Dissertations / Theses on the topic 'Cosmological hydrodynamics'

This thesis investigates the evolution of characteristic structures in neutrino or adiabatic baryon-dominated models of galaxy formation. We discuss the collapse of protocluster or protosupercluster clouds in terms of the behaviour of non-rotating, homogeneous triaxial ellipsoids, predicting that galaxies should populate filamentary or quasi-spherical structures rather than the generic flat structures (pancakes). Secondly we have designed a numerical code which allows us to do fast cosmological hydrodynamics. We investigate the effect of explosions on the standard pancake picture for galaxy formation. Blast waves created by the early evolution of galaxies can not produce the anti-biasing effect required to reconcile the rapid evolution of clustering in n-body simulations of neutrino models with observations of galaxies at redshifts greater than one.

Avec l’essor actuel de la sophistication et de l’efficacité des codes de cosmologie hydrodynamique,il est devenu possible d’inclure le transfert radiatif (RT) des photons ionisants dansles simulations cosmologiques, soit en post-traitement, soit en simulations couplées rayonnement+hydrodynamique (RHD). Malgré de nombreux obstacles, il y a eu cette derniéredécennie beaucoup de recherches menées sur les différentes stratégies et implémentations,dû au fait qu’un nombre de problèmes intéressants peuvent être désormais abordés par laRT et RHD, par exemple comment et quand l’Univers s’est réionisé, comment l’émissionradiative des étoiles et des noyaux actifs de galaxies se comportent pour réguler la formationdes structures à des échelles petites et grandes, et quelles prédictions et interprétationsnous pouvons faire des phénomènes observés, tels que la forêt Lyman-alpha et des sourcesdiffuses de rayonnement.Cela coïncide avec l’avènement du télescope spatial James Webb (JWST) et d’autresinstruments de pointe qui sont sur le point de nous donner un aperçu sans précédent sur lafin des âges sombres de l’Univers, quand le cosmos est passé d’un état froid et neutre à unétat chaud et ionisé, à la suite de l’apparition des sources radiatives.Notre préoccupation principale étant les rétroactions radiatives des premieres structures,nous avons mis en place une version RHD du code cosmologique Ramses, que nous appelonsRamsesRT, basée sur la méthode des moments. Ce code nous permet d’étudier les effets durayonnement ionisant dans les simulations cosmologiques RHD qui tirent pleinement profitdes stratégies de raffinement adaptif de grille et de parallélisation de Ramses. Pour rendreauto-cohérent le RHD nous avons également mis en oeuvre une thermochimie hors-équilibreincluant des espèces de l’Hydrogène et de l’Hélium qui interagissent avec le rayonnementtransporté.Je présente dans cette thèse une description détaillée de RamsesRT et de nombreux testscontribuant à sa validation.Jusqu’à présent nous avons utilisé RamsesRT pour étudier l’émission Lyman-alpha decourants d’accrétion, qui sont prédits à grand redshift par les simulations cosmologiques,mais n’ont jamais été clairement identifiés par les observations. Nous avons également étudiéle chauffage gravitationnel dans ces courants pour déterminer si ce dernier pouvait être lasource motrice principale des Lyman-alpha blobs, un phénomène observé qui a été beaucoupétudié et débattu au cours de la dernière décennie. Cet étudie nous permet de conclure queles Lyman-alpha blobs