The gaseous halo surrounding a galaxy disk is often referred to as the circum-galactic medium (CGM). The boundary of the CGM is loosely defined as the virial radius of the galaxy. Recent observations and simulations have shown that the CGM is massive, multiphase, clumpy, and metal-enriched. The CGM plays an important role in galaxy formation and evolution – it serves as a massive baryonic reservoir, from which the disk accretes gas fuel to sustain the star-formation activities, and to which the disk deposits feedback material. This dissertation focuses on the gas distribution in the CGM of the Milky Way (MW) and the baryon cycle between the CGM and disk of our neighbor – the Triangulum Galaxy (M33). Observations of the MW’s CGM are unavoidably contaminated by foreground gas since we reside in the MW’s disk. Conventionally, a velocity cut at |VLSR|~100 km/s is used as a proxy for distance, with low-velocity (|VLSR|<100 km/s) gas being more nearby to the Galactic disk than high-velocity gas. Using both a MW-mass simulation and all-sky QSO observations, I show that the low-velocity gas in the MW’s CGM is as massive as their high-velocity counterpart, and that the MW most likely hosts a massive CGM reservoir as those L~L* galaxies at z~0.2. I further study how baryons are cycled between the disk and CGM by observing gas accretion in M33. Using HST/COS to observe seven UV-bright stars in M33’s disk, I find that there is a layer of metal-enriched inflow moving towards M33 at a rate of dM/dt=2.9 Msun/yr. The gas inflow may be related to galactic fountain process or debris falling back down due to the potential past M31-M33 interaction. This work is among the first to unambiguously reveal the existence of a disk-wide, ionized galactic inflow beyond the Milky Way. In addition, with the same set of HST/COS sightlines, I make a serendipitous discovery of an ionized very-high-velocity cloud towards M33.

Thesis (Ph. D.)--University of Washington, 2003. / Vita. Includes bibliographical references (p. 201-212).

Thesis (Ph. D.)--University of Chicago, Department of Astronomy and Astrophysics, June 2001. / Includes bibliographical references. Also available on the Internet.

Galaxies, galaxy groups and galaxy clusters are embedded in large dark matter halos. Most galaxies in the local universe are found in the galaxy group environment. Locating the centres of galaxy groups is crucial in order to study their properties such as their halo masses. It is often assumed that the most massive galaxy (or brightest galaxy) resides at the centre of the gravitational potential. With the aim of evaluating the validity of this paradigm in galaxy groups, we used two different methods to probe the centres of galaxy group halos: the weak gravitational lensing and dynamical methods. We use these two methods to determine the best definition of galaxy group centres. Our sample is composed of 49 optically (spectroscopically) selected groups and 36 high quality X-ray-selected groups. In total our sample is composed of 78 distinct groups in the redshift range 0.1 < z < 0.9 from the GEEC sample. Our weak lensing analysis suggests that the weighted centre is a better definition than the most massive galaxy position. We address the question of whether or not the result is significantly different for X-ray and optically selected systems. For optically selected systems, the weighted centre is a significantly better assumption of the group centre than the most massive galaxies position. For the X-ray selected groups, the weighted centre and the most massive galaxy appear to trace the centre equally well, although the best definition is the location of the peak in X-ray emission. We evaluate, for the first time, the impact of dynamically complex groups on weak lensing analysis. Once we removed dynamically complex systems from our sample, the lensing signals for all centre definitions are in better agreement suggesting that groups with large offsets between the centre definitions are unevolved systems. For the dynamical method, velocity dispersion profiles suffer from large uncertainties and, therefore, we are unable to place any constraint on the centre definition from this technique. / Thesis / Master of Science (MSc)

