Le but global de cette thèse est de caractériser les processus qui régulent la formation d'étoiles à grande échelle dans les galaxies. Durant les quinze dernières années, le développement de l'astronomie infrarouge, portée par les satellites ISO, IRAS, Spitzer et Herschel, a révolutionné notre conception de l'évolution des galaxies. En observant le rayonnement émis par la poussière interstellaire, ces observatoires permettent de détecter l'énergie et la matière qui reste désespérément invisible aux télescopes optiques, et ont découvert ainsi une part conséquente et pourtant insoupçonnée de l'activité des galaxies. Les travaux de ma thèse reposent donc en grande partie sur les données acquises par le satellite Herschel, qui permet pour la première fois de détecter l'émission infrarouge des galaxies "normales" à de grandes distances (z=2). En m'appuyant sur ces nouvelles données, j'étudie statistiquement plusieurs milliers de galaxies à différentes époques de l'Univers. En particulier, j'apporte les meilleures contraintes disponibles à ce jour sur les propriétés de la "Séquence Principale" des galaxies. L'existence de cette séquence (la corrélation entre la masse stellaire, M* et le taux de formation d'étoile, SFR) s'est trouvée être un outil formidable pour comprendre l'évolution des galaxies. La faible dispersion observée autour de cette séquence suggère que la majorité des galaxies grandi par des épisodes de formation d'étoile longs et réguliers, et non par des processus violents comme ceux qui résultent de la collision (ou fusion) de deux galaxies. En développant une nouvelle technique d'analyse d'image, je montre en particulier que plus des deux tiers de la masse des étoiles observée aujourd'hui dans l'Univers ont été formées au sein de la Séquence Principale, et qu'il s'agit donc du mode dominant de croissance des galaxies. Dans un deuxième temps, je m'intéresse à caractériser l'évolution de la forme de cette séquence avec le temps, c'est à dire principalement la pente de la corrélation SFR-M*. Conformément aux résultats d'autres études publiées indépendamment, je trouve que cette pente évolue et décroit avec le temps, de sorte que les galaxies les plus massives forment relativement moins d'étoiles aujourd'hui que dans le passé. J'étudie les origines possibles de cette évolution, en quantifiant par exemple l'évolution morphologique des galaxies et la croissance des bulbes, ainsi que l'évolution du contenu en gaz d'hydrogène, le carburant de la formation d'étoile. J'en déduis que le changement de pente de la Séquence Principale peut être principalement expliqué par une variation de l'efficacité de formation d'étoile, et non par un processus morphologique ou par un manque de gaz. Les différentes observations que j'ai effectuées dans les travaux sus-cités me permettent d'établir des relations simples pour simuler les propriétés observables des galaxies, en particulier leurs spectres. J'utilise ces recettes pour créer une simulation réaliste d'un champ profond qui me sert à tester mes méthodes d'analyse, et qui reproduit correctement le fond diffus infrarouge. Enfin, j'introduis des résultats préliminaires sur la formation d'étoile dans l'Univers jeune (z=4) obtenus grâce à de nouvelles données acquises par le télescope ALMA. Je décris en particulier les contraintes apportées sur la Séquence Principale à cette époque, et j'étudie plus en détail deux galaxies extrêmement distantes que j'ai découvert par chance dans ces données. Ces galaxies sont parmi les plus lointaines connues à ce jour, et sont probablement les plus massives et poussiéreuses jamais détectées dans un Univers âgé de moins d'un milliard d'années. / The main goal of this thesis is to characterize the processes that regulate large-scale star formation in galaxies. During the last fifteen years, the development of infrared astronomy through the satellites ISO, IRAS, Spitzer and Herschel has revolutionized our conception of galaxy evolution. By observing the light emitted by the interstellar dust, these observatories allow us to detect the energy and matter that remain elusive to the best optical telescopes, and have thereby discovered a substantial yet unexpected part of the star formation activity of galaxies. The work of my thesis hence rely heavily on the data acquired by the Herschel satellite, which allow for the first time the detection in the infrared of "normal" galaxies at great distances (z=2).Taking advantage of these new data, I perform a statistical study of several thousands of galaxies at different epochs of the Universe. In particular, I bring forward the best constraints available today on the properties of the "Main Sequence" of galaxies. The existence of this sequence (the correlation between the stellar mass, M*, and the star formation rate, SFR) turned out to be a incredibly useful tool to understand galaxy evolution. The small dispersion that is observed around this sequence suggests that the majority of galaxies are growing through long and steady episodes of star formation, rather than intense bursts like those triggered by the collision (of merger) of two galaxies. By developing a new image analysis technique, I show in particular that more than two thirds of the mass of stars present in the Universe today has been formed within Main Sequence galaxies, hence that this is the dominant mode of galaxy growth.Then I approach another aspect of the Main Sequence, that is the characterization of the evolution of its shape, i.e., the slope of the SFR-M* correlation. In agreement with other studies that were published independently, I find that this slope evolves and decreases with time, so that the most massive galaxies are forming relatively fewer stars per year today than they used to in the past. I study the various possible causes for this evolution, by quantifying for example the morphological evolution of these galaxies and the growth of bulges, as well as the evolution in their hydrogen gas content, which is the fuel for star formation. I deduce from these observations that the change of slope of the Main Sequence can be mainly attributed to a decrease of the star formation efficiency, rather than by a morphological process or a lack of gas.The various observations I have made throughout the work described above allow me to establish simple prescriptions to simulate the observable properties of galaxies, in particular their spectrum. I use these recipes to create a realistic simulation of a deep field, that I use to test my analysis methods and that reproduces consistently the cosmic infrared background.Lastly, I introduce some preliminary results on star formation in the young Universe (z=4) obtained thanks to new data acquired with the ALMA telescope. I describe in particular the resulting new constraints on the Main Sequence at this epoch, and study in more detail two extremely distant galaxies that I have discovered by chance in these data. These two galaxies are among the most distant known today, and are probably the most massive and most dusty ever detected in a Universe that is less than a billion years old.

