de galaxies à z~0.9, ce qui pourrait indiquer la présence d'un proto-amas à haute formation d'étoiles à z~3.

Cette thèse porte sur le rôle des amas de galaxies dans la cosmologie et l'évolution des galaxies. J'utilise des observations photométriques et spectroscopiques multi-longueur d'onde (optiques, mm, proche/moyen-IR) que j'analyse avec des méthodes statistiques. Pour un échantillon d'amas détectés par le satellite Planck et ré-observés avec le télescope optique Gemini, j'étudie la dynamique des galaxies dans les amas afin de déterminer leur masse et de comprendre les erreurs systématiques sur ces estimations, d'une importance cruciale dans la cosmologie à l'heure actuelle. Dans le cadre de la collaboration CARLA (Clusters Around Radio Loud AGN), j’étudie les populations stellaires des galaxies dans les amas et les proto-amas dans leurs premières phases d'assemblage (z>1.4), afin d'étudier la suppression de la formation des étoiles et le rôle de l'environnement sur l'évolution des galaxies
This thesis focuses on the role of galaxy clusters in cosmology and galaxy evolution. I use multi-wavelenght photometric and spectroscopic observations (optical, mm, near/mid- IR) which I analyse with statistical methods. For a sample of clusters detected by the Planck satellite and re-observed with the Gemini optical telescope, I have studied the dynamics of member galaxies to probe their mass and understand possible systematics affecting such estimates, of crucial importance in cluster cosmology at present. Within the CARLA (Clusters Around Radio Loud AGN) collaboration, I examine morphology and stellar populations of galaxies in clusters and proto-clusters in their first assembly phases (z>1.4), with the aim of shedding light on the quenching of star formation and the role of the environment on galaxy evolution

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 detection of Mpc scale diffuse radio emission in galaxy clusters provides evidence that cosmic ray electrons, as well as cluster scale magnetic fields are present in clusters. As such, radio observations of clusters provide a unique opportunity to study the non-thermal populations of the intra-cluster medium. Observations of Faraday rotation in sources embedded in cluster and group environments offers an additional method for probing the cluster/group magnetic field. In this thesis I present low frequency radio observations of multiple galaxy clusters in order to investigate the nature of diffuse radio emission present in many clusters. I also present observations of the giant radio galaxy NGC 6251 and discuss both the source properties as well as the host group environment. In Chapter 1 of this thesis I review the current understanding of galaxy clusters, groups and radio galaxies. I also describe some of the astrophysical processes important to this thesis. In Chapter 2 I discuss the interferometry and the process of calibrating interferometric data. I also describe some of the techniques used later in the thesis such as QUfitting and RM synthesis. In Chapter 3 I present my observations of the massive merging galaxy cluster MACSJ2243.3-0935. I report the discovery of a radio halo in MACSJ2243.3-0935, as well as a new radio relic candidate, using the Giant Meterwave Radio Telescope and the KAT-7 telescope. The radio halo is coincident with the cluster X-ray emission and has a largest linear scale of approximately 0.9 Mpc. I measure a flux density of $10.0\pm 2.0\, \rm mJy$ at 610 MHz for the radio halo. I discuss equipartition estimates of the cluster magnetic field and constrain the value to be of the order of $1\, \rm \mu G$. The relic candidate is detected at the cluster virial radius where a filament meets the cluster. The relic candidate has a flux density of $5.2\pm 0.8\, \rm mJy$ at 610 MHz. I discuss possible origins of the relic candidate emission and conclude that the candidate is consistent with an infall relic. In Chapter 4 I present my GMRT observations at 610 MHz of 3 disturbed galaxy clusters, A07, A1235 and A2055. No diffuse emision was observed any of the three clusters. In order to place upper limits on the radio halo power in these clusters I have injected simulated halos at difffent radio powers into the uvdata. A07 has a radio halo upper limit of $P_{\rm 610MHz}=1.5\times10^{24}$ W Hz$^{-1}$. A2055 has a radio halo upper limit of $P_{\rm 610MHz}=1.8\times10^{24}$ W Hz$^{-1}$. A1235 has a radio halo upper limit of $P_{\rm 610MHz}=5.8\times10^{23}$ W Hz$^{-1}$. These limits are below the $P_{610}-L_{\rm X}$ relation and rule out bright radio halo in these clusters. I have identified these clusters as potential hosts for Ultra Steep Spectrum Radio Halo (USSRH). Observations with LOFAR should be capable of confirming whether or not these clusters host USSRH. In Chapter 5 I present observations of the giant radio galaxy NGC 6251 with LOFAR HBA. NGC 6251 is a giant radio galaxy with a borderline FRI/FRII morphology located in a poor group. The images presented in this chapter are the highest sensitivity and resolution images of NGC 6251 at these frequencies to date. Analysis of the low frequencies spectral index did not reveal any change in the low frequency spectra when compared with the higher frequency spectral index. NGC 6251 is found to be either at equilibrium or slightly electron dominated, similar to FRII sources. I calculated the ages of the low surface brightness extension of the northern lobe and the backflow of the southern lobe, which are only clearly visible at these low frequencies, to be 205 Myr$.

