Thèses sur le sujet « Bar formation, galaxy evolution »

The formation of stars from interstellar gas is the cornerstone of galaxy evolution. This thesis represents work undertaken in order to characterise the role of cool interstellar gas, and its relation to star formation, in galaxy evolution across cosmic time. In particular, it concentrates on star forming galaxies at the extremes of the galaxy assembly spectrum - extremely faint dwarfs, and extremely luminous starbursts - in an attempt to test the limits of galaxy evolution models. The thesis falls into two complimentary halves, addressing topics in the low redshift and high redshift Universe respectively. In the low redshift Universe, I discuss multi-wavelength studies of large samples of z rv O galaxies, which include extremely faint dwarf galaxies in the Local Volume. Using these samples, it is possible to derive a multitude of physical parameters (including star formation rates, stellar masses, and gas masses) which allow the interrelationship between star formation and gas content to be assessed in a statistically significant manner. In particular, modern wide field surveys (combined with deep, volume-limited data) allow trends to be analysed across many orders of magnitude in galaxy mass and star formation rate, shedding light on the global properties of galaxies in the local Universe. Moving to higher redshift, I discuss targeted observations of molecular gas in extreme star forming galaxies in the early Universe. These 'sub-millimetre' galaxies number amongst the most luminous objects ever discovered, and molecular gas observations have the power to uncover many of their physical properties, including their morphologies, kinematics, and star formation behaviour. I begin by presenting high-resolution observations of a small number of these galaxies at z rv 2, and discussing the implications for galaxy evolution studies. The final chapter of this thesis consists of the results of a survey for molecular gas in sub-millimetre galaxies conducted over the last decade, which represents the largest single study of molecular gas in the early Universe to date.

This thesis investigates various methods of dust obscuration measurements in order to derive accurate SFRs which I then use to investigate the SFR-density relation. I present self-consistent star formation rates derived through pan-spectral analysis of galaxies drawn from the Galaxy and Mass Assembly (GAMA) survey. I determine the most appropriate form of dust obscuration correction via application of a range of extinction laws drawn from the literature as applied to Ha, [On] and UV luminosities. I consider several different obscuration curves, including those of Milky Way, Calzetti (2001) and Fischera and Dopita (2005) and their effects on the observed luminosities. I find that the Fischera & Dopita (2005) obscuration curve with an Rv value of 4.5 gives the best agreement between the different SFR indicators. The 2200 A feature needs to be removed from this curve to obtain complete consistency between all SFR indicators suggesting that this feature may not be common in the average integrated attenuation of galaxy emission. The findings of this work indicate that incorporating a more direct measure of dust such as the far infrared (FIR) and near infrared (NIR) may help develop more accurate obscuration corrections particularly for the ultraviolet (UV) wavelength region as the UV radiation absorbed by dust is re-emitted in the infrared (IR). To carry out this analysis I combine the multiwavelength data from GAMA with data from the Herschel ATLAS (H­ATLAS) survey. I explore the connections between each of the following: the ultraviolet (UV) spectral slope, /3, the Balmer decrement, and the far infrared (IR) to 150 nm far ultraviolet (FUV) luminosity ratio. I reiterate the finding of other authors that there is a large scatter between the Balmer decrement and the /3 parameter, and that /3 may be poorly constrained when derived from only two broad passbands in the UV. While there is a stronger correlation between the IR to FUV luminosity ratio and the /3 parameter than with the Balmer decrement, neither of these correlations are particularly tight, and dust corrections based on /3 for high redshift galaxy SFRs must be treated with caution. I then used the SFRs that were derived using the above obscuration correction for­malism to investigate the known SFR-density relationship and explore in detail the de­pendence of SFR on stellar mass and density. I show that the SFR-density trend is only visible when I include the passive galaxy population along with the star-forming population. This SFR-density relation is absent when I consider only the star-forming population of galaxies, consistent with previous work. I find that stellar mass has the strongest influence on SFR and EWHa with the environment having no significant effect on the star-formation properties of the star forming population. The observation that the trends with density are due to the changing morphology fraction with density implies that the timescales must be very short for any quenching of the SFR in infalling galaxies. Alternatively galaxies may in fact undergo predominantly in-situ evolution where the in­fall and quenching of galaxies from the field into dense environments is not the dominant evolutionary mode.

Abstract The discs in galaxies are radially extended, rotationally supported, flattened systems. In the cosmological Lambda Cold Dark Matter model the formation of the discs is intimately connected with galaxy formation. Generally it is assumed that the discs have exponentially decreasing stellar surface brightness profiles, but completely satisfactory theoretical explanation for this has not been presented. Large number of studies in the past decade have challenged this view, and have found a change in the slope of the surface brightness profile in the outer regions of many galaxies discs: the surface brightness can decrease more, or less, steeply than in the inner regions. The transition between the two slopes is often called a disc break. Consequently, the discs are divided in three major categories: single exponential Type I, down-bending break Type II, and up-bending break Type III. Formation of these break features has been linked to the initial formation of the discs, internal evolution, and also with the interactions between galaxies. By studying the detailed properties of the disc break features, the evolutionary history of discs, and galaxies in general, can be better understood. The thesis work focuses on the structural analysis of the galaxies in the Spitzer Survey of Stellar Structure in Galaxies (S4G), which consists of 2352 galaxies observed in the 3.6 and 4.5 µm mid-infrared wavelengths with the Spitzer space telescope. Work has been carried out as a part of the data-analysis pipelines of the S4G survey, utilizing surface photometry. In addition, special emphasis has been put on the study of the disc and disc break properties in a wide range of galaxy morphological types and stellar masses. The thesis work attempts to at least partially understand how galaxy stellar mass and observed wavelength affect the properties of the discs and breaks, and how galaxy structural components are connected with the breaks. The data comprises mainly of the 3.6 µm infrared data, providing a view to the stellar mass distribution of galaxies. We find that the Type II breaks are the most common disc profile type, found in 45 ± 2% of the sample galaxies, consisting of 759 galaxies in the stellar mass range 8.5 ≲ log10(M*/M⊙) ≲ 11. Type I discs are found in 31 ± 2%, and the Type III breaks in 23 ± 2% of the sample. The fraction of the profile types also depends of the galaxy stellar mass: fractions of the Types II and III increase, while Type I fraction decreases, with increasing stellar mass. We attribute these changes with stellar mass to the increased frequency of bar resonance structures in higher mass galaxies, which are commonly associated with a Type II break, and to the increased fraction of Type III profiles in generally more massive early-type disc galaxies. In addition to the Type II breaks associated with bar resonance structures, we find that nearly half of these breaks relate to the visual spiral outer edge, confirming previous results of the Type II break connection with galaxy structure, and thus the internal evolution rather than initial formation of discs. Complementary data in optical wavelengths from the Sloan Digital Sky Survey shows a strong change in the properties of the discs inside the Type II breaks, indicating that the inner discs are evolving via star formation. In late-type spiral galaxies (T ≳ 4) with a Type II break, possible evidence of radial stellar migration is found in the outer disc: the slope of the surface brightness profile is shallower in the infrared compared to optical wavelengths, indicating that older stellar populations are more evenly spread throughout the disc. Formation of the Type I and III profiles remain poorly understood. However, indication that some of the Type III profiles are formed by environmentally driven processes is found, with a correlation between the properties of the local environment and the disc profile parameters. Furthermore, indication of star formation possibly causing the up-bends in spiral galaxies is found through a presence of young stellar population in the outer disc section.

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

In this thesis, we 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.