Galaxy clusters are the largest gravitationally-bound structures in the universe. Since clusters are comprised of hundreds of galaxies, hot X-ray emitting gas, and dark matter, they offer a unique laboratory in which to explore the evolution of large-scale structure and galaxies. To understand how massive, low-redshift galaxy clusters evolve to become what is observed in the modern universe, astronomers need to trace the evolution of progenitor clusters. Though there are thousands of well-studied low-redshift clusters, there are significantly fewer spectroscopically confirmed high-redshift clusters. Because most massive galaxies host supermassive black holes, one cluster tracer at both low and high redshift are active galactic nuclei (AGNs). Specifically, bent, double-lobed radio sources are commonly found in clusters. I find high-redshift clusters hosting bent AGNs and explore their evolution and red sequence galaxies over cosmic time. To characterize cluster evolution, I examine the galaxy populations surrounding each AGN to determine if bent AGNs are commonly found within clusters with evolved red sequence populations. I then identify evolution among the member galaxies in the clusters, I estimate cluster morphology, and I explore the relationship between bent radio source morphology and the surrounding cluster. By measuring the color of each galaxy and the overdensity of galaxies surrounding each AGN, I identify 39 red sequence cluster candidates, 17 of which are at redshifts of z > 1.0. Using my red sequence surface density measurement, I show that each bent AGN is not necessarily centrally located, but is generally within ~ 400 kpc of the cluster center. With this sample, I probe the dynamics of the host galaxies using the radio source morphology and find that most of the radio sources do not follow radial paths relative to the cluster center. By analyzing the morphology of the radio sources in my sample, I find that richer clusters host narrower bent sources. I also see a range of red sequence populations in the clusters, with variations, in particular, among the populations of faint red sequence galaxies. With my surface density measurements, I place preliminary constraints on cluster morphology, finding both relaxed and merging systems.

The cores of galaxy clusters contain the most massive galaxies in the Universe, the brightest cluster galaxies. These galaxies are unique compared to their counterpart galaxies outside of clusters as they have much brighter cores, and vast spatially- extended stellar envelopes. The theoretical picture of how they reached their huge masses relied on the idea of gradual stellar mass growth during the second half of the history of the Universe. However, recent observational evidence of highly-overdense protoclusters, the progenitors of these galaxies, demonstrates that some brightest cluster galaxies may have assembled within the first few billion years after the Big Bang – seemingly contradicting our theoretical predictions. I include my theoretical work that shows the short timescales over which these observed protoclusters trans- form into the brightest cluster galaxies and discuss the likelihood of finding these rare protoclusters in the early Universe. To push our understanding of the rapid evolution of these galaxies even further for- ward demands the use of numerical simulations due to the highly coupled, non-linear astrophysical processes that occur during the process. In this dissertation, I include improvements to our numerical models of hydrodynamical turbulence and supermas- sive black holes that I incorporated into a state-of-the-art hydrodynamical+gravity simulation code, in effort to provide the groundwork to improving our understanding of the build-up of the brightest cluster galaxies in the early Universe, and galaxy evolution in general. / Graduate