Cette thèse présente un certain nombre de résultats récents à propos de l'évolution des galaxies et la cosmologie, à partir de l'observation d'amas de galaxies en lumière visible. Nous introduisons d'abord les principales propriétés des amas de galaxies (Chapitre 1.1) et la façon dont ces objets permettent de contraindre le modèle cosmologique standard (Chapitre 1.2). Une grande partie des résultats présentés ici ont été obtenus à partir de l'étude du relevé d'amas DAFT/FADA, qui regroupe des amas dans la gamme de décalages spectraux 0.4
This thesis presents some recent results concerning galaxy evolution and cosmology,based on the observation of galaxy clusters at optical wavelengths. We first introduce the main properties of galaxy clusters (Sect. 1.1) and how they can be used for cosmology within the standard cosmological model (Sect. 1.2). A large fraction of the presented results comes from the study of the DAFT/FADA galaxy cluster survey at redshifts 0.4 < z < 0.9 (Sect. 1.3). We divide our study in two parts according to the observable that is considered: galaxy luminosity or galaxy shape. The distribution of galaxy luminosities is called the galaxy luminosity function (GLF), which can be used to probe the evolution of cluster galaxies (Sect. 2.1). Computing the GLFs for a sub sample of 25 DAFT/FADA clusters, we find that faint blue star forming galaxies are quenched into red quiescent galaxies from high redshift until today. Comparing to the field shows that this transformation is more efficient in high density environments.We also study the fraction of baryons in galaxy groups and clusters (Sect. 2.2). Wefind that in groups, the stars contained in galaxies can reach masses of the same order as those of the intra-cluster gas, while in clusters they are usually negligible relatively to the gas. Taking both stars and gas into account we constrain the matter density parameter Galaxy shapes are distorted by foreground objects that bend light in their vicinity. This lensing signal can be exploited to measure the mass distribution of a foreground cluster. We review the basic theory of weak lensing and shear measurement (Sect. 3.1), and then apply it to a subsample of 16 DAFT/FADA clusters, with Subaru/SuprimeCam or CFHT/MegaCam imaging (Sect. 3.2). We estimate the masses of these clusters, and take advantage of the large fields of view of our images to detect filaments and structures in the cluster vicinity, observationally supporting the hierarchical scenario of cluster growth. Finally, we detect shear peaks in Euclid-like simulations, and use their statistics as a cosmological probe, similarly to cluster counts (Sect. 3.3). We forecast the cosmological constraints that this technique will achieve when applied to the Euclid space mission, and develop a tomographic analysis that adds information from redshifts. We conclude with a discussion of our perspectives on future studies in all the fieldsinvestigated in the present thesis

We study the orientations of satellite galaxies in redMaPPer clusters constructed from the Sloan Digital Sky Survey at 0.1 < z < 0.35 to determine whether there is any preferential tendency for satellites to point radially towards cluster centres. We analyse the satellite alignment (SA) signal based on three shape measurement methods (re-Gaussianization, de Vaucouleurs, and isophotal shapes), which trace galaxy light profiles at different radii. The measured SA signal depends on these shape measurement methods. We detect the strongest SA signal in isophotal shapes, followed by de Vaucouleurs shapes. While no net SA signal is detected using re-Gaussianization shapes across the entire sample, the observed SA signal reaches a statistically significant level when limiting to a subsample of higher luminosity satellites. We further investigate the impact of noise, systematics, and real physical isophotal twisting effects in the comparison between the SA signal detected via different shape measurement methods. Unlike previous studies, which only consider the dependence of SA on a few parameters, here we explore a total of 17 galaxy and cluster properties, using a statistical model averaging technique to naturally account for parameter correlations and identify significant SA predictors. We find that the measured SA signal is strongest for satellites with the following characteristics: higher luminosity, smaller distance to the cluster centre, rounder in shape, higher bulge fraction, and distributed preferentially along the major axis directions of their centrals. Finally, we provide physical explanations for the identified dependences and discuss the connection to theories of SA.

Gravitational Lensing is a unique technique to investigate the dark matter distribution of structures in the Universe, from galaxies, through galaxy groups, clusters, up to the large-scale structure. It allows us to map the total projected mass density of structures acting as lenses, and thus to shed light on the distribution and properties of the otherwise-invisible dark matter. Clusters of galaxies are the largest virialized structures in the universe. Gravitational lensing analysis allows us to study their mass distribution in great detail. Weak lensing probes the mass distribution in the outskirts of clusters based on a statistical analysis of the shape distortion observed in hundreds of galaxies behind the cluster. Strong lensing, instead, allows us to reconstruct high resolution mass and magnification maps of the central region of the cluster. In addition, thanks to the lensing magnification of background sources, galaxy clusters act as "Gravitational Telescopes" and can be used to investigate the galaxy population of the early Universe at z>5. In the first part of my Thesis I use the CLASH and Frontier Fields cluster RXC J2248 to investigate sources at z~6. At such and higher redshift galaxies appear as optical dropouts, since the light they emit is redshifted to NIR wavelengths and no flux is observed in the UV and optical filters. I discovered a z~6 lensed galaxy in the core of RXC J2248 which appears as a quintuple lensed optical dropout in the 16 HST filters of the CLASH survey. I perform a detailed photometric analysis of these dropouts to verify that they present the same photometric properties and are actually multiple images of the same source. In addition, by performing the strong lensing analysis of the cluster core I verify that the lensing model supports the quintuple and z~6 nature of this system. In the second part of my Thesis I use strong gravitational analysis of the CLASH cluster A383 to probe the details of the mass distribution of galaxies in the cluster core. Well known luminosity scaling relations allow us to relate the physical properties as stellar velocity dispersion and size of the elliptical galaxies to their observed luminosity. However in clusters, galaxies suffer tidal stripping due to the interaction with other cluster members and the cluster dark matter halo. The goal of this work is to measure the galaxy halo sizes in a cluster core to quantify how much mass was stripped relative to field galaxies. Here I present a new approach to strong lensing analysis of clusters, in which I use measurements of cluster members' velocity dispersions as additional constraints in the lens modeling. I apply this analysis to Abell 383 to separate the galaxy mass content from the smooth dark matter mass component and investigate how the dark matter halo size scales with the galaxy luminosity in the cluster core. In addition I perform the surface brightness reconstruction of the southern giant arcs to improve constraints on close by individual galaxies and study possible deviations from the global scaling law measured for the cluster.