The current cosmological model, the Lambda CDM theory, describes with remarkable precision the assembly and growth of the large scale structures and of the dark matter halos in our Universe. A comprehensive theory for the baryon processes that take place within dark matter halos is, instead, still the subject of active research. The three major ingredients of this theory are known: accretion of hydrogen from the intergalactic medium, star formation, and feedback mechanisms in the form of galactic winds. However, the recipe to blend them together has not yet been found. This thesis focuses on the role that two of these ingredients have in the assembly and evolution of galaxies. The underlying questions that this work aims to address are how the accretion of hydrogen onto galaxies occurs and what the conditions needed to convert this raw fuel into stars are. The instruments used for this investigation are diverse, because of the multiplicity of physical processes, spatial scales, and cosmic epochs involved in the problem. Theory, or more specifically the analysis of hydrodynamic simulations to unveil gas accretion onto high-redshift galaxies, is the starting point for this work. In the second part, spectroscopy of bright quasars is used to probe the physical properties of gas and metals around and within distant galaxies. These observations are systematically compared to model predictions. Deep optical imaging is also used to connect the star formation rates of these galaxies to the gas properties that are measured in absorption. Finally, in the third part, the relationship between hydrogen and star formation on smaller scales is investigated by means of multiwavelength observations of local galaxies. This thesis contributes to the aforementioned open questions in four ways. First, it is shown that the accretion of gas onto galaxies as predicted by current simulations imprints characteristic signatures on the distribution of hydrogen and metals of a particular family of absorption line systems, the Lyman limit systems. Second, new spectroscopic observations that led to the discovery of gas clouds with physical properties that match predictions from simulations are presented, paving the way for establishing empirically how galaxies acquire their gas. Third, through a comparison of the hydrogen content and the star formation rates of distant galaxies, this thesis confirms how the presence of significant amounts of hydrogen is not a sufficient condition for the onset of star formation. Finally, after assessing the validity of star formation models in environments that are common to high redshift galaxies, these findings have been interpreted as inefficient star formation in regions with low gas column density and low metallicity.

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

The Apache Point Observatory Galactic Evolution Experiment (APOGEE), one of the programs in the Sloan Digital Sky Survey III (SDSS-III), has now completed its systematic, homogeneous spectroscopic survey sampling all major populations of the Milky Way. After a three-year observing campaign on the Sloan 2.5 m Telescope, APOGEE has collected a half million high-resolution (R similar to 22,500), high signal-to-noise ratio (>100), infrared (1.51-1.70 mu m) spectra for 146,000 stars, with time series information via repeat visits to most of these stars. This paper describes the motivations for the survey and its overall design-hardware, field placement, target selection, operations-and gives an overview of these aspects as well as the data reduction, analysis, and products. An index is also given to the complement of technical papers that describe various critical survey components in detail. Finally, we discuss the achieved survey performance and illustrate the variety of potential uses of the data products by way of a number of science demonstrations, which span from time series analysis of stellar spectral variations and radial velocity variations from stellar companions, to spatial maps of kinematics, metallicity, and abundance patterns across the Galaxy and as a function of age, to new views of the interstellar medium, the chemistry of star clusters, and the discovery of rare stellar species. As part of SDSS-III Data Release 12 and later releases, all of the APOGEE data products are publicly available.

The formation and evolution of galaxies is an interesting subject to study because it incorporates astrophysics from all scales, from the initial perturbations in the early universe creating the large scale structures that produce galaxies, right down to the evolution of stellar populations and their manipulation of the host galaxy. Simulations of galaxy formation allow us to test the various physical recipes against that which is observed in order to build a true and proper picture of what is happening in the real universe. L-Galaxies is a semi-analytic model of galaxy formation built on top of the merger trees from the Millennium dark matter simulation, and is constrained to match certain key observations at low redshift by applying a Monte Carlo Markov Chain (MCMC) method to constrain the free parameters. In using the model to make high redshift predictions of the stellar mass function, UV luminosity function and star formation rate distribution function we found that the model starts to deviate from observational constraints at the highest redshifts, particularly in high mass galaxies. In the case of the UV luminosity function, this is because the current dust model is calibrated at low redshift and lacks sophistication in that it only depends on the cold gas mass and the density of metals. To improve on this we implement a physically motivated dust model that traces the formation of dust from stellar sources, such as in the stellar winds of AGB stars and in the supernovae remnants of massive stars, the growth of dust inside molecular clouds, and the destruction of dust due to supernovae explosions. The model is fully integrated into L-Galaxies such that the evolution of dust is included in all the recipes relevant to the formation and evolution of galaxies, including: star formation; radiative feedback; cooling and reheating; and both major and minor mergers. Our results show a good fit to observations of the dust mass in galaxies both in the local universe and out to high redshift and we note a similar conclusion as in the literature that dust growth inside molecular clouds is not only necessary but the dominant source of the dust mass in these galaxies. However, stellar sources of dust can not be neglected as molecular clouds must first be seeded by dust grains in order for accretion to occur. This could be important in the very early universe, perhaps for the first galaxies that will hopefully be observed by JWST in the future, because these galaxies may not have had sufficient time to seed their molecular clouds and as such the dust produced by these stellar sources would be important for calculating the galaxies true observed luminosity. We finish by discussing the limitations of the model and discuss areas for possible improvement as well as the next steps in using this to better predict the luminosity of galaxies in future models.

