This edited volume presents the current state of gas accretion studies from both observational and theoretical perspectives, and charts our progress towards answering the fundamental yet elusive question of how galaxies get their gas. Understanding how galaxies form and evolve has been a central focus in astronomy for over a century. These studies have accelerated in the new millennium, driven by two key advances: the establishment of a firm concordance cosmological model that provides the backbone on which galaxies form and grow, and the recognition that galaxies grow not in isolation but within a “cosmic ecosystem” that includes the vast reservoir of gas filling intergalactic space. This latter aspect in which galaxies continually exchange matter with the intergalactic medium via inflows and outflows has been dubbed the “baryon cycle”. The topic of this book is directly related to the baryon cycle, in particular its least well constrained aspect, namely gas accretion. Accretion is a rare area of astrophysics in which the basic theoretical predictions are established, but the observations have been as yet unable to verify the expectations. Accretion has long been seen around the Milky Way in so-called High Velocity Clouds, but detecting accretion even around nearby galaxies has proved challenging; its multi-phase nature requires sensitive observations across the electromagnetic spectrum for full characterization. A promising approach involves looking for kinematic signatures, but accretion signatures are often confused with internal motions within galaxies. Accretion studies therefore touch a wide range of astrophysical processes, and hence a wide cross-section of the astronomical community. As observational facilities are finally able to access the wavelength ranges and depths at which accretion processes may be manifest, the time is right to survey these multiple lines of investigation and determine the state of the field in accretion studies of the baryon cycle.

This thesis focuses primarily on how two important processes --- galaxymergers and gas accretion from the surrounding intergalactic medium ---affect the evolution of galaxies. Using post-starburst, or E+A, galaxies as a marker sample that undergoesa rapid transition from gas-rich star-forming galaxies to quiescent, passively-evolving E/S0s, we study what triggers E+A evolution andwhat E+A galaxies will become after the fading of their young stellarpopulation. With high resolution HST WFPC2/ACS imaging, we investigatetheir small and large scale properties, including their detailedmorphologies, bulge fractions, color gradients, scaling relationships, and newly formed star-clusters. 70% of E+A galaxies show disturbancesand tidal features indicating a merger origin and all their propertiesare either consistent with those of E/S0s or, if left to evolve passively, will become like those of early-types. Using cosmological simulations, we study hydrogen and helium gravitationalcooling radiation from gas accretion by young galaxies, finding thatobserving optically thin cooling lines such as HeII 1640 and hydrogenHalpha is critical in understanding the nature of galaxies forming viagas-accretion. To obtain an unbiased sample of Lyman alpha blobs thatwill allow us to follow-up their optically thin Halpha lines in the NIR, we conduct a blind, wide-field, narrow-band imaging survey for Lymanalpha blobs. After searching over 4.82 deg2̂, we discover four blobsthat we spectroscopically confirm to lie at z=2.3. The properties ofthese blobs are diverse: two blobs are X-ray-detected and have broadoptical emission lines (e.g., CIV) characteristic of AGN. The other50\% of blobs are not X-ray or optically-detected as AGN down tosimilar limits. The number density of the four blobs is extremely low,3̃ x 10-̂6 Mpc-̂3, comparable to that of galaxy clusters at similarredshifts. The two X-ray undetected blobs are separated by only70"(550 kpc) and have almost identical redshifts (corresponding to

Despite significant advances in the numerical modeling of galaxy formation and evolution, it is clear that a satisfactory theoretical picture of how galaxies acquire their baryons across cosmic time remains elusive. In this thesis we present a computational study which seeks to address the question of how galaxies get their gas. We make use of new, more robust simulation techniques and describe the first investigations of cosmological gas accretion using a moving-mesh approach for solving the equations of continuum hydrodynamics.

We now understand theoretically that galaxy evolution involves inflows of “cold” gas from the cosmic web. But corresponding models grow galaxies with amounts of baryons larger than observed galaxies. To overcome this issue, theorists focus on making star formation inefficient by massively blowing gas out of star-forming disks. I explore a different road, investigating processes that may moderate gas accretion onto disks. We present a phenomenological scenario where gas accretion flows - if it is shocked - become biphasic and, as a result, turbulent. In this framework, we show that the formation of warm, turbulent clouds, embedded in a hot component, occurs in the important mass range of ∼ 10^11 - 10^13 Msun, where the bulk of stars have formed in galaxies. Gas accreted from intergalactic filaments (IGF) may eventually lose coherence and mix with the ambient halo gas. The direct interaction between galaxy feedback and accretion streams is thus more likely. Moderating the accretion efficiency may help to alleviate a number of significant challenges in theoretical galaxy formation. Using the code Ramses, I performed a zoom-in simulation and extracted the results for a particular accreting IGF into a halo of ∼ 3 10^11 Msun at z ∼ 2. I investigate the gas thermodynamics and structuration, along and across the filament, with respect to dark matter. I study several key quantities as they evolve along the filament and derive a refined paradigm to study filaments, as well as consequences regarding their fate after entering a halo. I finally make use of these results to extrapolate gas processes that the simulation may not have captured accurately.

The evolution of galaxies is closely linked to the exchange of gas between their disk and the circumgalactic medium (CGM) - the massive, extended, diffuse halo of gas in which galaxies are embedded. Recent advances in high-resolution spectroscopy have enabled observers to firmly establish the key role played by the CGM in the life cycle of galaxies: it is the hiding place of at least half of all galactic baryons, acting as a massive reservoir that replenishes the supply of fuel for star formation via gas accretion onto the disk. However, this nearly-invisible halo gas is challenging to observe, and we are still missing a complete picture of its distribution, kinematics, and multiphase structure. In this thesis, I use the Milky Way as a case study to shed light on the nature of cool and warm CGM gas flows, taking advantage of the abundance of quasar and stellar sightlines which probe the Galactic CGM. In particular, I focus on the behavior of low-velocity gas, which is often overlooked by CGM studies because it is difficult to measure in isolation. I show that local CGM gas is predominantly inflowing, place constraints on the inflowing cloud sizes, and determine that these clouds lie close to the disk. I use a novel spectral differencing technique to correct for foreground absorption along sightlines through the Galactic halo, and present the first unobscured measurements of the Milky Way's extended low-velocity CGM. The results demonstrate that either the warm CGM does not have a spherical morphology, as is often assumed for star-forming galaxies, or that the Milky Way is not a typical star-forming galaxy. Finally, I find that inflow velocities are higher for warmer gas, suggesting a picture in which warm accreting gas slows down and cools as it approaches the disk. The mass accretion rates of these inflows indicate that a significant fraction of star-formation fuel may accrete onto the disk at low velocities.