This thesis addresses the distributions of baryonic matter on three scales: the outskirts of the gas and galaxy profiles in galaxy clusters, the clustering of galaxies of galaxies on large scales and its relation to the underlying matter distribution, and the extremes of the galaxy distribution: the connections between the most distant galaxies ever discovered and the closest galaxies to our own, the Local Group Dwarfs. We begin with investigations of the outskirts of galaxy clusters, where long-standing analytical models of structure formation as well as recent simulations predict the existence of steep density jumps in the gas (the 'virial shock') and dark matter profiles near the virial radius. We describe a new method for deriving models for the gas distribution in galaxy clusters, which relies on a few basic assumptions --- including the existence of the virial shock and a coincident density jump in the dark matter --- and show a resulting profile for the gas that is in good agreement both with X-ray observations of cluster interiors and simulations of the outskirts, and requires fewer parameters than the traditional three-parameter beta-model. Recent simulations have strengthened the arguments in favor of the existence of a dark matter density jump, arising from the accumulation of particles at the apocenter of their first orbit. Since cluster member galaxies are expected to follow similar collisionless dynamics as the dark matter, the galaxy density profile should show a steep density jump as well. We present evidence for a feature consistent with a density jump in galaxy density profiles constructed from photometry from the Sloan Digital Sky Survey and Hectospec (MMT) spectroscopy of cluster members and discuss avenues for probing the density jumps with future data sets. Moving to larger scales where massive galaxies of different types are expected to trace the same large-scale structure, we present a test of this prediction by measuring the clustering of red and blue galaxies at z~0.6 using the CMASS sample of galaxies from the 12th Data Release of SDSS-III. The stochasticity between these two samples is quantified via the correlation coefficient r, which can be constructed from two different statistics. Both statistics indicate that on intermediate scales (20 < R < 100 Mpc/h) there is low stochasticity between the two samples of galaxies, providing a constraint on a key systematic in using large galaxy redshift surveys for cosmology. In cosmology, dense redshift surveys permit the measurement of the scale of baryon acoustic oscillations (BAO), which appear as a modest amplification at scales of about R = 105 Mpc/h in the two-point auto-correlation function of galaxies, provided that there is a sufficiently high density of galaxies with accurately measured three-dimensional positions. As a result, due to the expense of spectroscopic observations, to date most BAO analyses have been performed at fairly low redshifts where present surveys can attain the requisite densities without sacrificing efficiency. We present a new method of measuring the BAO using the cross-correlation of a sparse spectroscopic sample with a denser, photometric sample of galaxies that will allow us to extend BAO measurements to higher redshifts than are presently accessible with spectroscopy alone. We discuss applications of this new method to current and upcoming datasets. Finally, we connect galaxies both near --- the Local Group dwarf galaxies --- and far --- the high-redshift galaxies discovered by space-based observatories like Hubble and Spitzer. We evolve the local dwarfs back in time using stellar population synthesis code and juxtapose the properties of their ancient selves against those of the galaxies already discovered at high redshift. We additionally compare the properties of the dwarfs' progenitors with the detection limits of the future James Webb Space Telescope (JWST), finding that JWST should be able to detect the progenitors of galaxies similar to a few of the brightest local galaxies.
Physics

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.

Philosophiae Doctor - PhD
In this thesis, we present a multi-wavelength analysis of star-forming galaxies to shed new light on the evolution of the far-IR-radio relations in intermediate redshift (0.3 < z < 0.6) galaxy clusters and galaxy groups. The far-infrared (far-IR) emission from galaxies is dominated by thermal dust emission. The radio emission at 1.4 GHz is predominantly produced by non-thermal synchrotron radiation. The underlying mechanisms, which drive the far-IR-radio correlation, are believed to arise from massive star formation. A number of studies have investigated the relationship as a function of redshift in the field and have found no evolution out to at least z _ 2, however few works have been done in galaxy clusters. In nearby clusters, the median logarithmic ratio of the far-IR to radio luminosity is qFIR = 2.07_0.74, which is lower than the value found in the field, and there is an indication of an enhancement of radio emission relative to the far-IR emission. Understanding the properties of the far-IR-radio correlation in a sample of distant and massive cluster and groups plays an important role in understanding the physical processes in these systems. We have derived total infrared luminosities for a sample of cluster, group, and field galaxies through an empirical relation based on Spitzer MIPS 24 _m photometry. The radio flux densities were measured from deep Very Large Array 1.4 GHz radio continuum observations. We have studied the properties of the far-IR-radio correlation of galaxies at intermediate redshift clusters by comparing the relationship of these galaxies to that of low redshift clusters. We have also examined the properties of the galaxies showing radio excess to determine the extent that galaxy type or environment may explain the radio excess in galaxy clusters. We find that the ratio of far-IR to radio luminosity for galaxies in an intermediate redshift cluster to be qFIR = 1.72_0.63. This value is comparable to that measured in low redshift clusters. A higher fraction of galaxies in clusters show an excess in their radio fluxes when compared to low redshift clusters, and corroborates previous evidence of a cluster enhancement of radio excess sources at this earlier epoch as well. We have also investigated the properties of the far-IR-radio correlation for a sample of galaxy groups in the COSMOS field. We find a lower percentage of radio-excess sources in groups as compared to clusters. This provides preliminary evidence that the number of radioexcess sources may depend on galaxy environment. We also find that a larger fraction of radio-excess sources in clusters are red sequence galaxies.