A major step forward in our understanding of the formation mechanism and evolution with cosmic time of galaxies is given by the recent development of powerful astronomical tools able to detect exceedingly faint signals from remote distances. One of the latter developments of particular signicance was the opening of wavelengths longwards the classical visual band to the astronomical observation from space. From the mid '80 with the pioneering IRAS survey, until today, with the operation of the Herschel Space Observatory in the far-infrared and the Large Atacama Millimetre Array (ALMA) in the millimetre, a variety of initiatives have been put in operation to observe the Cosmo at long wavelengths. Among the many novelties emerging from all this, it became more and more evident the role of diuse media in galaxies in shaping their spectral emission and modifying the flux emerging from stars. A particular eect is that due to the mixing of dust and gas in the galaxy interstellar media (ISM): as dust particles have a large cross section with respect to the optical and ultraviolet photons typically emitted by stars, a large fraction of stellar emission is absorbed by dust and re-emitted at long wavelengths in the IR and millimeter bands. According to observational evidence and theoretical expectation, this re-shaping of galaxy emission spectra by the dust content of their ISM becomes more and more important the richer the galaxy content is of primeval gas and dust. This happens particularly when we consider the early phases of their formation and evolution. Important discoveries in this sense have been made using large millimetric telescopes on ground (like SCUBA, IRAM, APEX among others), revealing the existence of a population of extremely luminous galaxies at high redshift, with luminosities comparable to those of quasars. Most of the light emitted by these objects falls in the far-infrared and sub-mm and is visible strongly redshifted at millimeter wavelengths. In the optical these galaxies appear as extremely faint and red, due to the combined eect of dust extinction and redshift. This discovery, later conrmed by space infrared observatories, proved the existence of phases of enormous star formation activity already at high redshift, very likely related to the formation of massive spheroidal galaxies (ellipticals, S0's). Altogether, all these recent developments require new approaches to investigate the physical properties of high-redshift galaxies, hence constraining the history of their assembly and early evolution. At the time when only visual (optical-UV) data were available on moderate-redshift galaxies the modellistic requirements to interprete their spectro-photometric data were most simply to add linearly the contributions of all stars present in the galaxy, quite independently on the geometry of the stellar distribution or dust opacities. Now the new data on dust re-radiations oered by infrared observatories showed that, particularly during major episodes of stellar formation, large or even dominant fractions of the emission by young stars are absorbed by dust and re-emitted as large far-infrared bumps. There is thus a clear association of star formation and dust extinction, because high star formation rates require highly opaque media for gas to collapse and form stars. In a real star-forming galaxy stars, gas and dust are mixed in a very complicated way, and dust extinction is a strong function of the age of the stellar population in the galaxy. New data then force us to consider a new generation of models of galaxy synthetic spectra, with radical complications with respect to previous classical modelling, occurring at two levels: on one side dust extinction can not be neglected and should be considered as a function of the age of the stellar population in the galaxy. In addition, the eect of dierential extinction entails that geometrical eects in the distribution of stars and dust play a fundamental role and have to be carefully modelled. Various attempts have been made during the last decade to provide simplied approaches to the above problems. The simplest widely used is to calculate the summed spectrum of all stars and extinguish it with an average and possibly representative extinction law. Virtually all results of physical analyses of galaxy populations in the universe are based on simplied numerical codes based on this approach. This can provide sensible results in relation to galaxy populations characterized by the presence of negligible amounts of interstellar gas and modest extinction, like in early type galaxies in the local universe or moderately starforming spirals. But we already know that this is an improper approach when considering more actively star-forming galaxies or objects detected during early phases of gas collapse hence in the presence of a very rich interstellar medium. These correspond to the most important and interesting phases of the formation and evolution of galaxies. The aim of the present work is to contribute to overcome the diculties and limitations described above, with a new eort of modelling and physical analysis of populations of galaxies, both in the local universe and at high redshifts. More specically, the main focus of my PhD research is to investigate the nature and mass assembly history of dusty star forming galaxies at high redshift (0.8< z <2.5), observed with Herschel, through a self-consistent modelling of their main physical properties like stellar, gas and dust mass, SFR and SFH, and dust attenuation. With total infrared luminosities between 1011 1012 L and 1012 L, respectively, Luminous and Ultra luminous infrared galaxies [(U)LIRGs hereafter], are among the most luminous and complex extragalactic objects we can conceive, including all varieties of young and old stellar populations, dust absorption, scattering, grain thermal re-radiation, and AGN emission (Lonsdale et al., 2006). Although they are quite rare in the local universe, they dominate the cosmic star formation rate and the FIR background at z > 1. Therefore they are suitable laboratories to study the main physical processes which drive galaxy formation and evolution. The physical characterization of the ULIRG phenomenon requires a multi-wavelength approach and a detailed treatment of dust eects. Galaxy Evolution Synthesis Technique seems to be a powerful tool to interpret galaxy spectra. The spectral energy distribution (SED) of a galaxy contains valuable information about its physical properties, including the stellar, gas and dust content, the age and abundance distribution of the stellar populations resulting from the SFH, and their interaction with the interstellar medium (ISM). The study of the SED therefore oers the most direct way to investigate galaxy formation and evolution, both through direct observations and corresponding theoretical modelling. To model the emission from stars and dust consistently and get reliable estimates for the main galaxy physical parameters we need to solve the radiative transfer equation for idealised but realistic geometrical distributions of stars and dust, as well as take advantage of the full SED coverage from UV to sub-mm. The power of our approach, here, lies in the combination of a full multi-wavelength coverage for our selected galaxies, including the FIR from 70 to 500 micron by Herschel PACS and SPIRE and the IRS Spitzer spectra where available, with a self-consistent spectral synthesis code GRASIL (Silva et al., 1998) used to interpret galaxy SEDs. This code computes the SEDs of galaxies from far-UV to radio including a state-of-the-art treatment of dust extinction and reprocessing based on a full radiative transfer solution. The characteristics of this model (e.g. accounting of diuse and clumped dust and stars, realistic geometry, both giving rise to an age-dependent dust attenuation, and a full computation of dust temperature as a function of grain size and composition) together with the large spectral coverage, allows a thorough physical analysis of the observed SEDs. Our analysis demonstrates that a correct and self-consistent treatment of dust extinction and reprocessing together with a full multiwavelength coverage (from far-UV to radio), is essential to get reliable estimates of the main physical parameters like stellar mass, dust extinction and SFRs. We show that such a physical approach can have strong impact on the claimed relation between the SFR and stellar mass. This is due to the uncertainties related to the interpretation of the optical to far-IR emission depending on the age of stars responsible for the dust heating and reprocessing. We observe that the addition of radio emission to the spectral multi-band tting oers a tight constraint on the current SFR, considering that only stars younger than about 10 Myr produce the galactic cosmic rays responsible for the non-thermal radio emission. Moreover as the radio emission probes the SFH on dierent timescales with respect to the IR emission, our analysis also allows us to better understand and constrain the source's past SFHs, in particular the number of massive stars contributing to the NT component of radio emission through core-collapse SNe.
Un importante passo avanti nella comprensione del meccanismo di formazione ed evoluzione delle galassie e dato dal recente sviluppo di potenti strumenti astronomici in grado di rilevare segnali estremamente deboli provenienti da distanze remote. Uno degli ultimi sviluppi di particolare rilevanza e stata l'apertura di lunghezze d'onda maggiori della banda visibile all'osservazione astronomica dallo spazio. Dalla meta degli anni '80 con la pionieristica survey IRAS, no ad oggi, con il telescopio spaziale Herschel operante nel lontano infrarosso e l'Atacama Large Millimetre Array (ALMA) nel millimetrico, una serie di iniziative sono state messe a punto per osservare il Cosmo a lunghezze d'onda lunghe. Tra le tante novita che emergono da tutto questo, e diventato sempre piu evidente il ruolo del mezzo diuso nelle galassie nel `modellare' la loro emissione spettrale modicando il flusso emergente dalle stelle. Un eetto particolare e quello dovuto alla combinazione di polveri e gas nel mezzo interstellare della galassia (ISM): poiche le particelle di polvere hanno una elevata sezione d'urto rispetto ai fotoni ottici e UV tipicamente emessi dalle stelle, una frazione signicativa dell' emissione stellare e assorbita dalla polvere e riemessa a lunghezze d'onda nella banda IR e millimetrica. Secondo l'evidenza osservativa e le previsioni teoriche, questo `re-shaping' degli spettri di emissione delle galassie ad opera del contenuto di polvere del loro ISM diventa sempre piu importante quanto piu ricco e il contenuto di gas e polvere primordiale della galassia. Cio accade soprattutto quando si considerano le prime fasi della loro formazione ed evoluzione. Scoperte importanti in questo senso sono state fatte utilizzando grandi telescopi millimetrici da terra (come SCUBA, IRAM, APEX tra gli altri), rivelando l'esistenza di una popolazione di galassie ad alto redshift estremamente luminose, con luminosita confrontabili a quelle dei quasar. La maggior parte della radiazione emessa da questi oggetti cade nel lontano infrarosso e sub-mm ed e visibile fortemente spostata verso il rosso alle lunghezze d'onda millimetriche. Nell'ottico queste galassie appaiono come estremamente deboli e rosse, per l'effetto combinato di estinzione da polvere e redshift. Questa scoperta, in seguito confermata da osservatori spaziali infrarossi, ha dimostrato l'esistenza di fasi di intensa attivita di formazione stellare gia ad alto redshift, molto probabilmente legate alla formazione di galassie sferoidali massive (ellittiche, S0). Complessivamente, tutti questi recenti sviluppi richiedono nuovi approcci per studiare le proprieta fisiche delle galassie ad alto redshift, quindi vincolare la storia del loro assemblaggio e l'evoluzione iniziale. Al tempo in cui erano disponibili solo i dati ottici e UV, per galassie a basso redshift, i requisiti modellistici per interpretare i loro dati spettro-fotometrici consistevano semplicemente nel sommare linearmente i contributi di tutte le stelle presenti nella galassia, indipendentemente dalla geometria della distribuzione stellare o opacita della polvere. Ora i nuovi dati a disposizione sulla ri-emissione della polvere forniti da osservatori infrarossi hanno dimostrato che, in particolare durante i principali episodi di formazione stellare, grandi o addirittura dominanti frazioni della emissione da stelle giovani vengono assorbite dalla polvere e riemesse come grandi picchi nel lontano infrarosso. Vi è quindi una chiara associazione tra la formazione stellare e l'estinzione della polvere, in quanto elevati tassi di formazione stellare richiedono mezzi altamente opachi anche il gas possa collassare e formare stelle. In una galassia `star forming' reale stelle, gas e polvere sono mescolati in modo molto complesso, e l'estinzione della polvere e una forte funzione dell' eta della popolazione stellare nella galassia. I nuovi dati a disposizione ci obbligano a considerare una nuova generazione di modelli di spettri sintetici di galassie, con radicali complicazioni rispetto al precedente modelling classico, che si vericano a due livelli: da un lato l'eetto di estinzione della non puo essere trascurato e deve essere considerato in funzione dell'eta della popolazione stellare nella galassia. Inoltre, l'eetto di estinzione dierenziale fa s che gli eetti geometrici nella distribuzione di stelle e polvere svolgano un ruolo fondamentale e debbano essere accuratamente modellati. Vari tentativi sono stati fatti nel corso degli ultimi dieci anni per fornire approcci semplicati ai problemi di cui sopra. Il piu semplice ampiamente utilizzato consiste nel calcolare lo spettro totale sommando i contributi di tutte le stelle estinguendolo poi con una legge di estinzione media possibilmente rappresentativa. Praticamente tutti i risultati delle analisi siche di popolazioni di galassie nell'universo si basano su codici numerici semplicati basati su questo approccio. Questo puo fornire risultati ragionevoli per popolazioni di galassie caratterizzate da quantita trascurabili di gas interstellare e moderata estinzione, come nelle galassie del primo tipo nell'universo locale o nelle spirali `moderately star forming'. Ma sappiamo gia che si tratta di un approccio non corretto quando si considerano galassie caratterizzate da una piu attiva formazione stellare o oggetti osservati durante le prime fasi di collasso del gas e quindi in presenza di un mezzo interstellare molto ricco. Queste corrispondono alle fasi piu importanti ed interessanti della formazione ed evoluzione delle galassie. Lo scopo del presente lavoro e quello di contribuire a superare le dicolta e le limitazioni appena descritte, attraverso l'implementazione di una nuova modellistica e analisi sica delle popolazioni di galassie sia nell' universo locale che ad alto redshift. Piu nello specico il fulcro di questo progetto di ricerca di dottorato e quello di studiare la natura sica e la storia di assemblaggio in massa di galassie oscurate dalla polvere, ad alto redshift (0.8< z <2.5) e con attivita recente di formazione stellare, osservate con Herschel, attraverso una modellizzazione auto-consistente delle loro principali proprieta siche tra cui massa stellare, del gas e della polvere, SFR e SFH e l'oscuramento da polvere. Con tipiche luminosita infrarosse negli intervalli 1011 1012 L e 1012 L, le galassie luminose e ultraluminose nell'IR [(U)LIRGs nel testo], rispettivamente, sono tra gli oggetti extra-galattici piu luminosi e complessi che si possano concepire, comprendenti un'ampia varieta di popolazioni stellare giovani e vecchie, assorbimento da polvere, scattering, riemissione termica da parte dei grani e emissione AGN (Lonsdale et al., 2006). Sebbene questi oggetti siano abbastanza rari nell'Universo locale, essi dominano la rate di formazione stellare cosmica e il FIR background a z > 1. Percio' essi rappresentano i laboratori piu adatti ove studiare i principali processi sici che regolano la formazione ed evoluzione delle galassie. La caratterizzazione sica della fenomenologia delle (U)LIRGs richiede un approccio multi lunghezza d'onda e una trattazione dettagliata degli eetti della polvere. La tecnica di sintesi evolutiva delle galassie costituisce un potente strumento per interpretare gli spettri delle galassie. La distribuzione spettrale d'energia di una galassia (SED) contiene preziose informazioni sulle sue proprieta siche, che includono il contenuto in gas e in stelle, la distribuzione di eta e di abbondanze della popolazione stellare che viene dalla storia di formazione stellare e la loro interazione con il mezzo interstellare. Lo studio della SED quindi costituisce il metodo piu diretto per investigare la formazione ed evoluzione delle galassie, sia attraverso osservazioni dirette che attraverso corrispondenti modelli teorici. Per modellare l'emissione da stelle e polvere in maniera consistente e ottenere stime af-dabili dei principali parametri sici della galassia e necessario risolvere le equazioni del trasferimento radiativo per distribuzioni geometriche idealizzate ma realistiche di stelle e polvere, cos come trarre vantaggio da una copertura in lunghezza d'onda completa dal far-UV al radio. La forza del nostro approccio sta nella combinazione di una completa copertura multilunghezza d'onda per la nostra selezione di galassie, che include il FIR 70-500 micron da Herschel PACS e SPIRE e gli spettri IRS di Spitzer, ove disponibili, con un codice di sintesi spettrale auto-consistente, GRASIL citep Silva1998, utilizzato per interpretare le SED delle galassie. Questo codice calcola le SED delle galassie, dal lontano UV al radio, includendo un trattamento dettagliato degli eetti di estinzione e ri-emissione della polvere basato sulla risoluzione delle equazioni del trasferimento radiativo. Le caratteristiche di questo modello, prima tra tutte l'attenuazione da polvere dipendente dall'eta delle popolazioni stellari, diverse geometrie per la distribuzione di stelle e polvere nella galassia, calcolo della distribuzione delle temperature della polvere in funzione della tipologia dei grani insieme con la completa copertura spettrale, consentono un' approfondita analisi sica delle SED osservate. La nostra analisi dimostra quindi che un trattamento corretto e auto-consistente dell' estinzione e riemissione da polvere assieme ad una copertura Multiwavelength completa (dal lontano-UV al radio), e essenziale per ottenere stime adabili dei principali parametri sici come la massa stellare, l'estinzione da polvere e la SFR. Mostriamo che un approccio sico di questo tipo puo avere un forte impatto sulla rivendicata relazione tra SFR e la massa stellare. Cio e dovuto alle incertezze legate all'interpretazione dell' emissione dall' ottico al lontano IR in funzione dell'eta delle popolazioni stellari responsabili del riscaldamento della polvere e ri-emissione. Osserviamo che l'aggiunta dell' emissione radio al t spettrale multibanda fornisce uno stretto vincolo alla SFR recente, dato che soltanto le stelle piu giovani di circa 10 milioni di anni producono i raggi cosmici galattici responsabili dell'emissione radio non termica. Inoltre, poiche l'emissione radio sonda la SFH su tempi scala diversi rispetto all'emissione IR, la nostra analisi ci permette anche di capire meglio e vincolare SFH della sorgente, in particolare il numero di stelle massicce che contribuiscono alla componente non termica di emissione radio attraverso le `core-collapse SNe'.

