The cosmological constant + cold dark matter (ΛCDM) theory is the 'standard model' of cosmology. Encoded in it are extremely accurate descriptions of the large scale structure of the Universe, despite a very limited number of degrees of freedom. The model struggles, however, to explain some measurements on galactic and smaller scales. The shape of the dark matter distribution toward the centres of galaxies is predicted to be steeply increasing in density ('cuspy') by the theory, yet observations of the rotation curves of some galaxies suggest that it instead reaches a central density plateau (a 'core'). This discrepancy is termed the 'cusp-core problem'. I propose a new way of quantifying this problem as a diversity in the central mass content of galaxies. This characterization does not distinguish between dark and ordinary ('baryonic') matter, but the apparent problem is so severe that the signature of the cusp-core discrepancy is still obvious. By formulating the problem in this way, several uncertain modelling steps are effectively removed from the discussion, allowing for a more narrowly focussed examination of remaining steps in the analysis. My subsequent comparison of recent results from galaxy formation simulations and observed galaxies in the space of the baryonic Tully-Fisher relation (BTFR) reveals some galaxies with an apparent anomalously low dark matter content not only in the centre, but out to the largest measurable radii. These objects are very difficult to explain within the ΛCDM framework; the most plausible interpretation which emerges is that the effect of systematic uncertainties in modelling the kinematics in these galaxies – particularly in the estimate of their inclinations – has been substantially underestimated. This motivates a re-examination of rotation curve measurement methods. I use a collection of simulated galaxies to demonstrate that, when these are synthetically 'observed' and modelled analogously to real galaxies, non-circular motions present in the gas discs give the appearance of cores, even though all of the simulated galaxies have central cusps. The errors are large enough to reproduce the full width of the observed scatter in rotation curve shapes. Provided the simulations produce sufficiently faithful models of real galaxies, these modelling errors could constitute a solution to the cusp-core problem within the ΛCDM paradigm. Regardless, the kinematic models must be better understood before drawing any strong cosmological conclusions. / Graduate

Quasars are galaxies hosting accreting supermassive black holes; due to their brightness, they are unique probes of the early universe. To date, only a few quasars have been reported at z > 6.5 (< 800 Myr after the big bang). In this work, we present six additional z greater than or similar to 6.5 quasars discovered using the Pan-STARRS1 survey. We use a sample of 15 z greater than or similar to 6.5 quasars to perform a homogeneous and comprehensive analysis of this highest-redshift quasar population. We report four main results: (1) the majority of z greater than or similar to 6.5 quasars show large blueshifts of the broad C IV lambda 1549 emission line compared to the systemic redshift of the quasars, with a median value similar to 3x higher than a quasar sample at z similar to 1; (2) we estimate the quasars' black hole masses (M-BH similar to (0.3-5) x. 10(9) M circle dot) via modeling of the Mg II lambda 2798 emission line and rest-frame UV continuum and find that quasars at high redshift accrete their material (with <(Lbol L-Edd)> = 0.39) at a rate comparable to a luminosity-matched sample at lower redshift, albeit with significant scatter (0.4 dex); (3) we recover no evolution of the Fe II/Mg II abundance ratio with cosmic time; and (4) we derive near-zone sizes and, together with measurements for z similar to 6 quasars from recent work, confirm a shallow evolution of the decreasing quasar near-zone sizes with redshift. Finally, we present new millimeter observations of the [C II] 158 mu m emission line and underlying dust continuum from NOEMA for four quasars and provide new accurate redshifts and [C II]/infrared luminosity estimates. The analysis presented here shows the large range of properties of the most distant quasars.

Analytical models and simulations predict a rapid decline in the halo density profile associated with the transition from the "infalling" regime outside the halo to the "collapsed" regime within the halo. Using data from SDSS, we explore evidence for such a feature in the density profiles of galaxy clusters using several different approaches. We first estimate the steepening of the outer galaxy density profile around clusters, finding evidence for truncation of the halo profile. Next, we measure the galaxy density profile around clusters using two sets of galaxies selected on color. We find evidence of an abrupt change in galaxy colors that coincides with the location of the steepening of the density profile. Since galaxies that have completed orbits within the cluster are more likely to be quenched of star formation and thus appear redder, this abrupt change in galaxy color can be associated with the transition from single-stream to multi-stream regimes. We also use a standard model comparison approach to measure evidence for a " splashback"-like feature, but find that this approach is very sensitive to modeling assumptions. Finally, we perform measurements using an independent cluster catalog to test for potential systematic errors associated with cluster selection. We identify several avenues for future work: improved understanding of the small-scale galaxy profile, lensing measurements, identification of proxies for the halo accretion rate, and other tests. With upcoming data from the DES, KiDS, and HSC surveys, we can expect significant improvements in the study of halo boundaries.