Radio halos are Mpc scale diffuse sources located at the center of a fraction of galaxy clusters. In the current theoretical picture, they form via the re-acceleration of electrons in the ICM by means of turbulence, injected during cluster mergers. This scenario allows basic predictions on the formation history of radio halos that can only be tested by analysing large samples of galaxy clusters with adequate radio and X-ray data. The main goal of this Thesis is to study the first complete large sample of mass-selected galaxy clusters to obtain solid statistical constraints on the connection between radio halos and the dynamics and mass of the host clusters. We used the Planck SZ catalogue to select a sample of 75 massive galaxy clusters (M500>6x10^{14}Msun) at redshift z=0.08-0.33 and we collected information on the presence or absence of diffuse emission from the literature and from the large observational (GMRT and JVLA) campaign carried out during this PhD project. We analysed X-ray Chandra and XMM-Newton data to investigate the dynamical properties of clusters. We updated the radio power-mass scaling relation for radio halos and we found clear evidence for a bimodal behaviour of clusters in both the radio power-mass plane and, for the first time, in the radio emissivity-mass diagram, with radio halos and non-radio halo clusters following two distinct distributions. Similarly to previous studies, we found that this bimodality is clearly connected to the cluster dynamics. For the very first time, we found an increase of the radio halo fraction with the cluster mass, which is remarkably in agreement with theoretical models. In addition to the statistics of radio halos, the amount of data available in this Thesis led to the discovery of new radio relics, mini halos and head tail radio galaxies in our clusters.

We study the influence of environment on galaxy evolution by focusing on two galaxy types known for their connection to dense environments, S0s and Brightest Cluster Galaxies (BCGs). Our goal is to identify the mechanisms responsible for the properties of galaxies in groups and clusters. We first examine the effects of environment on S0 formation over the past ~7 Gyr by tracing the increasing S0 fraction in clusters at two mass scales. We find the build-up of S0s driven by groups/clusters with velocity dispersions σ ≲ 750 km s⁻¹, suggesting mechanisms that operate most efficiently via slow encounters (e.g., mergers and tidal interactions) form S0s.With less-massive halos identified as the site for S0 formation, we test whether another route to S0 formation exists, not in isolated groups but rather in a system of four merging groups (SG1120). We place limits on how recent the S0s in that system could have formed, and finding no star formation, conclude they formed ≳ 1 Gyr prior to SG1120's current configuration, when they were in more isolated groups. We next explore cluster outskirts to constrain the number of infalling galaxies that need to be transformed and whether that process has already begun. We find the red fraction of infalling galaxies is elevated relative to the field, and that red galaxies are more clustered than blue ones, a signature of "pre-processing". We disentangle the relative strength of global versus local environment on galaxy transformation by comparing the correlation of red fraction with radius and local density. We find that both parameters are connected with the red fraction of galaxies. Finally, we measure the frequency of galaxies falling into the cluster that are bright enough to supplant the current BCG and compare the results to models. We find in ~ 85% of our clusters that the BCG is secure and remains in its priviledged state until z ~ 0.From these analyses, we find that intermediate density environments (groups and cluster outskirts) are the key site to forming S0 galaxies, and that BCGs, while not exclusively a cluster phenomenon, are well established by the redshifts we explore.

Les amas de galaxies sont des outils cosmologiques et astrophysiques essentiels, car ce sont les objets les plus grands et les plus massifs gravitationnellement liées dans l'Univers. L'étude de leur fonction de masse, de leur fonction de corrélation et des relations d'échelle entre leur masse et différentes observables nous permettent de tester les prévisions des modèles cosmologique et les scenarii de formation des structures. Ils sont aussi d'intéressants laboratoires pour l'étude de la formation et de l'évolution des galaxies, et de leur interactions avec le milieu qui les entourent, dans d’environnements denses. Pour y parvenir, estimer précisément leur masse revêt une importance fondamentale. J’ai étudié la précision de la richesse optique calculée par l’algorithme de détection d’amas RedGOLD (Licitra et al. 2016) en tant que mass proxy, en utilisant des mesures de lentilles gravitationnelles (weak lensing) et des observations en rayon X. J’ai mesuré les masses cumulées d’un échantillon de 1323 amas de galaxies dans le CFHTLS et NGVS à 0.2
Galaxy clusters are essential cosmological and astrophysical tools, since they represent the largest and most massive gravitationally bound structures in the Universe. Through the study of their mass function, of their correlation function, and of the scaling relations between their mass and different observables, we can probe the predictions of cosmological models and structure formation scenarios. They are also interesting laboratories that allow us to study galaxy formation and evolution, and their interactions with the intra-cluster medium, in dense environments. For all of these goals, an accurate estimate of cluster masses is of fundamental importance. I studied the accuracy of the optical richness obtained by the RedGOLD cluster detection algorithm (Licitra et al. 2016) as a mass proxy, using weak lensing and X-ray mass measurements. I measured stacked weak lensing cluster masses for a sample of 1323 galaxy clusters in the CFHTLS W1 and in the NGVS at 0.2