We investigate the possibility that present-day galaxies and their dark matter halos contain a population of massive black holes (MBHs) that form by hierarchical merging of the black hole remnants of the first stars in the Universe. Some of the MBHs may be large enough or close enough to the centre of the galactic host that they merge within a Hubble time. We estimate to what extent this process could contribute to the mass of the super-massive black holes (SMBHs) observed in galactic centres today. The relation between SMBH and galactic bulge mass in our model displays the same slope as that found in observations. Many MBHs will not reach the centre of the host halo, however, but continue to orbit within it. In doing so MBHs may remain associated with remnants of the satellite halo systems of which they were previously a part. Using a semi-analytical approach that explicitly accounts for dynamical friction, tidal disruption and encounters with galactic disks, we follow the hierarchical merging of MBH systems and their subsequent dynamical evolution inside the respective host halos. In this context two types of dynamical processes are examined in more detail. We predict the mass and abundance of MBHs in present-day galactic halos and also estimate the MBH mass accretion rates considering two different accretion scenarios. On this basis we determine the bolometric, optical and X-ray luminosity functions for the accreting MBHs using thin disk and advection dominated accretion flow models. Our predicted MBH X-ray emissions are then compared with observations of ultra-luminous X-ray sources in galaxies. We find that the slope and normalisation of the predicted X-ray luminosity function are consistent with the observations. We also estimate the rate of gravitational wave events received from MBH mergers across all redshifts. At the end of their lives the first stars may explode in supernovae that are associated with gamma ray bursts (GRBs). Provided these are in principle detectable we have estimated the expected rate of events observed.

We present the first results from an ongoing survey to characterize the circumgalactic medium (CGM) of massive high-redshift galaxies detected as submillimeter galaxies (SMGs). We constructed a parent sample of 163 SMGQSO pairs with separations less than similar to 36" by cross-matching far-infrared-selected galaxies from Herschel with spectroscopically confirmed QSOs. The Herschel sources were selected to match the properties of the SMGs. We determined the sub-arcsecond positions of six Herschel sources with the Very Large Array and obtained secure redshift identification for three of those with near-infrared spectroscopy. The QSO sightlines probe transverse proper distances of 112, 157, and 198. kpc at foreground redshifts of 2.043, 2.515, and 2.184, respectively, which are comparable to the virial radius of the similar to 10(13) M circle dot halos expected to host SMGs. High-quality absorption-line spectroscopy of the QSOs reveals systematically strong H I Ly alpha absorption around all three SMGs, with rest-frame equivalent widths of similar to 2-3 A. However, none of the three absorbers exhibit compelling evidence for optically thick H I gas or metal absorption, in contrast to the dominance of strong neutral absorbers in the CGM of luminous z similar to 2 QSOs. The low covering factor of optically thick H I gas around SMGs tentatively indicates that SMGs may not have as prominent cool gas reservoirs in their halos as the coeval QSOs and that they may inhabit less massive halos than previously thought.

Caracterizamos estatisticamente a estrutura dos halos de matéria escura extraídos da Simulação do Millennium, uma das maiores simulações do modelo cosmológico CDM realizadas até hoje, com 10 bilhões de partículas num volume de 500h−1Mpc3. Nossos resultados confirmam que na época atual, z = 0, o parâmetro de concentração dos halos se correlaciona com a massa. Esta correlação é ajustada por uma lei de potência no intervalo de massas que corresponde a halos de galáxias (1012h−1M) até aglomerados de galáxias (1015h−1M), log10 c ∞ −α log10M, com α = 0.1. Comparamos a previsão de três modelos existentes na literatura para esta correlação. Medimos a distribuição de probilidade do parâmetro de concentração, que é ajustada com boa aproximação por uma função lognormal com dispersão σlog10 c = 0.1 em todo o intervalo de massas. Verificamos que o parâmetro de concentração é independente do spin do halo quando sistemas claramente fora de equilíbrio são removidos da análise. Comparamos modelos que relacionam o parÂmetro de concentração com histórico de formação extraído da simulação. Mostramos que quando definimos a época de formação levando em conta a distribuição de progenitores do halo e não apenas o progenitor mais massivo, obtemos uma correlação melhor entre densidade média do universo nessa época e a densidade característica do halo. Finalmente, testamos a validade de dois modelos que predizem o parâmetro de concentração a z = 0 com base no histórico de acreção de massa do progenitor mais massivo do halo. / We use the Millennium Simulation (MS) to study the statistics of CDM halo concentrations at z = 0. Our results confirm that the average halo concentration declines monotonically with mass; the concentration-mass relation is well fit by a power-law over 3 decades in mass, up to the most massive objects that form in a CDM universe (~1015h−1M). We compare this relation with three models in the literature. The large volume surveyed, together with the unprecedented numerical resolution of the MS, allow us to estimate with confidence the distribution of concentrations and, consequently, the abundance of systems with unusual properties. We find that halo concentrations are independent of spin once haloes manifestly out of equilibrium have been removed from the sample. The concentrations of out-ofequilibrium haloes tend to be lower and have more scatter, while their spins tend to be higher. A number of previously noted trends within the halo population are induced primarily by these properties of unrelaxed systems. Finally, we compare the result of predicting halo concentrations using the mass assembly history of the main progenitor with predictions based on simple arguments regarding the assembly time of all progenitors. The latter are typically as good or better than the former, suggesting that halo concentration depends not only on the evolutionary path of a halo’s main progenitor, but on how and when all of its constituents collapsed to form non-linear objects.