The extension of the concepts of Newtonian cosmology to a universe consisting of a hierarchy of metagalaxies is fairly straightforward. However, in general relativistic cosmology, the construction of such a hierarchical universe is a difficult problem. It is the purpose of this work to examine some aspects of hierarchical cosmology in both the Newtonian and general relativistic cases. It is suggested that the metagalaxy may be a black hole or Schwarzschild object,(to account for the fact that no objects which could be identified as metagalaxies have been, as yet, observed. Some features of this concept are discussed. Tidal forces exerted on a metagalaxy, due to others distributed around it, are estimated in the Newtonian case. Such tidal forces may or may not be detectable, depending on the distance between metagalaxies. The interior of a metagalaxy is represented by a Fried-mann model, with given values of k and A. The Friedmann model is matched at the boundary to a Schwarzschild spacetime. The consequences of this and related calculations suggest that in most cases, a metagalaxy may be a black hole for only part of its lifetime, i.e., for other times, it may be optically detectable to an exterior observer. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 1976 / Bibliography: leaves 235-239. / Ph. D. / Ph. D. Massachusetts Institute of Technology, Department of Physics

Répondre aux questions fondamentales concernant notre compréhension de l’Univers, comme la cause de son expansion accélérée ou la nature de la matière noire, requiert de confronter les théories aux observations. Dans ce contexte, les amas de galaxies peuvent être utilisés comme de puissantes sondes observationnelles. Cependant, à l’heure actuelle, leur utilisation est limitée par des incertitudes et des effets systématiques, qui affectent notamment la mesure de leur masse, que l’on présume dominée par la matière noire. Les amas de galaxies peuvent être étudiés à différentes longueurs d’onde : le gaz chaud qui compose le milieu intra-amas (ICM en anglais) émet des rayons X et est observable dans le domaine millimétrique via l’effet Sunyaev Zel’dovich (SZ), alors que les galaxies rayonnent principalement en optique et infrarouge. Combiner et comparer ces observables permet de réduire les incertitudes et les effets systématiques des contraintes cosmologiques issues des amas. Dans ce contexte, cette thèse a pour but de préparer les grands relevés observationnels du futur comme Euclid et le Large Synoptic Survey Telescope (LSST). Elle présente les analyses multi-longueurs d’onde d’un échantillon d’amas détectés en X dans le relevé XXL, couvrant une large gamme de masses et de redshifts. La première partie de cette thèse introduit le contexte cosmologique et présente les propriétés observationnelles des galaxies et amas de galaxies, ainsi que les ingrédients pour construire des échantillons cosmologiques d’amas. La deuxième partie traite de la caractérisation optique des amas XXL et des propriétés de leurs galaxies membres. Nous commençons par la présentation de XXL et du Canada-France-Hawaii Telescope Legacy Survey (CFHTLS), un relevé optique associé. Ensuite, nous nous concentrons sur la caractérisation de la qualité des redshifts photométriques du CFHTLS et sur leur utilisation pour construire les fonctions de luminosité (LF en anglais) optiques des galaxies d’amas XXL. Il apparaît que la LF des galaxies satellites dépend légèrement de la richesse des amas, le principal proxy de masse en optique, mais ne montre pas d’évolution significative avec le redshift. Ensuite, nous entreprenons l’étude de la couleur et de la fraction de galaxies à noyaux actifs (AGN en anglais) dans les galaxies d’amas XXL et montrons que la masse joue un rôle clé dans la régulation de l’activité de formation stellaire dans les amas. Pour finir, l’algorithme de détection d’amas WaZP est utilisé pour étudier la contrepartie optique des amas XXL. La troisième partie de cette thèse est consacrée au projet observationnel dédié à la cartographie du signal SZ de trois amas XXL distants, avec la camera à haute résolution angulaire NIKA2. La préparation du projet est discutée, en se servant des données optiques et X afin de prédire le signal SZ attendu. Ensuite, nous présentons la procédure d’observation au télescope et la réduction des données, dédiée à la production des cartes SZ étalonnées. Le projet est en cours et un amas, XLSSC102, à z = 0.97, a été observé partiellement. Nous développons ensuite une méthode de détection en aveugle des potentielles galaxies qui peuvent contaminer le signal SZ, permettant la découverte fortuite de galaxies poussiéreuses à haut taux de formation stellaire dans le champ de XLSSC102. La morphologie et l’état dynamique de XLSSC102 sont ensuite caractérisés grâce à la combinaison des données optiques, SZ et X et les profils radiaux de masse et de propriétés thermodynamiques de l’ICM sont mesurés en associant les données X et SZ. Cela permet de montrer que XLSSC102 est un amas en coalescence avec une masse de ∼ 3 × 10^14 Msol et est compatible avec le scénario d’évolution standard de la formation des amas. / Addressing fundamental questions regarding our understanding of the Universe, such as the cause of its accelerated expansion or the nature of dark matter, requires to confront theories and observations. In this context, galaxy clusters can be used as powerful observational probes. However, their current utilisation is limited by uncertainties and systematic effects, notably affecting the measurement of their mass, which is presumably dominated by dark matter.Galaxy clusters can be studied at different wavelengths: the hot gas composing the Intra Cluster Medium (ICM) shines in X-ray and is observable at millimetre wavelengths via the Sunyaev-Zel’dovich (SZ) effect, whereas galaxies emit principally in the optical and infrared. Combining and comparing these observables allows us to reduce the uncertainties and systematics in the cosmological constraints obtained from clusters. In this context, this thesis aims at paving the way of future large surveys such as Euclid and the Large Synoptic Survey Telescope. It presents the multi-wavelengths analyses of a sample of clusters detected in X-ray in the XXL survey, spanning a wide range of masses and redshifts. The first part of the thesis introduces the cosmological context and presents the observational properties of galaxies and clusters, and the ingredients to build cosmological cluster samples. The second part concentrates on the optical characterisation of XXL clusters and the properties of their member galaxies. It starts by presenting XXL and the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS), an optical counterpart survey. Then, it focuses on the characterisation of the CFHTLS photometric redshifts quality and their use to construct the optical galaxy luminosity functions (LF) of XXL clusters. The LF of satellite galaxies is found to slightly depend on cluster richness, the main optical mass proxy, but no significant redshift evolution is observed. Then, the study of the colour and active galactic nuclei (AGN) fraction in XXL cluster galaxies is performed, finding that the mass plays a key role in shaping AGN and star formation activity in clusters. Finally, the WaZP optical cluster finder algorithm is used to investigate the optical counterparts of XXL clusters. The third part of this thesis is dedicated to the observational project dedicated to the mapping of the SZ signal in three distant XXL clusters, with the high angular resolution NIKA2 camera. The preparation of the project is discussed, making use of the X-ray and optical data to predict the expected SZ signal. Then, the observation procedure at the telescope and the data reduction, dedicated to produce calibrated SZ maps, are presented. The project is still ongoing and one cluster, XLSSC102, at z = 0.97, has been partially observed. The development of the blind detection of galaxies potentially contaminating the SZ signal is developed, allowing for the serendipitous discovery of dusty star forming galaxies in the field of XLSSC102. The morphology and dynamical state of XLSSC102 are then characterised using optical, SZ and X-ray data and the radial ICM thermodynamics and mass profiles are measured combining SZ and X-ray data. This allows us to show that XLSSC102 is a merging cluster with a mass ∼ 3 × 10 14 M, and is compatible with the standard evolution scenario of cluster formation.