peuvent, en principe, être alimentés par le chauffage gravitationnel,et que d’autre part, les courants d’accrétion sont sur le point d’être directement détectablesavec des instruments à venir.Mes intentions futures sont d’utiliser RamsesRT dans les simulations cosmologiques àhaute résolution, de la formation des premiéres galaxies jusqu’à l’époque de la réionisation,et ainsi étudier comment la rétroaction radiative affecte la formation et l’évolution de cesgalaxies et de faire des prévisions d’observation qui peuvent être testées avec des instrumentssophistiqués tels que le JWST
With the increasing sophistication and efficiency of cosmological hydrodynamics codes, ithas become viable to include ionizing radiative transfer (RT) in cosmological simulations,either in post-processing or in full-blown radiation-hydrodynamics (RHD) simulations. Inspite of the many hurdles involved, there has been much activity during the last decade or soon different strategies and implementations, because a number of interesting problems canbe addressed with RT and RHD, e.g. how and when the Universe became reionized, howradiation from stars and active galactic nuclei plays a part in regulating structure formationon small and large scales, and what predictions and interpretations we can make of observedphenomena such as the Lyman-alpha forest and diffuse sources of radiation.This coincides with the advent of the James Webb space telescope (JWST) and otherstate-of-the-art instruments which are about to give us an unprecedented glimpse into theend of the dark ages of the Universe, when the cosmos switched from a cold and neutralstate to a hot and ionized one, due to the turn-on of ionizing radiative sources.With a primary interest in the problem of radiative feedback in early structure formation,we have implemented an RHD version of the Ramses cosmological code we call RamsesRT,which is moment based and employs the local M1 Eddington tensor closure. This code allowsus to study the effects of ionizing radiation on-the-fly in cosmological RHD simulationsthat take full advantage of the adaptive mesh refinement and parallelization strategies ofRamses. For self-consistent RHD we have also implemented a non-equilibrium chemistry ofthe atomic hydrogen and helium species that interact with the transported radiation.I present in this thesis an extensive description of the RamsesRT implementation andnumerous tests to validate it.Thus far we have used the RHD implementation to study extended line emission fromaccretion streams, which are routinely predicted to exist at early redshift by cosmologicalsimulations but have never been unambiguously verified by observations, and to investigatewhether gravitational heating in those streams could be the dominant power source ofso-called Lyman-alpha blobs, an observed phenomenon which has been much studied anddebated during the last decade or two. Our conclusions from this investigation are thatLyman-alpha blobs can in principle be powered by gravitational heating, and furthermorethat accretion streams are on the verge of being directly detectable for the first time withupcoming instruments.My future intent is to use RamsesRT for high-resolution cosmological zoom simulations ofearly galaxy formation, up to the epoch of reionization, to study how radiative feedbackaffects the formation and evolution of those galaxies and to make observational predictionsthat can be tested with upcoming instruments such as the JWST