High redshift galaxy protoclusters are the precursors of today’s massive clusters; the sites of formation of the most massive galaxies in the present-day Universe. By studying these immature structures we can directly analyse the formation of galaxies in the densest environments without relying on extrapolations from low redshift. Finding protoclusters is challenging due to the need for very wide and deep surveys. Radio-loud active galactic nuclei (RLAGN) have been shown to preferentially reside in overdense environments at z > 1. By using these bright radio sources as beacons, protoclusters may be efficiently selected, without the need for large, blind surveys. In this thesis I study the properties of galaxies in high redshift (z > 1.3) clusters and protoclusters selected around RLAGN. Using a sample of 37 clusters and protoclusters from the Clusters Around Radio-Loud AGN (CARLA) survey, I show that the protocluster galaxies have an approximately unevolving, red observed-frame i'-[3.6] colour across 1.3 < z < 3.2. This is at odds with the simple models which are commonly used to explain the cluster red sequence in the local Universe, which predict cluster galaxy colours to become more blue at higher redshifts. Taking the full cluster population into account, I show that the formation of stars within the majority of massive cluster galaxies occurs over at least 2 Gyr, and peaks at z ~ 2–3. This is consistent with the cosmic star formation history, with star formation ending in clusters at 1 < z < 2. I further show that massive galaxies at z > 2 must have assembled within 0.5 Gyr of them forming a significant fraction of their stars. This means that few massive galaxies in z > 2 protoclusters could have formed via dry mergers. Some of the CARLA structures exhibit signs of being mature, collapsed clusters. In a pilot project, I report on the discovery of a z = 1.58 cluster with a strong red sequence around the RLAGN 7C 1753+6311. I demonstrate that the cluster has an enhanced quiescent galaxy fraction that is three times that of the control field. I also show that this enhancement is mass dependent: 91 +/- 9% of the M* > 10^10.5 Msun cluster galaxies are quiescent, compared to only 36 +/- 2% of field galaxies, whereas the fraction of quiescent galaxies with lower masses is the same in the cluster and field environments. This is in contrast to low redshift studies which have shown that mass and environmental effects on quenching star formation are separable. In the literature there is some debate as to whether RLAGN preferentially reside in clusters of a certain stage of collapse. The presence of a dense core and a well-formed, quiescent red sequence suggest that 7C1753+6311 resides within a mature cluster. This means that distant RLAGN do not solely reside in young, uncollapsed protoclusters, rather they can be found in clusters in a wide range of evolutionary states. Finally I present results from surveys of Halpha emitters in the fields around three high redshift RLAGN. I find that there is more dust-obscured star formation in protocluster galaxies than in similarly-selected control field galaxies at z ~ 2.5 and there is tentative evidence of a higher fraction of starbursting galaxies in the denser environment. However, on average I do not find a difference between the star formation rate (SFR)-mass relations of the protocluster and field galaxies and so conclude that the SFR of these galaxies at z ~ 2.5 is governed predominantly by galaxy mass and not the host environment. The stellar mass distribution of the protocluster galaxies is also skewed towards higher masses and there is a significant lack of low mass (M < 10^10 Msun) galaxies within the protocluster core. These results have implications for future protocluster surveys. The lack of low mass galaxies affects the level of overdensity which is detected. If only high mass galaxies are considered, the density of the protocluster field may be over-estimated. This means that it is important when quantifying protoclusters to compare their mass functions, rather than simply number overdensities. I also find that some radio galaxies do not reside in the centre, or densest region of the surrounding structure, meaning the overdensity measured in an aperture centred on the RLAGN will be underestimated. This means that future studies of (proto)clusters around RLAGN should use larger fields of view in order to establish the existence of a (proto)cluster.

In this thesis, I explore the evolution of the brightest cluster galaxies (BCGs) over the last 10 billion years through detailed studies of both local BCGs from SDSS and their high-z progenitors from CANDELS UDS. First, I study a large sample of local BCGs and link their morphologies to their structural properties. We derive visual morphologies for these BCGs and find that ~57% of local BCGs are cD galaxies, ~13% are ellipticals, and ~21% belong to intermediate classes, mostly between E and cD. There is a continuous distribution in the properties of the BCG's envelopes, ranging from undetected (elliptical BCGs) to clearly detected (cD galaxies), with intermediate classes showing increasing degrees of the envelope presence. A minority (~7%) of BCGs have disk morphologies, with spirals and S0s in similar proportions, and the rest (~2%) are mergers. After carefully fitting the galaxy light distributions using Sersic models, I find a clear link between BCG morphology and structure, such that cD galaxies are typically larger than elliptical BCGs, and the visually extended envelope of cD galaxies is a distinct structure differing from the central bulge. Based on this BCG morphology--structure correlation, I develop a statistically robust way to separate cD from non-cD BCGs, by which cD galaxies can be selected with reasonably high completeness and low contamination. Next, I investigate the effect of environment on the properties of local BCGs by studying the relationship between the BCG's internal properties (stellar mass, structure and morphology) and their environment (local density and cluster halo mass). I find that the size of BCGs is determined by the intrinsic BCG stellar mass, with a weak correlation with the cluster environment. Additionally, more massive BCGs tend to inhabit denser regions and more massive clusters than lower mass BCGs. The growth of the BCGs seems to be linked to the hierarchical growth of the structures they inhabit: as the groups and clusters became denser and more massive, the BCGs at their centres also grew. Moreover, I demonstrate that cD galaxies are ~40% more massive than elliptical BCGs, and prefer denser regions and more massive haloes. My results, together with the findings of previous studies, suggest an evolutionary link between elliptical and cD BCGs. I propose that most present-day cDs started their life as ellipticals at z~1, which subsequently grew in stellar mass and size due to mergers. In this process, the cD envelope developed. This process is nearing completion since the majority of the local BCGs have cD morphology. However, the presence of BCGs with intermediate morphological classes suggests that the growth and morphological transformation of BCGs is still ongoing. Finally, I present a new method for tracing the evolution of BCGs from z~2 to z~0. I conclude, on the basis of semi-analytical models, that the best method to select BCG progenitors at z~2 is a hybrid environmental density and stellar mass ranking approach. Ultimately I am able to retrieve 45% of BCG progenitors. Although the selected high-z progenitor sample is a mixture of BCG and non-BCG progenitors, I demonstrate that their properties can be used to trace BCG evolution. Applying this method to the CANDELS UDS data, I construct an observational BCG progenitor sample at z~2. A local BCG comparison sample is constructed using the SDSS data, taking into account the likely contamination from non-BCGs to ensure a fair comparison between high-z and low-z samples. Using these samples I demonstrate that BCG sizes have grown by a factor of ~3.2 since z~2, and BCG progenitors are mainly late-type galaxies, exhibiting less concentrated profiles than their early-type local counterparts. I also find that BCG progenitors have more disturbed morphologies, while local BCGs have much smoother profiles. Moreover, I find that the stellar masses of BCGs have grown by a factor of ~2.5 since z~2, and the SFR of BCG progenitors has a median value of ~14 Msun/yr, much higher than their quiescent local descendants. I demonstrate that at 1 < z < 2 star formation and merging contribute approximately equally to BCG mass growth. However, merging plays a dominant role in BCG assembly at z < 1. I also find that BCG progenitors at high-z are not significantly different from other galaxies of similar mass at the same epoch. This suggests that the processes which differentiate BCGs from normal massive elliptical galaxies must occur at z < 2.

Magister Scientiae - MSc
We determine the stellar mass of star forming galaxies in the X-ray luminous cluster MS 0451.6-0305 at z ∼ 0.54. The stellar masses are estimated from fitting model spectral energy distributions (SEDs) to deep, optical UBRIz observations obtained from WIYN 3.5m telescope and public NIR K-band image from Palomar Observatory telescope. The model SEDs are based on the stellar population synthesis (SPS) model of Bruzual & Charlot (2003) and Conroy et al. (2009) that span a wide range of age, star formation history, Initial Mass Function (IMF), metallicity and dust content. We measure stellar masses for galaxies down to M∗∼2×10⁸M(.) We find a tight correlation between stellar masses derived from the two SPSs. We compare the derived stellar masses to the dynamical masses for a set of 25 star forming galaxies. The dynamical masses are derived from high resolution, spectroscopic observations of emission lines from the DEIMOS spectrograph on the Keck telescope. A strong correlation is seen between the dynamical and stellar mass for the galaxies; and the star forming galaxies show fairly constant ratio between stellar and dynamical mass. When comparing to the field sample of Guzm ́an et al. (2003) of luminous compact blue galaxies, we see an excess of low mass galaxies in the cluster.
South Africa

In this thesis, I explore how the environments of both galaxy and cluster-scale strong gravitational lenses affect studies of cosmology and the properties of the earliest galaxies. Galaxy-scale lenses with measured time delays can be used to determine the Hubble constant, given an accurate lens model. However, perturbations from structures along the line of sight can introduce errors into the measurement. I use data from a survey towards known lenses in group environments to calculate the external shear in these systems, which is typically marginalized over in standard lens analyses. In three of six systems where I compare the independently-calculated environment shear to lens model shears, the quantities disagree at greater than 95% confidence. We explore possible sources of this disagreement. Using these data, I generate fiducial lines of sight and insert mock lenses with assumed input physical and cosmological parameters and find that those parameters can be recovered with ∼ 5-10% scatter when uncertainties in my characterization of the environment are applied. The lenses in groups have larger bias and scatter. I predict how well new time delay lenses from LSST will constrain H₀ and find that an ensemble of 500 quad lenses will recover H₀ with ∼ 2% bias with ∼ 0.3% precision. On larger scales, galaxy cluster lenses can magnify the earliest galaxies into detectability. While past studies have focused on single massive clusters, I investigate the properties of lines of sight, or "beams", containing multiple cluster-scale halos in projection. Even for beams of similar total mass, those with multiple halos have higher lensing cross sections on average. The optimal configurations for maximizing the cross section are also those that maximize faint z ∼ 10 detections. I present a new selection technique to identify beams in wide-area photometric surveys that contain high total masses and often multiple clusters in projection as traced by luminous red galaxies. I apply this technique to the Sloan Digital Sky Survey and present the 200 most promising beams. Several are confirmed spectroscopically to be among the highest mass beams known with some containing multiple clusters. These are among the best fields to search for faint high-redshift galaxies.