We present new N-body, hydrodynamical simulations of the interaction between the starburst galaxy NGC 7714 and its poststarburst companion NGC 7715, focusing on the formation of the collisional features, including (1) the gas-rich star-forming bridge, (2) the large gaseous loop (and stellar tails) to the west of the system, (3) the very extended H I tail to the west and north of NGC 7714, and (4) the partial stellar ring in NGC 7714. Our simulations confirm the results of earlier work that an off-center inclined collision between two disk galaxies is almost certainly responsible for the peculiar morphologies of this system. However, we have explored a wider set of initial galaxy and collisional encounter parameters than previously and have found a relatively narrow range of parameters that reproduce all the major morphologies of this system. The simulations suggest specific mechanisms for the development of several unusual structures. We find that the complex gas bridge has up to four distinct components, with gas contributed from two sides of NGC 7715, as well as from NGC 7714. The observed gas-star offset in this bridge is accounted for in the simulations by the dissipative evolution of the gas. The models suggest that the most recently formed gas bridge component from NGC 7715 is interacting with gas from an older component. This interaction may have stimulated the band of star formation on the north side of the bridge. The models also indicate that the low surface brightness H I tail to the far west of NGC 7714 is the end of the NGC 7715 countertail, curved behind the two galaxies. The sensitivity of the tidal structures to collision parameters is demonstrated by comparisons between models with slightly different parameter values. Comparison of model and observational (H I) kinematics provides an important check that the morphological matches are not merely fortuitous. Line-of-sight velocity and dispersion fields from the model are found to match those of the observations reasonably well at current resolutions. Spectral evolutionary models of the NGC 7714 core by Lançon et al. suggest the possibility of multiple starbursts in the last 300 Myr. Our hydrodynamic models suggest that bursts could be triggered by induced ringlike waves and a postcollision buildup of gas in the core of the galaxy.

Philosophiae Doctor - PhD / In recent history there have been several advances in cosmology, which has significantly shaped our understanding of the Universe. The current leading theory is called ΛCDM, which can successfully model the expansion of the Universe from a primordial state and describe the dynamics of its contents, thereby resulting in the large-scale structure present today. The model is based on general relativity, that describes gravitational interaction as the curvature of a four-dimensional manifold called space-time. However, despite the many successes of ΛCDM, there are a number of things that need further investigation. The Cosmic Microwave Background (CMB) is the oldest observable radiation in the Universe, and this cosmological relic contains a detectable structure. The process leading up to the CMB determines the initial conditions of ΛCDM, but is still poorly understood. It is widely accepted that inflation was responsible for the rapid expansion after the Big Bang, although this is yet to be verified experimentally. The distribution of the primordial potential is imprinted on ultra-large scales of the matter distribution, which offers an important insight into uncovering this mystery. In addition to the primordial Universe, there are other concepts that still puzzle us in ΛCDM itself. The fact that we have been unable to directly detect and explain these dark components (that make up around 96% of the Universe) has prompted several theorists to consider alternative cosmological models. Therefore, testing general relativity and ΛCDM is still an essential part of cosmological research. A key observational discriminant between general relativity and modified theories of gravity is the rate at which the large-scale structure grows from small perturbations. The relativistic effects (or light-cone effects) expected in general relativity also offer an independent test of the gravitational model.

No description available.

This thesis aims to better constrain the high-mass end of the globular cluster system (GCS) mass - Halo mass relation. This relation between the total mass contained in the globular clusters in a galaxy and the mass of its dark matter halo has been found to be nearly linear. The measured mass ration between the mass of the GCS and the halo mass, defined as eta, has been found to be constant over a large range of galaxy masses, however there is comparatively less data for galaxies with dark matter halo masses >10^13 solar masses. This research analyzes the GCSs of a sample of eleven high-mass brightest cluster galaxies (BCGs) through the use of the photometry program DOLPHOT on Hubble Space Telescope (HST) images in the F814W filter. The mass of the galaxies' GCSs were calculated from their GC radial distributions, and the dark matter halo masses were determined from the known relationship between total stellar mass and halo mass for BCGs. This research utilizes a new standardization technique to calculate the size of the GCS based on the galaxy's virial radius. These GC and halo masses were then compared to determine eta at this high-mass end, which was found to be eta=(6.84+/- 1.47)\times10^-5. When adding the sample BCGs to a catalogue of 303 lower-mass galaxies a total value for eta was found to be eta=(2.99 +/- 0.06)\times10^-5, which is within the literature range for of eta, and is both slightly higher and has a reduced uncertainty compared to the value for the catalogue galaxies alone which was found to be eta=(2.87 +/- 0.11)\times10^-5. This consistency between the eta-values of high-mass and lower-mass galaxies implies that in order for these BCGs to have such massive GCSs, a large proportion of these GCs must be accreted from galaxy mergers at late redshift. / Thesis / Master of Science (MSc)