We present new optical and radio spectroscopic observations of the remarkable galaxy Malin 1. This galaxy has unique features that include an extremely low surface brightness disk with an enormous mass of neutral hydrogen, and a low luminosity Seyfert nucleus. Malin 1 is exceptional in its values of MHO, LB, and MHI /Ln, and modest in its surface mass density of gas and stars. Spirals with large Min /LB tend to have low mean column densities of HI, and are close to the threshold for star formation due to instabilities in a rotating gas disk. In these terms, Malin 1 has a disk with extremely inefficient star formation. The bulge spectrum is dominated by the absorption features of an old, metal rich stellar population, although there is some evidence for hot (young) stars. The emission line excitations and widths in the nucleus are typical of a Seyfert galaxy; but Malin 1 is in the lowest 5% of the luminosity function of Seyferts, despite a copious fuel supply. Malin 1 is in a low density region of the universe. We propose it as an unevolving disk galaxy, where the surface mass density is so low that the chemical composition and mass fraction in gas change very slowly over a Hubble time. Its properties are similar to those of the damped Lyman -a absorption systems seen in the spectra of high redshift quasars. We emphasize that there are strong observational selection effects against finding gas -rich galaxies that are both massive and diffuse. Finally, we suggest that large and massive HI disks may have formed as early as z - 2, and remained quiescent to the present day. Subject headings : individual (Malin 1) - galaxies : photometry - galaxies : Seyfert - galaxies : stellar content - radio sources : 21 cm radiation - stars : formation

This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2019
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 239-254).
In this thesis, I present the development of various models for dust physics suited for galaxy formation simulations. I begin by introducing a model to evolve the spatial distribution of dust in galaxies, with dust treated as a passive scalar advected according to hydrodynamic flow. This model accounts for processes that affect the interstellar dust budget, like stellar dust production, accretion of gas-phase metals, and supernova-driven destruction. Using the moving-mesh hydrodynamics code arepo, I perform cosmological zoom-in simulations of Milky Way-sized galaxies to study the evolution of interstellar dust. Predictions from this model compare favorably to a number of observed low-redshift dust scaling relations and suggest that galactic dust-to-gas ratios can strongly increase with cosmic time. I also present simulations of uniformly sampled cosmological volumes to analyze the behavior of dust statistics on large scales.
While these simulations predict a reasonable present-day cosmic dust density, they are unable to produce the abundance of dust-rich galaxies observed at high redshift. Next, I develop a model to more realistically track the dynamics and sizes of interstellar grains. This novel framework handles dust using live simulation particles, each representing a population of dust grains of different sizes and subject to dynamical forces like aerodynamic drag. I implement and validate a second-order semi-implicit integrator for the drag coupling between dust and gas, and I outline how the local size distribution of interstellar grains can be evolved using a second-order piecewise linear discretization. Using simulations of idealized galaxies, I illustrate how different physical processes affecting dust grain sizes would impact galactic extinction curves. Finally, I describe an extension of these methods to couple dust physics and radiation hydrodynamics in arepo-rt.
This enables simulations to directly model radiation pressure on, photon absorption by, and thermal emission from dust grains. The framework introduced in this thesis can be used in the future to model other physics relevant for interstellar dust.
by Ryan Michael McKinnon.
Ph. D.
Ph.D. Massachusetts Institute of Technology, Department of Physics

