Universit`adegli Studi di Trieste Department of Physics PhD Thesis On the Role of Cold Gas in Galaxy Evolution PhD Student: Supervisor: Anna Zoldan Dr. Gabriella De Lucia Contents Introduction2 1 Thesis context3 1.1 The role of gas in galaxy evolution . .3 1.2 Observational framework . .6 1.3 Theoretical framework . .9 1.4 Thesis objectives . 12 2 Background on cosmology and simulations 15 2.1 Cosmology and Large Scale Structure . 15 2.1.1 Cosmology and astrophysics: different views of the same Universe . 15 2.1.2 Cosmological principle and cosmological measurements . 16 2.1.3 The Big Bang and the baryons' origin . 18 2.1.4 Perturbations and growth of structures . 18 2.1.5 Statistical description of perturbations . 20 2.2 Cosmology and astrophysics in simulations . 23 2.2.1 Dark Matter only N-body cosmological simulations . 24 2.2.2 Halo and subhalos identification . 25 2.2.3 Halo Occupation Distribution . 26 2.2.4 Hydrodynamical simulations . 27 2.2.5 Semi-Analytic Models: state of the art . 28 3 Semi-Analytic Models 33 3.1 The Millennium Simulation . 33 3.2 SUBFIND and the merger trees . 34 3.3 The Semi-Analytic Model . 35 3.3.1 Re-ionization . 38 3.3.2 Cooling . 38 3.3.3 Star Formation . 39 3.3.4 Stellar feedback . 41 3.3.5 Metal enrichment . 42 3.3.6 AGN feedback . 43 3.3.7 Mergers and disk instabilities . 44 1 2 CONTENTS 4 HI-selected galaxies 47 4.1 Introduction . 47 4.2 Simulations and galaxy formation models . 49 4.2.1 Galaxy formation models . 49 4.2.2 Dark matter simulation and merger trees . 51 4.2.3 Light-cones algorithm . 51 4.3 Neutral hydrogen distribution and scaling relations . 54 4.3.1 Neutral hydrogen mass estimates . 54 4.3.2 Stellar and HI Mass Functions . 55 4.3.3 Scaling relations . 57 4.4 Two-point correlation function . 59 4.4.1 The projected correlation function . 60 4.4.2 Model predictions . 60 4.4.3 Halo occupation distribution . 62 4.5 The role of satellite galaxies . 66 4.6 Relations with the dark matter halo . 69 4.6.1 HI galaxy content and maximum halo mass . 69 4.6.2 HI galaxy content and halo spin . 71 4.7 Conclusions . 72 5 Sizes and specific angular momenta 77 5.1 Introduction . 77 5.2 The model . 79 5.2.1 The cosmological simulation and the merger tree . 80 5.2.2 The fiducial semi-analytic model . 80 5.3 The size-mass relation . 84 5.3.1 The size of galactic components . 89 5.3.2 Early Type Central and Satellite Galaxies . 91 5.3.3 The size-mass relation for X17 modifications . 92 5.4 The specific angular momentum . 94 5.4.1 Specific angular momentum determination . 94 5.4.2 Comparison with observations . 100 5.4.3 Specific angular momentum in X17CA3 and X17G11 . 102 5.5 Evolution of galaxies dynamics . 104 5.5.1 Morphological dependencies of the size-mass relation . 104 5.5.2 Disk instability in central and satellite galaxies . 107 5.5.3 The role of cold gas in the dynamical evolution . 109 5.5.4 Evolution in X17CA3 and X17G11 . 112 5.6 Conclusions . 113 6 Conclusions and future perspectives 117 6.1 Ongoing work: mock catalogs . 118 6.1.1 Mock catalogs for VANDELS . 119 6.1.2 Mock catalogs for SKA . 122 6.2 Future plans . 124 6.2.1 Dedicated cosmological simulations . 124 6.2.2 A consistent treatment for disk instabilities . 126 7 Bibliography 131 Chapter 1 Thesis context Galaxy evolution results from a complex interplay of physical processes, that act on a wide range of scales and times. One crucial piece of this complex puzzle is cold gas. Indeed, all physical processes occurring within galaxies, as well as those related to interactions with the external environment, directly influence (or are regulated by) the dynamics and the content of cold gas. In this work, I take advantage of state-of-the-art models of galaxy formation and evolution embedded in a cosmological context, and use them to explore the role of cold gas in the physical processes driving galaxy evolution. This approach allows a controlled analysis of individual galaxy histories, and of the contributions of the specific processes involved. I can, in this way, follow the origin of the observed relations, providing a physical interpretation for existing data and predictions for higher redshift. This Thesis work provides results useful for the interpretation of data col- lected by ongoing and upcoming surveys, such as those planned for ASKAP and MeerKAT, precursors of the Square Kilometre Array. In this Chapter I provide an overview of the Thesis context, describing the physical processes that rule the cold gas content evolution, and the current status of cold gas observations and related theoretical studies. In the final part of this chapter, I describe in detail the objectives of this study and the methods adopted. 