In the present thesis a thourough multiwavelength analysis of a number of galaxy clusters known to be experiencing a merger event is presented. The bulk of the thesis consists in the analysis of deep radio observations of six merging clusters, which host extended radio emission on the cluster scale. A composite optical and X–ray analysis is performed in order to obtain a detailed and comprehensive picture of the cluster dynamics and possibly derive hints about the properties of the ongoing merger, such as the involved mass ratio, geometry and time scale. The combination of the high quality radio, optical and X–ray data allows us to investigate the implications of the ongoing merger for the cluster radio properties, focusing on the phenomenon of cluster scale diffuse radio sources, known as radio halos and relics. A total number of six merging clusters was selected for the present study: A3562, A697, A209, A521, RXCJ 1314.4–2515 and RXCJ 2003.5–2323. All of them were known, or suspected, to possess extended radio emission on the cluster scale, in the form of a radio halo and/or a relic. High sensitivity radio observations were carried out for all clusters using the Giant Metrewave Radio Telescope (GMRT) at low frequency (i.e. ≤ 610 MHz), in order to test the presence of a diffuse radio source and/or analyse in detail the properties of the hosted extended radio emission. For three clusters, the GMRT information was combined with higher frequency data from Very Large Array (VLA) observations. A re–analysis of the optical and X–ray data available in the public archives was carried out for all sources. Propriety deep XMM–Newton and Chandra observations were used to investigate the merger dynamics in A3562. Thanks to our multiwavelength analysis, we were able to confirm the existence of a radio halo and/or a relic in all clusters, and to connect their properties and origin to the reconstructed merging scenario for most of the investigated cases. • The existence of a small size and low power radio halo in A3562 was successfully explained in the theoretical framework of the particle re–acceleration model for the origin of radio halos, which invokes the re–acceleration of pre–existing relativistic electrons in the intracluster medium by merger–driven turbulence. • A giant radio halo was found in the massive galaxy cluster A209, which has likely undergone a past major merger and is currently experiencing a new merging process in a direction roughly orthogonal to the old merger axis. A giant radio halo was also detected in A697, whose optical and X–ray properties may be suggestive of a strong merger event along the line of sight. Given the cluster mass and the kind of merger, the existence of a giant radio halo in both clusters is expected in the framework of the re–acceleration scenario. • A radio relic was detected at the outskirts of A521, a highly dynamically disturbed cluster which is accreting a number of small mass concentrations. A possible explanation for its origin requires the presence of a merger–driven shock front at the location of the source. The spectral properties of the relic may support such interpretation and require a Mach number M < ∼ 3 for the shock. • The galaxy cluster RXCJ 1314.4–2515 is exceptional and unique in hosting two peripheral relic sources, extending on the Mpc scale, and a central small size radio halo. The existence of these sources requires the presence of an ongoing energetic merger. Our combined optical and X–ray investigation suggests that a strong merging process between two or more massive subclumps may be ongoing in this cluster. Thanks to forthcoming optical and X–ray observations, we will reconstruct in detail the merger dynamics and derive its energetics, to be related to the energy necessary for the particle re–acceleration in this cluster. • Finally, RXCJ 2003.5–2323 was found to possess a giant radio halo. This source is among the largest, most powerful and most distant (z=0.317) halos imaged so far. Unlike other radio halos, it shows a very peculiar morphology with bright clumps and filaments of emission, whose origin might be related to the relatively high redshift of the hosting cluster. Although very little optical and X–ray information is available about the cluster dynamical stage, the results of our optical analysis suggest the presence of two massive substructures which may be interacting with the cluster. Forthcoming observations in the optical and X–ray bands will allow us to confirm the expected high merging activity in this cluster. Throughout the present thesis a cosmology with H0 = 70 km s−1 Mpc−1, m=0.3 and =0.7 is assumed.

In the present thesis a thourough multiwavelength analysis of a number of galaxy clusters known to be experiencing a merger event is presented. The bulk of the thesis consists in the analysis of deep radio observations of six merging clusters, which host extended radio emission on the cluster scale. A composite optical and X–ray analysis is performed in order to obtain a detailed and comprehensive picture of the cluster dynamics and possibly derive hints about the properties of the ongoing merger, such as the involved mass ratio, geometry and time scale. The combination of the high quality radio, optical and X–ray data allows us to investigate the implications of the ongoing merger for the cluster radio properties, focusing on the phenomenon of cluster scale diffuse radio sources, known as radio halos and relics. A total number of six merging clusters was selected for the present study: A3562, A697, A209, A521, RXCJ 1314.4–2515 and RXCJ 2003.5–2323. All of them were known, or suspected, to possess extended radio emission on the cluster scale, in the form of a radio halo and/or a relic. High sensitivity radio observations were carried out for all clusters using the Giant Metrewave Radio Telescope (GMRT) at low frequency (i.e. ≤ 610 MHz), in order to test the presence of a diffuse radio source and/or analyse in detail the properties of the hosted extended radio emission. For three clusters, the GMRT information was combined with higher frequency data from Very Large Array (VLA) observations. A re–analysis of the optical and X–ray data available in the public archives was carried out for all sources. Propriety deep XMM–Newton and Chandra observations were used to investigate the merger dynamics in A3562. Thanks to our multiwavelength analysis, we were able to confirm the existence of a radio halo and/or a relic in all clusters, and to connect their properties and origin to the reconstructed merging scenario for most of the investigated cases. • The existence of a small size and low power radio halo in A3562 was successfully explained in the theoretical framework of the particle re–acceleration model for the origin of radio halos, which invokes the re–acceleration of pre–existing relativistic electrons in the intracluster medium by merger–driven turbulence. • A giant radio halo was found in the massive galaxy cluster A209, which has likely undergone a past major merger and is currently experiencing a new merging process in a direction roughly orthogonal to the old merger axis. A giant radio halo was also detected in A697, whose optical and X–ray properties may be suggestive of a strong merger event along the line of sight. Given the cluster mass and the kind of merger, the existence of a giant radio halo in both clusters is expected in the framework of the re–acceleration scenario. • A radio relic was detected at the outskirts of A521, a highly dynamically disturbed cluster which is accreting a number of small mass concentrations. A possible explanation for its origin requires the presence of a merger–driven shock front at the location of the source. The spectral properties of the relic may support such interpretation and require a Mach number M < ∼ 3 for the shock. • The galaxy cluster RXCJ 1314.4–2515 is exceptional and unique in hosting two peripheral relic sources, extending on the Mpc scale, and a central small size radio halo. The existence of these sources requires the presence of an ongoing energetic merger. Our combined optical and X–ray investigation suggests that a strong merging process between two or more massive subclumps may be ongoing in this cluster. Thanks to forthcoming optical and X–ray observations, we will reconstruct in detail the merger dynamics and derive its energetics, to be related to the energy necessary for the particle re–acceleration in this cluster. • Finally, RXCJ 2003.5–2323 was found to possess a giant radio halo. This source is among the largest, most powerful and most distant (z=0.317) halos imaged so far. Unlike other radio halos, it shows a very peculiar morphology with bright clumps and filaments of emission, whose origin might be related to the relatively high redshift of the hosting cluster. Although very little optical and X–ray information is available about the cluster dynamical stage, the results of our optical analysis suggest the presence of two massive substructures which may be interacting with the cluster. Forthcoming observations in the optical and X–ray bands will allow us to confirm the expected high merging activity in this cluster. Throughout the present thesis a cosmology with H0 = 70 km s−1 Mpc−1, m=0.3 and =0.7 is assumed.