<p>  Understanding the formation and evolution of galaxies is one of the most fundamental questions in modern astronomy. While it is widely accepted that galaxy formation needs to be understood in the context of cosmic structure formation of dark matter, a complex interplay of different physical processes that drive galaxy formation makes it challenging to elucidate how the large-scale environment of dark matter influences galaxies, particularly in their formative epoch (z > 2). </p> <p>    As the most luminous nebular emission arising from star formation,  Lyalpha provides a promising and effective tool to study the young universe and nascent galaxies.</p> <p>  At z>2, Lyalpha emission is redshifted into the visible window that is detectable by ground-based telescopes. Existing studies also suggest that strong Lyalpha-emitting galaxies represent a young and low-mass galaxy population and therefore are the best visible tracers of the large-scale structure of the distant universe. </p> <p>    In this thesis, I present two complementary studies designed to address these questions using Lyalpha emission as a cosmological tool. In Chapter 2, I investigate the kinematics and spatial distribution of the gas-phase interstellar and circumgalactic media using compact and diffuse Lyalpha emission in and around distant galaxies. I also carry out a comprehensive characterization of how Lyalpha properties correlate with other galaxy properties and the environment that galaxies reside in. In Chapter 3, I explore how Lyalpha-emitting galaxies trace the large-scale structure characterized by other means; I also conduct a detailed investigation of the distribution of different `types' of galaxies and H i gas  in  and around the most massive cosmic structure known to date. These investigations are informative in  building clear  expectations for the ongoing and upcoming experiments -- including the Legacy Survey for Space and Time, James Webb Space Telescope, Dark Energy Spectroscopic Instruments, and Hobby-Eberly Telescope Dark Energy eXperiment -- in obtaining a detailed picture of galaxy evolution in the context of their environments. </p>

Large observational surveys have compiled substantial galaxy samples with an array of different properties across cosmic time. While we have a broad understanding of how galaxies grow and build their observable properties, the details of galaxy growth and evolution pose a fundamental challenge to galaxy evolution theories. Nonetheless, galaxy evolution is ultimately regulated by the properties of the gas reservoir. In this thesis I use numerical simulations to answer key questions related to the galactic gas reservoir, and galaxy mergers: a major transformational process. In Chapter 2 I present an analysis of 28 simulated L* galaxies to understand the physical processes that shape the massive gas reservoir surrounding galaxies (i.e. the circum-galactic medium; CGM). I show that (1) the gas and metal content of the CGM is driven by galaxy growth and the strength/presence of feedback processes, and (2) the ionisation and internal structures of the CGM are shaped by galactic outflows, and active galactic nucleus luminosity. Albeit dependent on internal galactic properties and the physical processes that shape them, the CGM remains greatly diverse, thus posing a challenge for observational surveys. As a follow-up to my study of normal L* galaxy gas halos, in Chapter 3 I present a theoretical study of the effect of galaxy mergers on the CGM. I demonstrate that galaxy mergers can leave a strong imprint on the CGM's gas and metal content, metallicity, and size. The merger can increase (1) the CGM's metallicity by 0.2-0.3 dex within 0.5 Gyr post-merge, and (2) the metal covering fractions by factors of 2-3. In spite of the increase in the CGM's metal content, the hard ionising field during the merger can drive a decline in the covering fractions of commonly observed ions. In Chapter 4 I shift focus to star formation, particularly the effects of galaxy mergers on star formation. While the effects of galaxy mergers have been proven observationally, theoretical predictions are limited to small binary merger suites and cosmological zoom-in studies. I present a statistical study of 27,691 post-merger galaxies from IllustrisTNG to quantify the effect of galaxy mergers on galactic star formation. I report a dependence in the merger-induced star formation rate (SFR) on mass ratio, stellar mass, gas fraction, and galaxy SFR. I also track the evolution of the effects of galaxy mergers demonstrating their decay over ~500 Myr. In Chapter 6, I leverage galactic scaling relations to extend my work on the effects of galaxy mergers to resolved scales. However, before using the simulated resolved scaling relations, I first examine their existence and robustness. In Chapter 5, I demonstrate the emergence of the kpc-scale star forming main sequence (rSFMS) in the FIRE-2 simulations. Nonetheless, the slope of the rSFMS is dependent on the (1) star formation tracer's timescale, and (2) observed resolution, which I propose is caused by the clumpiness of star formation. I develop a toy model that quantitatively captures the effects of clumpy star formation. I then illustrate how the model can be used to characterise the mass of star-forming clumps. Having demonstrated the existence and robustness of known scaling relations in numerical simulations, I explore the effects of galaxy mergers on resolved scales in Chapter 6. I generate synthetic observations for 1,927 post-mergers in IllustrisTNG and examine the radially-dependent merger-driven SFR enhancement, and metallicity suppression in post-mergers. Galaxy mergers preferentially boost star formation in the centres and suppress metallicities globally. The effects of the merger depends on galaxy properties such as stellar mass, SFR, mass ratio, and gas fraction. / Graduate