The formation and evolution of spiral galaxies is a research topic central to modern Astronomy. In this context, the Milky Way offers a unique opportunity for astronomers to study a spiral galaxy in detail and thus informs many aspects of galaxy formation theory. The observational signatures of Galactic stellar components provide clues to its assembly history. This thesis is focused on two main components of the Galaxy: the stellar disk and bulge. In particular, we examine the chemical properties of these components and their implications for Galactic evolution. The data in this thesis were obtained with HERMES, a new high-resolution optical spectrograph on the Anglo Australian Telescope. The disk sample consists of over 3000 giant-branch stars, extending up to 4 kpc in height from the Galactic plane. The thin disk (low-α population) exhibits a steep negative vertical metallicity gradient, a signature observed in galaxy evolution models where radial migration plays an important role. The thick disk (high-α population) has a weaker vertical metallicity gradient, which could have arisen from a settling phase of the primordial disk. The [α/Fe] ratios of the thin and thick disk populations are distinct and nearly constant with height. This indicates the two populations were formed in very different conditions, and although the high-α population likely experienced a settling phase, its formation timescale was fast still, in the order of a few Gyrs. To investigate the chemistry of the Galactic bulge and its connection to the disk, we obtained abundance ratios of 18 elements for more than 800 red giants. The [α/Fe] abundance ratios show vertical variations that are consistent with the distribution of bulge metallicity components: at high latitudes [α/Fe] is enhanced as the metal-poor component dominates; closer to the plane, the metal-rich components contribute lower [α/Fe]. However, at fixed metallicity, all elements show uniform abundance ratios with latitude. We observe normal [Na/Fe] ratios that do not vary as a function of latitude at fixed metallicity, indicating that the bulge does not contain strongly helium-enhanced populations as observed in globular clusters. By comparing our results with that of the GALAH survey, we conclude that there are similarities between the bulge and disk in terms of their chemistry. However, the more metal-poor bulge population ([Fe/H] ≲ -0.8) shows enhanced abundance ratios compared to the disk for some light, alpha, and iron-peak elements that are associated with core-collapse supernovae (SNeII). This population may have experienced a different evolution to bulge stars of disk origin. Moreover, the [La/Eu] abundance ratios suggest higher r-process contribution in the bulge, which indicates that overall the bulge experienced a higher star formation rate than the disk. Keywords: Galaxy, stellar populations, stellar abundances, disk, bulge, galaxy formation, 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.

We introduce a statistical exploration of the parameter space of the Munich semi-analytic model built upon the Millennium dark matter simulation. This is achieved by applying a Monte Carlo Markov Chain (MCMC) method to constrain the 6 free parameters that define the stellar mass function at redshift zero. The model is tested against three different observational data sets, including the galaxy K-band luminosity function, B −V colours, and the black hole-bulge mass relation, to obtain mean values, confidence limits and likelihood contours for the best fit model. We discuss how the model parameters affect each galaxy property and find that there are strong correlations between them. We analyze to what extent these are simply reflections of the observational constraints, or whether they can lead to improved understanding of the physics of galaxy formation. When all the observations are combined, the need to suppress dwarf galaxies requires the strength of the supernova feedback to be significantly higher in our best-fit solution than in previous work. We interpret this fact as an indication of the need to improve the treatment of low mass objects. As a possible solution, we introduce the process of satellite disruption, caused by tidal forces exerted by central galaxies on their merging companions. We apply similar MCMC sampling techniques to the new model, which allows us to discuss the impact of disruption on the basic physics of the model. The new best fit model has a likelihood four times better than before, reproducing reasonably all the observational constraints, as well as the metallicity of galaxies and predicting intra-cluster light. We interpret this as an indication of the need to include the new recipe. We point out the remaining limitations of the semi-analytic model and discuss possible improvements that might increase its predictive power in the future.

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

We present a study of the photometric and spectroscopic properties of galaxies in a sample of six nearby, rich galaxy clusters. We examine the variations of fundamental galaxy properties, such as luminosity, morphology, and star formation rates with environment, providing new constraints on the mechanisms that drive the evolution of galaxies. This study also introduces a new maximum likelihood algorithm to recover the true distribution function of galaxies from an incomplete sample. This algorithm is ideally suited for modern-day surveys that gather a large amount of information about each object. The R-band luminosity function (GLF) shows no variation among clusters or between the field and clusters, with the exception of an enhancement of the luminous tip of the GLF in clusters. However, the GLF of quiescent galaxies steepens significantly between the field and clusters and is not universal in clusters either, suggesting that star formation properties may be more strongly correlated than the luminosity function with environment. The U-band GLF in clusters is slightly steeper than the R-band GLF, indicating that cluster galaxies are bluer at fainter magnitudes and that the GLF is thus weakly sensitive to star formation, dust, or metallicity effects. To constrain the mechanisms that shape the morphologies of cluster galaxies, we have calculated separate R-band luminosity functions for galaxy bulges and disks. Their distribution as a function of morphology and environment indicates that intermediate- and early-type galaxies can be generated from late-type galaxies by increasing the luminosity of the bulge, but not by fading the disks alone, favoring galaxy-galaxy interactions or mergers as the primary morphological transformation mechanism. Finally, we find a residual correlation of star formation with environment even after accounting for environmental variations of morphology, stellar mass, and stellar age. Thus, the star formation gradient in clusters is not just another aspect of the morphology-density relation, and cannot be solely the result of initial conditions, but must partly be due to subsequent evolution through a mechanism (or mechanisms) sensitive to environment. These results thus constitute a true "smoking gun" pointing to the effect of environment on the later evolution of galaxies.

Despite the progress made towards a more comprehensive knowledge of galaxy evolution, a global picture of the mechanisms regulating the formation of stars in galaxies, of how galaxy evolutionary properties correlate with stellar masses and star formation rates (SFRs) and of the processes suppressing the star formation in galaxies and their timescales is still lacking. In this thesis work, we attempt to address some of these open questions, inspecting galaxy evolution back in cosmic time. In particular, we start from the archaeological analysis of passive local galaxies (1), reconstructing their past star formation histories. Then we take a step back towards the phase in which galaxies quench their star formation (2), defining a new methodology able to identify the quenching progenitors of passive galaxies. Finally, we move back to the star-forming phase (3), investigating the properties of high-redshift galaxies which could be the star-forming progenitors of the passive local ones. Our investigations mainly rely on the spectral analysis of galaxies. In particular, we study both the passive and star-forming phase by exploiting the information contained in the galaxy full-spectrum, whose shape depends on the properties of the underlying stellar populations. The quenching phase is instead investigated by means of emission line ratios, which are associated to the Interstellar Medium (ISM) and its ionization state during or just after the star formation has stopped.
Nonostante i progressi fatti verso una più profonda comprensione dell’evoluzione delle galassie, manca ancora una visione completa di quali siano i meccanismi che regolano la formazione sellare nelle galassie, di come le proprietaà evolutive delle galassie correlino con le loro masse e tassi di formazione stellare e quali processi siano respons- abili dello spegnimento della formazione stellare e i loro tempi-scala. In questo lavoro di tesi, si è cercato di rispondere ad alcune di queste domande aperte, studiando l’evoluzione delle galassie a ritroso nel tempo. In particolare, siamo partiti dallo studio archeologico di galassie passive locali (1), ricostruendo le loro storie di formazione stellare. Abbiamo poi fatto un passo indietro verso la fase in cui le galassie spengono la loro formazione stellare (2), definendo una nuova metodologia che ci permetta di identificare i progenitori delle galassie passive nella fase immediatamente successiva all’interruzione della formazione stellare. Infine, siamo andati ancora più indietro nel tempo, studiando la fase in cui le galassie formano stelle (3), analizzando le proprietà di galassie ad alto redshift che potrebbero essere i progenitori delle galassie passive locali. I nostri studi si sono basati sull’analisi spettrale delle galassie. In particolare, abbiamo studiato la fase passiva e quella star-forming sfruttando l’informazione contenuta nella totalità degli spettri delle galassie analizzate, la cui forma dipende dalle caratteristiche delle popolazioni stellari. La fase di spegnimento della formazione stellare è stata in- vece analizzata usando rapporti tra righe di emissione, che sono collegate al mezzo interstellare e al suo stato di ionizzazione durante o subito dopo lo spegnimento della formazione stellare.