This thesis attempts to test several frameworks of non-Newtonian gravity in the context of galaxies and galaxy clusters. The theory most extensively discussed was that of Modified Newtonian Dynamics (MOND) with Galileon gravity, Emergent Gravity (EG) and Modified Gravity (MOG) mentioned to a lesser extent. Specifically, the main focus of this thesis was to determine whether MOND and MOND-like theories were compatible with galaxy cluster data, without the need to include cold dark matter. To do this, the paradigms of Extended MOND (EMOND), Generalised MOND (GMOND) and superfluid dark matter were investigated. The theories were outlined and applied to galaxy cluster data. The main findings of this were that EMOND and GMOND had some success with explaining galaxy cluster mass profiles, without requiring an additional dark matter component. The superfluid paradigm also enjoyed some success in galaxy clusters, which was expected as it behaves in a similar manner to the standard cold dark matter paradigm in cluster environments. However, the superfluid paradigm may have issues in the very centre of galaxy clusters due to the theory predicting constant density cores, whereas the cold dark matter paradigm predicts density cores which are cuspier. The EMOND paradigm was also tested against ultra-diffuse galaxy (UDGs) data as they appear in cluster environments, where EMOND becomes important. It was found that EMOND can reproduce the inferred mass of the UDGs, assuming they lie on the fundamental manifold (FM). The validity of the assumptions used to model the UDGs are discussed in the text. A two-body problem was also conducted in the Galileon gravity framework. The amount of additional gravitational force, compared to Newtonian was determined for a small galaxy at the edge of a galaxy cluster.

In questa tesi di laurea triennale vengono esposte le conoscenze fondamentali che descrivono gli ammassi di galassie. I galaxy clusters sono strutture gravitazionalmente legate composte di galassie, gas denominato ICM (Intra Cluster Medium) e materia oscura. Queste 3 diverse componenti sono responsabili rispettivamente del 5%, 15% e 80% circa della massa totale dell’ammasso; per la maggior parte degli ammassi la massa totale è 10^{14-15} masse solari. Nella prima parte della tesi si illustrano brevemente queste 3 componenti e le si inquadrano nelle diverse classificazioni morfologiche degli ammassi. Nella seconda parte ho passato in rassegna alcune delle funzioni più importanti per descrivere un ammasso di galassie. Nella terza ed ultima parte sono esposti i principali meccanismi grazie ai quali conosciamo gli ammassi di galassie.

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.

The evolution of radio loud quasars is found to be strongly dependent upon their galaxy cluster environment. Previous studies (Yee and Green 1987) have shown that bright quasars at z ∼ 0.6 are found in clusters as rich as Abell richness class 1, while high luminosity quasars at lower redshifts are found only in poorer environments. An observational study of the environments of 66 low luminosity quasars with 0.3 < z < 0.6 yields several objects in rich clusters of galaxies. This result implies that radio loud quasars in these environments have faded approximately 3 magnitudes in the interval between redshifts 0.6 and 0.4, corresponding to a luminosity e-folding fading time of 900 million years, similar to the dynamical timescale of these environments. The analysis of low luminosity radio quiet quasars indicate that they are never found in rich environments, suggesting that they are a physically different class of objects. Properties of the quasar environment are investigated to determine constraints on the physical mechanisms of quasar formation and evolution. The optical cluster morphology indicates that the cluster cores have smaller radii and higher galaxy densities than are typical for low redshift clusters of similar richness. Radio morphologies may indicate that the formation of a dense intra-cluster medium is associated with the quasars' fading at these epochs. Galaxy colors appear to be normal, but there may be a tendency for clusters associated with high luminosity quasars to contain a higher fraction of gas-rich galaxies than those associated with low luminosity quasars, a result consistent with the formation of an ICM. Multislit spectroscopic observations of galaxies associated with high luminosity quasars indicate that quasars are preferentially located in regions of low relative velocity dispersion, either in rich clusters of abnormally low velocity dispersion, or in poor groups which are dynamically normal. This suggests that galaxy-galaxy interactions may play a role in quasar formation and sustenance. Virialization of rich clusters and the subsequent increase in galaxy velocities may therefore be responsible for the fading of quasars in rich environments.

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.

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.