Using data from the Next Generation Virgo Cluster Survey and high-resolution simulations of Virgo cluster-like halos, we determine the stellar-to-halo mass relation (SHMR) for subhalos, using the technique of abundance matching. The subhalo SHMR differs markedly from its field galaxy counterpart, regardless of how the subhalo mass is defined (mass at z = 0, mass at infall, or maximum mass while in the field). The slope of the relation at low mass (M⋆<10^10 Msun) is in all cases steeper than the same for the field. We find conflicting indicators of whether this difference in slope indicates an increasing or decreasing dark-to-stellar ratio; further modelling is required to reach a definitive conclusion. We also find evidence for the existence of a measurable age gradient in velocity, such that older subhalos have lower velocities than their younger peers. This opens the possibility that good quality redshifts of the lower mass galaxies of the Virgo cluster might provide additional constraints on the SHMR at high redshift and its evolution. Finally, we investigate the degree to which mergers, particularly major mergers, cause mixing of old and new material in halos, which has implications for the robustness of any implied radial age gradient. We find only a slight increase in mixing for major mergers over minor mergers, and little evidence for any large amount of mixing being induced by mergers of any ratio.

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Within the past decade, the CDM model has emerged as a standard paradigm of structure formation. While it has been very successful in explaining the structure of the Universe on large scales, on smaller (galactic) scales problems have surfaced. In this thesis, we investigate several of these problems in more detail. The thesis is organized as follows. In Chapter 1, we give a brief introduction about structure formation in the universe and discuss some of the problems being faced by the current CDM paradigm of galaxy formation.In Chapter 2, we analyze the angular momentum properties of virialized halos obtained from hydrodynamical simulations. We describe an analytical function that can be used to describe a wide variety of angular momentum distributions (AMDs), with just one parameter α. About $90-95% of halos turn out to haveα < 1.3, while exponential disks in cosmological halos would require 1.3 < α < 1.6. This implies that a typical halo in simulations has an excess of low angular momentum material as compared to that of observed exponential disks, a result which is consistent with the findings of earlier works.In Chapter 3, we perform controlled numerical experiments of merging galactic halos in order to shed light on the results obtained in cosmological simulations. We explore the properties of shape parameter α of AMDs and the spin ratio λGas/λDM in merger remnants and also their dependence on orbital parameters. We find that the shape parameter α is typically close to 1 for a wide range of orbital parameters, less than what is needed to form an exponential disk.The last chapter of the thesis (Chapter 4) is devoted to the analysis of phase space structure of dark matter halos. We first present a method to numerically estimate the densities of discretely sampled data based on a binary space partitioning tree. We implement an entropy-based node splitting criterion that results in a significant improvement in the estimation of densities compared to earlier work. We use this technique to analyze the phase space structure of halos.