We find two chemically distinct populations separated relatively cleanly in the [Fe/H]-[Mg/Fe] plane, but also distinguished in other chemical planes, among metal-poor stars (primarily with metallicities [Fe/H] < -0.9) observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and analyzed for Data Release 13 (DR13) of the Sloan Digital Sky Survey. These two stellar populations show the most significant differences in their [X/Fe] ratios for the alpha-elements, C+N, Al, and Ni. In addition to these populations having differing chemistry, the low metallicity high-Mg population (which we denote "the HMg population") exhibits a significant net Galactic rotation, whereas the low-Mg population (or "the LMg population") has halo-like kinematics with little to no net rotation. Based on its properties, the origin of the LMg population is likely an accreted population of stars. The HMg population shows chemistry (and to an extent kinematics) similar to the thick disk, and is likely associated with in situ formation. The distinction between the LMg and HMg populations mimics the differences between the populations of low-and high-a halo stars found in previous studies, suggesting that these are samples of the same two populations.

Les galaxies passives présentent des morphologies et propriétés structurelles différentes des galaxies de masse similaire formant des étoiles. La preuve d'une distribution bimodale dans propriétés des galaxies suggère un lien entre les processus de quenching et les structures des galaxies. Contraindre les mécanismes et la chronologie de la formation du bulbe s'avère fondamental pour comprendre l'origine de cette corrélation. Les bulbes grossissent-ils au cours de la séquence principale? Les galaxies ré-accrètent-elles un disque formant des étoiles? Les galaxies stoppent-elles leur formation d'étoile à partir des régions internes? etc. Répondre de manière pertinente à ces questions nécessite de résoudre les parties internes des galaxies à différentes époques. Grâce aux données de haute résolution en multi-longueur d'onde fournies par CANDELS, j'ai réalisé une décomposition séparant le bulbe du disque à partir des courbes de brillance de surface de 17'300 galaxies (F160W<23,00. J'ai utilisé le catalogue ainsi obtenu pour comprendre comment les galaxies stoppent leur formation d'étoile et déterminer l'impact que le quenching peut avoir sur les composantes internes. Les propriétés structurelles des bulbes et des disques, bien que différentes, dépendent peu de la morphologie globale de la galaxie hôte et de son activité de formation d'étoile. Si il existe un seul mécanisme de formation pour tous les types de galaxie ou plusieurs mécanismes contribuant à l'augmentation de la densité centrale, aucune trace dans la structure de la composante interne n'est gardée. De plus, les bulbes et les disques évoluant dans des galaxies soit éteintes, soit formant des étoiles (SF), bien qu'ils présentent des propriétés structurelles similaires, possèdent des distributions de couleurs différentes. Le processus de quenching ne semble pas avoir un impact significatif sur les propriétés des composantes internes.La seconde question clé est de savoir à quel moment les bulbes se forment. La distribution en morphologie le long du graphe SFR-masse montre un manque de galaxie calme (quiescent) avec B/T<0.3 alors que les galaxies avec B/T>0.3 sont présentes tout au long de la séquence principale. Cela suggère que la formation du bulbe doit commencer au cours de la séquence principale. De plus, nous n'avons aucune preuve d'un quelconque processus quenching sans qu'il y ait grossissement du bulbe. Nous n'excluons cependant pas la possibilité que les bulbes de la séquence principale correspondent à des galaxies ayant ré-accrété un disque formant des étoiles. La connaissance des âges est à ce niveau nécessaire pour réellement contraindre ce scénario. Une analyse élargie qui inclurait de l'imagerie à bande étroite (SHARDS) permettrait d'explorer les âges typiques des bulbes et des disques afin de placer des contraintes sur leur temps de formation
Passive galaxies have different morphologies and structural properties than star-forming galaxies of similar mass. The evidence of a bimodal distribution of galaxy properties suggests a link between the quenching process and and galaxy structure. Understanding the origin of this correlation requires establishing constraints on the mechanisms as well as on the timing of bulge formation. How are bulges formed?Do bulges grow in the main sequence? Are galaxies re-accreting a star forming disk? Do galaxies start to quench from the inside? etc.Proper answers to these questions require resolving the internal components of galaxies at different epochs.Thanks to the CANDELS high-resolution multi-wavelength data, I performed 2-D bulge-disk decompositions of the surface brightness profile of $simeq 17'300$ galaxies (F160W < 23, 0 < z < 2) in 4-7 filters, covering a spectral distribution of 430-1600 nm. A novel approach, based on deep-learning, allowed us to make an a-priori selection of the best profile. Stellar parameters are computed trough the SED fitting. The final catalog contains structural/morphological informations together with the stellar population properties for a large sample of bulges and disks within galaxies. This is the largest and more complete catalog of bulge-disc decompositions at $z>0$.The catalog is then used to investigate how galaxies quench and transform their morphologies.The size of disks and massive bulge is independent of the bulge-to-total ratio ($M_{*}>10^{10} M_{odot}$). It suggests a unique formation process for massive bulges and also that disk survival/regrowth is a common phenomenon after bulge formation. However pure bulges (B/T>0.8), are ~30% larger than bulges embedded in disks at fixed stellar mass and have larger Sersic indices. This is compatible with a later growth of these systems through minor mergers.Bulges in star-forming galaxies are found to be 30% larger than bulges in quenched systems, at fixed stellar mass. Regarding the disks the systematic difference is only a factor of $sim 0.1$. This can be interpreted as a signature that galaxies experience an additional morphological transformation during or after quenching. However, this result is not free of progenitor bias.Moreover, the vast majority (if not all) of pure disks (B/T<0.2) in our sample lie in the main-sequence. It suggests that quenching without any bulge growth is not a common channel at least in the general field environment probed by our data. Pure "blue" bulges (B/T>0.8) do exist however, suggesting that the formation of bulges happens while galaxies are still star forming.Finally, in order to put constraints on the formation times of bulges and disks I analyzed the UVJ colors rest frame. Almost all galaxies in our sample present negative color gradients. Bulges are always redder than the disks at all redshifts. This is compatible with a scenario of inside-out quenching put forward by previous works. However rejuvenation through disk accretion could lead to similar signatures