This thesis presents a theoretical investigation of supermassive black holes and the quasars they power through cosmological hydrodynamic simulations on petascale supercomputers. As the size of simulations increase, visualization and interaction with the data become difficult. We developed interactive visualization software on top of existing image deep-zoom and tagging infrastructure libraries and platforms, specifically in the context of cosmological hydrodynamic simulations. The visualization tools were instrumental in establishing the cold flow fueling model of high redshift quasar growth. We then proceed to further study this growth model with high resolution zoom-in re-simulations, and report that the cold flows feeding supermassive blackholes are numerically stable. We studied the morphology of HII regions with a parallel implementation of the ray tracing scheme SPHRAY, and found that the HII regions due to high redshift quasars are smoother and more extended that those due to galaxies. Finally, we developed a set of tools to produce and verify mocked correlated quasars and Lyman-α forest. We fit the linear theory correlation function of the Quasar-Quasar and Forest-Forest auto-correlation and Quasar-Forest cross-correlation of the mocked transmission fraction and quasar locations up to 160 Mpc/h.

Despite years of study, the origins of red galaxies are not fully understood in a cosmological context. We develop new models for quenching star-formation and producing red galaxies in cosmological hydrodynamic simulations. We start with phenomenological models applied in post-processing to previously-run simulations. We focus separately on mergers and hot haloes – akin to “quasar mode” and “radio mode” feedback – as the drivers shutting down star-formation. With appropriate parameter choices, each model can produce a reasonably good match observed color-magnitude diagrams and red galaxy luminosity functions at redshift zero. We uncover some difficulties with these models in general, including red galaxy stellar populations that appear too blue by 0.1 magnitudes in g − r due to a metallicity deficit. Building on the post-processing models, we develop quenching models for simulations that run on-the-fly. Again, we test merger quenching and hot halo quenching separately. We model merger feedback as > 1000 km s⁻¹ winds motivated by observations of post-starburst galaxies, and wemodel hot halo feedback by continuously adding thermal energy to circum-galactic gas in haloes dominated by gas above 250, 000 K. Merger quenching temporarily shuts down starformation, butmerger-remnant galaxies typically resume star-formation with 1− 2 Gyrs thanks to accretion of newfuel fromthe IGM.Hot halo quenching successfully produces a realistic red sequence, providing a good match to the observed red galaxy luminosity function. Despite some minor difficulties with hot halo quenching, we examine its effects in more detail. Specifically, we study the evolution of the simulated red sequence over time. We find that galaxies with stellar mass ∼ 10¹¹M⊙ are the first to populate the red sequence at z ≳ 2, with significantly fewer red galaxies around 10¹⁰M⊙ until z ≈ 0. We show that massive galaxies grow substantially after moving onto the red sequence, primarily through minor mergers. We also examine the relationship between quenching and environment.

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 evolution of galaxies across cosmic time, from the first galaxies to the local Universe, are studied using cosmological hydrodynamical simulations. It is demonstrated that, for the first time, a hydrodynamical simulation can reproduce the observed evolution of galaxy stellar masses and the trends in star formation rates. The success of the simulation in producing galaxies with similar histories to those observed increases the potential for using hydrodynamical simulations to explore galaxy formation physics. With this intention, we consider the effects of the environment and active galactic nuclei (AGN) quenching on the galaxy population and the shape of the galaxy stellar mass function (GSMF). We find environmental processes are effective at quenching galaxies in the simulation and operated on short timescales. AGN feedback, which produces a large passive fraction at high stellar masses, drives the exponential break in the GSMF. Specific star formation rates (SFRs) in simulations, both hydrodynamical and semi-analytical, have been shown to be discrepant with observations. We investigate proposed solutions to this problem using a suite of cosmological hydrodynamical simulations. The offset in the simulations, at the level of 0.2 to 0.4 dex, can only be resolved by employing an extreme model that does not recover any of the observed trends in stellar mass or the cosmic star formation rate density. This study implies that the observed star formation rates across comic time are inconsistent with the growth of stellar mass. Two galaxy populations are then explored in more detail. We examine the first galaxies and their potential to reionize the Universe. We find that low-mass galaxies, undergoing extreme star formation for their stellar mass, produce the majority of ionizing photons at redshifts 6 and above. The second study considers the most highly star forming galaxies in the simulation, which represent an extreme population. These galaxies have similar stellar and gas masses to the observed sub-mm population, however, their SFRs are lower. On further investigation, the selection of galaxies based on SFRs is not adequate to compare the simulation to the sub-mm population, in particular at high redshift. These results highlight the importance of dust temperature in the selection.

Simulations including solely dark matter performed over the last three decades have delivered an accurate and robust description of the cosmic web and dark matter structures. With the advent of more precise cosmological probes, planned and ongoing, and dark matter detection experiments, this numerical modelling has to be improved to incorporate the complex non-linear and energetic processes taking place during galaxy formation. We use the ``Evolution and Assembly of GaLaxies and their Environment'' (EAGLE) suite of cosmological simulations to investigate the effects of baryons and astrophysical processes on the underlying dark matter distribution. Many effects are expected and we investigate (i): the modification of the profile of halos from the Navarro-Frenk-White profile shape found in collisionless simulations, including the changes in the dark matter profiles themselves, (ii) the changes of the inner density profiles of rich clusters, where observations have suggested a deviation from the standard cold dark matter paradigm, (iii) the offset created by astrophysical process between the centre of galaxies and the centre of the dark matter halo in which they reside and, (iv) the changes in the shape of the dark matter profile due to baryons in the centre of Milky Way halos and the impact these changes have on the morphology of the annihilation signal that could be observed as an indirect proof of the existence of dark matter. In all cases we find that the baryons play a significant role and change the results found in collisionless simulations dramatically. This highlights the need for more simulations like EAGLE to better understand and analyse future cosmology surveys. We also conduct a thorough study of the hydrodynamics solver parameters used in these simulations, assess their impact on the simulated galaxy population and show how robust some of the EAGLE results are against such variations.