The evolution of galaxies rely on a wide variety of physics, and numerical simulations are one of the main tools used to study them. In this thesis we develop a framework for what models can be used to realistically simulate galaxies and study their evolution. We begin with setting specific requirements on the numerical resolution of galaxies, and then test the effects of different stellar feedback models on isolated disk galaxies. We then investigate the addition of magnetic fields into the simulations, and what role they play in determining the contents, behaviour, and star formation, within the interstellar medium of galaxies. / Thesis / Master of Science (MSc) / We develop a framework used to realistically simulate the evolution of galaxies. Specifically we investigate the addition of supernova and magnetic field models, and provide solutions to eliminate the dependence of those models on numerical resolution.

No description available.

At higher redshift galaxies exhibit increasingly irregular and clumpy morphologies, withstar-forming clumps dominating the FUV output of their host galaxies thus being inti-mately related to star formation and the formation and evolution of galaxies. This workexamines star-forming clumps in a remarkable, young spiral galaxy at Cosmic Noon witha redshift ofz= 2.584, lensed by the galaxy cluster Abell 2744. To this aim, NIRCamobservations in 7 filter bands are utilized to determine photometry of clumps and performbroadband SED fitting to determine characteristic sizes, ages and masses and infer theirdynamical ages and mass surface densities. The clump within this galaxy spans a widerange of properties with sizes between 20 to 200 pc and masses between 105M⊙and109M⊙. While clumps are not resolved down to scales of individual star clusters, small,dense clumps may host star clusters. A number of clumps exhibits agestage&gt;100 Myr,thus being able to survive feedback up to these timescales. This population of clumpsalso appears to be dynamically evolved and gravitationally bound as well as the denseststructures within the investigated sample, with roughly∼20% of clumps exhibiting masssurface densities comparable to bound stellar clusters in the local Universe.

Aiming at understanding the role of stellar feedback in galaxy evolution, I present a study of the hot interstellar medium in several representative galaxies, based primarily on X-ray observations as well as theoretical modelling. I find that, in the massive disk galaxies NGC2613 and M104, the observed amount of hot gas is much less than that predicted by current galaxy formation models. Such a discrepancy suggests a lack of appropriate treatments of stellar/AGN feedback in these models. I also find that stellar feedback, primarily in the form of mass loss from evolved stars and energy released from supernovae, and presumably consumed by the hot gas, is largely absent from the inner regions of M104, a galaxy of a substantial content of evolved stars but little current star formation. A natural understanding of this phenomenon is that the hot gas is in the form of a galactic-scale outflow, by which the bulk of the stellar feedback is transported to the outer regions and perhaps into the intergalactic space. A comparison between the observed sub-galactic gas structures and model predictions indicate that this outflow is probably subsonic rather than being a classical supersonic galactic wind. Such outflows are likely prevalent in most early-type galaxies of intermediate masses in the present-day universe and thus play a crucial role in the evolution of such galaxies. For the first time I identify the presence of diffuse hot gas in and around the bulge of the Andromeda Galaxy (M31), our well-known neighbor. Both the morphology and energetics of the hot gas suggest that it is also in the form of a large-scale outflow. Assisted with multiwavelength observations toward the circumnuclear regions of M31, I further reveal the relation between the hot gas and other cooler phases of the interstellar medium. I suggest that thermal evaporation, mostly likely energized by Type Ia supernovae, acts to continuously turn cold gas into hot, a process that naturally leads to the inactivity of the central supermassive blackhole as well as the launch of the hot gas outflow. Such a mechanism plays an important role in regulating the multi-phase interstellar medium in the circumnuclear environment and transporting stellar feedback to the outer galactic regions.

No description available.