La formation et l'évolution du disque épais de la Voie Lactée restent controversées. Nous avons utilisé un modèle de synthèse de la population de la Galaxie, le Modèle de la Galaxie de Besançon (Robin et al., 2003), qui peut être utilisé pour l'interprétation des données, étudier la structure galactique et tester différents scénarios de formation et évolution Galactique. Nous avons examiné ces questions en étudiant la forme et la distribution de métallicité du disque mince et du disque épais en utilisant l'approche de synthèse de la population. Nous avons imposé sur des simulations les erreurs d'observation et les biais afin de les rendre directement comparables aux observations. Nous avons corrigé les magnitudes et les couleurs des étoiles de la simulation, en utilisant un modèle d'extinction. Les modèles d'extinction disponibles ne reproduisent pas toujours la quantité exacte d'extinction le long de la ligne de visée. Un programme a été développé pour corriger la distribution de l'extinction en fonction de la distance le long de ces lignes. Les extinctions correctes ont ensuite été appliquées sur les simulations du modèle. Nous avons étudié la forme du disque mince en utilisant des données photométriques aux basses latitudes du sondage SDSS-SEGUE. Nous avons comparé qualitativement et quantitativement les observations et les simulations et nous avons essayé de contraindre la fonction de masse initiale. En utilisant la spectroscopie du relevé SEGUE, nous avons sélectionné les étoiles du turn-off de la séquence principale (MSTO) (Cheng et al 2012) et des géantes K pour étudier la distribution de métallicité du disque mince et du disque épais. Nous avons calculé une estimation de distance pour chaque étoile à partir de la relation entre les températures effectives et magnitudes absolues pour les catalogues observés et simulés. Ces deux catalogues ont les mêmes biais sur les distances, elles sont donc comparables. Nous avons développé un outil basé sur une méthode MCMC-ABC pour déterminer la distribution de la métallicité et étudier les corrélations entre les paramètres ajustés. Nous avons confirmé la présence d'un gradient de métallicité radiale de -0.079 ± 0.015 dex kpc−1 pour le disque mince. Nous avons obtenu une métallicité du disque épais au voisinage solaire de -0.47 ± 0.03 dex, compatible avec les résultats obtenus par les études précédentes. De plus, le disque épais ne montre pas de gradient, mais les données sont compatibles avec un gradient positif intérieur suivi d'un négatif extérieur. Nous avons ensuite appliqué les outils développés au relevé spectroscopique Gaia-ESO et calculé la distribution de métallicité des étoiles F/G/K dans le disque mince et épais en supposant une formation en deux époques du disque épais de la Voie Lactée. Nous avons obtenu une métallicité locale dans le disque épais de -0.23 ± 0.04 dex légèrement plus élevée que celle obtenue avec SEGUE mais en accord avec Adibekyan et al. (2013) et un gradient de métallicité radiale du disque épais en accord avec notre analyse précédente des données de SEGUE et la littérature. La métallicité locale est en accord avec la littérature au niveau de 3σ mais parce que les données GES sont préliminaires, une analyse plus approfondie avec plus de données et de meilleurs calibrations doit être faite. L'existence d'un gradient plat dans le disque épais peut être une conséquence d'une formation à partir d’un gaz turbulent et bien homogène, ou bien un fort mélange radial a brassé après coup les étoiles
The formation and evolution of the thick disc of the Milky Way remain controversial. We made use of a population synthesis model of the Galaxy, the Besançon Galaxy Model (Robin et al. 2003), which can be used for data interpretation, study the Galactic structure and test different scenarios of Galaxy formation and evolution. We examined these questions by studying the shape and the metallicity distribution of the thin and thick disc using the population synthesis approach. We imposed on simulations observational errors and biases to make them directly comparable to observations. We corrected magnitudes and colors of stars, from the simulation, using an extinction model. The available extinction models do not always reproduce the exact quantity of extinction along the line of sight. A code to correct the distribution of extinction in distance along these lines have been developed and the corrected extinctions have been applied on model simulations. We studied the shape of the thin disc using photometric data at low latitudes from the SDSS-SEGUE survey. We compared qualitatively and quantitatively observations and simulations and try to constrain the Initial Mass Function. Using the spectroscopic survey SEGUE we selected Main Sequence Turnoff (MSTO) stars (Cheng et al 2012) and K giants to study the metallicity distribution of the thin and thick discs. We computed a distance for each star from the relation between effective temperatures and absolute magnitudes for the observed and simulated catalogs. These two catalogues have the same biases in distances, therefore are comparable. We developed a tool based on a MCMC-ABC method to determine the metallicity distribution and study the correlations between the fitted parameters. We confirmed a radial metallicity gradient of -0.079 ± 0.015 dex kpc−1 for the thin disc. We obtained a solar neighborhood metallicity of the thick disc of -0.47 ± 0.03 dex similar to previous studies and the thick disc shows no gradient but the data are compatible with an inner positive gradient followed by a outer negative one. Furthermore, we have applied the developed tools to the Gaia-ESO spectroscopic survey and computed the metallicity distribution of F/G/K stars in the thin and thick disc assuming a two epoch formation for the thick disc of the Milky Way. We obtained a local metallicity in the thick disc of -0.23 ± 0.04 dex slightly higher than the one obtained with SEGUE but in agreement with Adibekyan et al. (2013) and a radial metallicity gradient for the thick disc in agreement with our previous analysis of SEGUE data and the literature. The local metallicity is in fair agreement with literature at the 3σ level but because the GES data is an internal release under testing further analysis with more data and better calibrations have to be done. The existence of a flat gradient in the thick disc can be a consequence of an early formation from a highly turbulent homogeneous well mixed gas, unless it has suffered heavy radial mixing later on

We present a series of numerical galaxy formation studies which apply new numerical methods to produce increasingly realistic galaxy formation models. We first investigate the metallicity evolution of a large set of idealized hydrodynamical galaxy merger simulations of colliding galaxies. We find that inflows of metal--poor interstellar gas triggered by galaxy tidal interactions can account for the systematically lower central oxygen abundances observed in local interacting galaxies. We show the central metallicity evolution during merger events is determined by a competition between the inflow of low--metallicity gas and enrichment from star formation. We find a time-averaged depression in the galactic nuclear metallicity of ~0.07 dex for gas--poor disk--disk interactions, which explains the observed close pair mass-metallicity and separation-metallicity relationships.
Astronomy

>Magister Scientiae - MSc
We quantitatively examine the effects of merger and environment within a cosmological hydrodynamic simulation. We show that our simulation model broadly reproduces the observed scatter in H I at a given stellar mass as quantified by the HI mass function in bins of stellar mass, as well as the H I richness versus local galaxy density. The predicted H I fluctuations and environmental effects are roughly consistent with data, though some discrepancies are present at group scales. For satellite galaxies in & 1012Mhalos, the H I richness distribution is bimodal and drops towards the largest halo masses. The depletion rate of H I once a galaxy enters a more massive halo is more rapid at higher halo mass, in contrast to the specific star formation rate which shows much less variation in the attenuation rate versus halo mass. This suggests that, up to halo mass scales probed here (. 1014M), star formation is mainly attenuated by starvation, but H I is additionally removed by stripping once a hot gaseous halo is present. In low mass halos, the H I richness of satellites is independent of radius, while in high mass halos they become gas-poor towards the center, confirming the increasing strength of the stripping with halo mass. By tracking the progenitors of galaxies, we show that the gas fraction of satellite and central galaxiesdecreases from z =5 ! 0, tracking each other until z⇠1 after which the satellites’ H I content drops much more quickly, particularly for the highest halo masses. Mergers somewhat increase the H I richness and its scatter about the mean relation, but these variations are consistent with arising form inflow fluctuations, unlike in the case of star formation where mergers boost it above that expected from inflow fluctuations. In short, our simulations suggest that the H I content in galaxies is determined by their ability to accrete gas from their surroundings, with stripping effects playing a driving role once a hot gaseous halo is present.

In extragalactic astronomy, a central challenge is that we cannot directly watch what happens to galaxies before and after they are observed. This dissertation focuses on linking predictions of galaxy time-evolution directly with observations, evaluating how interactions, mergers, and other processes affect the appearance of elliptical galaxies. The primary approach is to combine hydrodynamical simulations of galaxy formation, including all major components, with dust radiative transfer to predict their observational signatures. The current paradigm implies that a quiescent elliptical emerges following a formative starburst event. These trigger accretion onto the central supermassive black hole (SMBH), which then radiates as an active galactic nucleus (AGN). However, it is not clear the extent to which SMBH growth is fueled by these events nor how important is their energy input at setting the appearance of the remnant. This thesis presents results drawing from three phases in the formation of a typical elliptical: 1) I evaluate how to disentangle AGN from star formation signatures in mid-infrared spectra during a dust-enshrouded starburst, making testable predictions for robustly tracing SMBH growth with the James Webb Space Telescope ; 2) I develop a model for the rate of merger-induced post-starburst galaxies selected from optical spectra, resolving tension between their observed rarity and merger rates from other estimates; and 3) I present results from Hubble Space Telescope imaging of elliptical galaxies in galaxy clusters at 1 < z < 2, the precursors of present-day massive clusters with \(M \sim10^{15}M_{\odot}\), demonstrating that their stars formed over an extended period and ruling out the simplest model for their formation history. These results lend support to a stochastic formation history for ellipticals driven by mergers or interactions. However, significant uncertainties remain in how to evaluate the implications of galaxy appearance, in particular their morphologies across cosmic time. In the final chapter, I outline an approach to build a "mock observatory" from cosmological hydrodynamical simulations, with which observations of all types, including at high spatial and spectral resolutions, can be brought to bear in directly constraining the physics of galaxy formation and evolution.
Astronomy

In this Thesis we attempted to answer to some of the fundamental questions concerning galaxy evolution. In particular when and how galaxies got their present-day stellar content and how this process depends on their mass. In order to address this key issues, we developed a new semi-analytic model of galaxy formation (GECO, Galaxy Evolution COde), that couples a Monte Carlo representation of the hierarchical clustering of dark matter haloes with analytic recipes for the baryonic physics, such as the cooling of the gas, the star formation, the feedback from SN and AGN. We set the model on observations in the local universe and then we predict and compare results for the high redshift galaxies.