Feedback from the most massive components of a young stellar cluster deeply affects the surrounding ISM driving an expanding over-pressured hot gas cavity in it. In spiral galaxies these structures may have sufficient energy to break the disk and eject large amount of material into the halo. The cycling of this gas, which eventually will fall back onto the disk, is known as galactic fountains. We aim at better understanding the dynamics of such fountain flow in a Galactic context, frame the problem in a more dynamic environment possibly learning about its connection and regulation to the local driving mechanism and understand its role as a metal diffusion channel. The interaction of the fountain with a hot corona is hereby analyzed, trying to understand the properties and evolution of the extraplanar material. We perform high resolution hydrodynamical simulations with the moving-mesh code AREPO to model the multi-phase ISM of a Milky Way type galaxy. A non-equilibrium chemical network is included to self consistently follow the evolution of the main coolants of the ISM. Spiral arm perturbations in the potential are considered so that large molecular gas structures are able to dynamically form here, self shielded from the interstellar radiation field. We model the effect of SN feedback from a new-born stellar cluster inside such a giant molecular cloud, as the driving force of the fountain. Passive Lagrangian tracer particles are used in conjunction to the SN energy deposition to model and study diffusion of freshly synthesized metals. We find that both interactions with hot coronal gas and local ISM properties and motions are equally important in shaping the fountain. We notice a bimodal morphology where most of the ejected gas is in a cold $10^4$ K clumpy state while the majority of the affected volume is occupied by a hot diffuse medium. While only about 20\% of the produced metals stay local, most of them quickly diffuse through this hot regime to great scales.

Le galassie star-forming, come la Via Lattea, hanno formato stelle durante tutta la propria vita (~ 12 Gyr). Tuttavia, tipicamente la massa di gas contenuta nel disco di tali galassie può sostenere la formazione stellare solo per pochi Gyr. Tale problema (gas-consumption dilemma) suggerisce la necessità di un continuo accrescimento di gas freddo da parte del disco della galassia ad un tasso di ~ 1 M_Sol /yr. Modelli teorici cosmologici e recenti osservazioni in banda X mostrano che tali galassie sono circondate da corone di gas caldo alla temperatura viriale (T ~ 10^6 K per la MW). Queste corone, se in grado di raffreddare, potrebbero sostenere il processo di formazione stellare agli attuali tassi osservati. Simulazioni idrodinamiche in 2D realizzate negli ultimi anni hanno mostrato che il gas coronale caldo può raffreddare efficientemente in seguito all’interazione con nubi di fontana galattica, processo sostenuto dalle esplosioni di supernove. La coda turbolenta, formatasi in seguito allo sviluppo di un'instabilità di Kelvin-Helmholtz, causa la diminuzione del tempo di cooling di parte del gas coronale e la sua condensazione. Questo gas condensato (10-20% della massa iniziale della nube) ricade sul disco alimentando la formazione stellare. In questa tesi estendiamo la trattazione della nube di fontana galattica ad una geometria 3D. I codici finora utilizzati, a griglia fissa, non permettono tale trattazione per via del proibitivo costo computazionale. Abbiamo quindi utilizzato ENZO, un codice a griglie adattive che raffina la risoluzione solo in zone soddisfacenti alcuni criteri. Abbiamo realizzato una serie di simulazioni idrodinamiche in 2D sia nel caso in cui il gas non possa raffreddare radiativamente sia nel caso contrario e per entrambi è stato individuato il criterio di raffinazione più performante. Tale criterio è stato utilizzato per una simulazione 3D, ricavando un valore per la condensazione quasi doppio rispetto alla trattazione precedente (~ 30%).