Doctor en Ciencias, Mención Astronomía
En el contexto del modelo de formacion jerárquico, las galaxias son esculpidas por una secuencia de colisiones y eventos de acreción. En algunas de estas colisiones los núcleos de cada galaxia migran a la región central del nuevo sistema y se fusionan, forman- do un nuevo núcleo virializado. Dentro de este nuevo núcleo los agujeros negros super masivos (SMBHs) de cada galaxia migran hacia el centro debido a la fricción dinámi- ca, formando un sistema binario de SMBHs. Entender la evolución de estas binarias es crucial ya que si la separacion de los SMBHs se reduce a un tamaño comparable con aGW ∼ 10−3 (MMBHs /10^6 M ) pc, entonces la binaria se convierte en una fuente intensa de ondas gravitacionales (GW) lo cual permite la coalescencia de los SMBHs en 10^10 años. Por lo tanto, si somos capaces de determinar que le ocurrirá a las binarias de SMBH después de una colisión de galaxias, seremos capaces de determinar la cantidad de fuentes intensas de GW en el Universo y comprenderemos mejor la evolución cósmica de la población de SMBHs. Si las galaxias involucradas en una colisión tienen una fracción de gas de al menos 1 %, esperamos que se forme un disco de gas masivo en el kiloparsec central del remanente de la colisón, con una masa ∼ 1 − 10 veces la masa de los SMBHs. Este gas puede extraer eficientemente el momento angular de la binaria, haciendo que su separación disminuya hasta un valor comparable con aGW , en una escala de tiempo del orden de 10^7 años. Sin embargo, si el gas no es capaz de redistribuir de manera eficiente el momento angular extraído de la binaria entonces se alejara de esta, generando un vacío de baja densidad (gap) alrededor de la binaria. En este caso la binaria entrara en un ré- gimen de contracción lenta cuya escala de tiempo es comparable con la edad del Universo. Motivado por este escenario, en esta tesis derivo un criterio analítico para determinar la formación de gap en estos sistemas, es decir, bajo que condiciones una binaria expe- rimentará una contracción rápida o una lenta. Las estimaciones derivadas de mi criterio son concordantes con los resultados de simulaciones numéricas de sistemas binaria/disco. Realice simulaciones numéricas de colisiones de galaxias para determinar la probabi- lidad de que se cumplan las condiciones para una contracción rapida de la binaria, en sistemas astrofísicos reales. En todas las simulaciones observe que la formación de un gap es poco probable. Estime que la formación de gap sería posible sólo si el gas tiene una velocidad turbulenta igual o menor a la del centro de galaxias espirales locales (10 km s^−1 ). Otra posibilidad sería que los SMBHs acreten una masa mayor al 2 % de la masa del núcleo de la galaxia remanente, lo que implica que los SMBHs deberían acretar a un ritmo mucho mayor que el derivado de observaciones. Además, use simulaciones numéricas para estudiar el efecto de la formación estrelar en la evolución dinámica de un par de SMBHs en la época pre-binaria y concluí que si la eficiencia de la formación estrelar cambia en un factor ∼ 20, entonces el tiempo de migración de los SMBHs cambia sólo en un factor 2. De mi resultados concluyo que es probable que las binarias de SMBHs experimenten una contracción rápida. Esto implica que el número de binarias de SMBHs en el Universo debiera ser muy bajo. Esta restricción es muy importante para la evolución de la población cósmica de SMBHs, el número esperado de binarias de SMBH en el Universo y la cantidad de fuentes de GW que esperamos observar con futuras misiones.

We use cosmological hydrodynamic simulations to study the impact of out-flows and radiative feedback on high-redshift galaxies. For outflows, we consider simulations that assume (i) no winds, (ii) a .constant-wind. model in which the mass-loading factor and outflow speed are constant, and (iii) "momentum driven" winds in which both parameters vary smoothly with mass. In order to treat radiative feedback, we develop a moment-based radiative transfer technique that operates in both post-processing and coupled radiative hydrodynamic modes. We first ask how outflows impact the broadband spectral energy distributions (SEDs) of six observed reionization-epoch galaxies. Simulations reproduce five regardless of the outflow prescription, while the sixth suggests an unusually bursty star formation history. We conclude that (i) simulations broadly account for available constraints on reionization-epoch galaxies, (ii) individual SEDs do not constrain outflows, and (iii) SED comparisons efficiently isolate objects that challenge simulations. We next study how outflows impact the galaxy mass metallicity relation (MZR). Momentum-driven outflows uniquely reproduce observations at z = 2. In this scenario, galaxies obey two equilibria: (i) The rate at which a galaxy processes gas into stars and outflows tracks its inflow rate; and (ii) The gas enrichment rate owing to star formation balances the dilution rate owing to inflows. Combining these conditions indicates that the MZR is dominated by the (instantaneous) variation of outflows with mass, with more-massive galaxies driving less gas into outflows per unit stellar mass formed. Turning to radiative feedback, we use post-processing simulations to study the topology of reionization. Reionization begins in overdensities and then .leaks. directly into voids, with filaments reionizing last owing to their high density and low emissivity. This result conflicts with previous findings that voids ionize last. We argue that it owes to the uniqely-biased emissivity field produced by our star formation prescriptions, which have previously been shown to reproduce numerous post-reionization constraints. Finally, preliminary results from coupled radiative hydrodynamic simulations indicate that reionization suppresses the star formation rate density by at most 10.20% by z = 5. This is much less than previous estimates, which we attribute to our unique reionization topology although confirmation will have to await more detailed modeling.

The stellar populations harboured by some of the Universe’s earliest galaxies are within observational reach. Determining the details of these stellar populations and their formation histories within the first billion years after the Big Bang is crucial for both understanding the earliest stages of galaxy evolution and for assessing the contribution of early star-forming galaxies to cosmic reionization. This thesis presents observational measurements of the rest-frame UV and optical colours of star-forming Lyman Break galaxies (LBGs) at redshifts 4 < z < 9, and their inferred stellar population parameters. By combining ground-based ~1 deg² surveys with deeper, narrower space-based deep-field surveys, we have constrained the rest-frame UV spectral slope of galaxies over a wide-range of cosmic time (4 < z < 9) and luminosity (−23 < MUV < −17) in a self-consistent way. To do so, we developed simulations to allow the inference of intrinsic colours from noisy, potentially biased observations. With these simulations, a robust UV colour measurement method was devised in preparation for the Hubble Ultra Deep Field 2012 (UDF12) survey. Then, after delivery of the UDF12 data, our technique and simulations were applied to yield the first bias-free measurements of the UV spectral slope of galaxies at z ≈ 7 and 8. We found no support for the previously claimed dominant sub-population of exotically blue, faint galaxies at z ≈ 7. In fact with careful consideration of their errors and selection biases, even the most extreme galaxies we observed can have their colours explained by stellar population synthesis models of unremarkable parameters. Expanding this study to brighter, rarer, galaxies required the inclusion of wide-area ground-based survey data, and consequently a more focused examination of galaxies at z ≈ 5. We selected high signal-to-noise galaxies from four fields, with absolute magnitudes spanning MUV = −22.5 to −17.5, and measured their rest-frame UV spectral slopes. Coupling these measurements with our simulated observations, we were able to determine the width of the intrinsic colour distribution of galaxies at z ≈ 5. We found that brighter galaxies are not only on average redder than their fainter counterparts, but they are also less self-similar in their colours. The redder average UV colours of brighter galaxies can be attributed to those galaxies being either older, or more dust reddened. By pairing these measurements, which are primarily a probe only of the presently forming portion of the stellar population, with those of LBG’s Balmer Breaks, which are more sensitive to bygone star formation, we were able to break this age–dust degeneracy and conclude that, at z ≈ 5, brighter galaxies are more heavily reddened than fainter galaxies even though their stars are no older.