1.1 The role of gas in galaxy evolution In the current standard picture of structure formation, galaxies are believed to form from gas condensation at the center of dark matter halos. The gas, trapped within the potential wells of collapsing structures, is shock heated to high temperatures. It can then cool and be accreted on the central regions, where it settles in a rotating disk. The accretion of gas on central galaxies takes place very efficiently in the case of accretion along filamentary, anisotropic structures (the so-called \cold mode"), or less efficiently when the accretion is isotropic and undergoes shocks when interacting with the medium (the so- called \hot mode"; Silk, 1977; Rees and Ostriker, 1977; Binney, 1977; White and Rees, 1978). The difference between the two accretion modes is set by the time scale at which the accreted gas cools, compared to the dynamical 3 4 CHAPTER 1. THESIS CONTEXT time of the halo. The latter is the typical time scale for a particle to cross 12 the halo. In the case of hot accretion, typical of massive halos (& 10 M ) at late times, the cooling time is usually longer than the dynamical time, and the shocked gas forms a quasi-static hot atmosphere that extends out to the virial radius. In cold accretion, instead, typical of small halos at early times, the cooling time is short compared to the dynamical one. In this case, time is not enough to form a quasi-static hot atmosphere, and the shocks occur very near to the halo center, followed by a very rapid cooling of the gas. This two-modes description of gas accretion was introduced in early semi-analytic models (White and Frenk, 1991a), and is included in all recently published models (e.g. Benson et al., 2001b; Croton et al., 2006). The validity of this cooling model has been confirmed by dedicated hydrodynamical simulations (Yoshida et al., 2002; Helly et al., 2003; Saro et al., 2010; Lu et al., 2011; Monaco et al., 2014, and references therein). Observationally, hot gas halos are observed through their X-ray emission around individual galaxies, groups and clusters of galaxies (Crain et al., 2010; Anderson and Bregman, 2011). The existence of cold inflows (gas accreting in cold mode) has not been confirmed observationally yet. Indirect evidences are inferred from observations of atomic hydrogen absorbers in proximity of high redshift galaxies (Giavalisco et al., 2011), or from high-velocity clouds around the Milky Way (Sancisi et al., 2008), whose interpretation is, however, not unequivocal. Due to conservation of angular momentum, cold gas is expected to settle in a rotating disk. There is general agreement that the hot gas halos statistically follow the same distribution of spin of DM halos (van den Bosch et al., 2002, 2003), and that the angular momenta of the hot gas and of the DM halo are strictly correlated (Sharma and Steinmetz, 2005). Hydrodynamical simulations show that the retention of the hot gas angular momentum in the cold gas disk depends on the assembly history of galaxies (Zavala et al., 2008). In the case of smooth accretion histories, typical of disk dominated galaxies, the angular momentum of the parent halo is transferred to the gaseous disk and preserved. In the case of bulge dominated galaxies, large fractions of the angular momentum are lost during the merger events that lead to the formation of the central bulge component. The dynamical state of the cold gas disk is important, because it determines its typical size and stability. The cold gas disk is the place where stars form. Proto-stars are observed to form in giant molecular clouds (GMC), that are cold (T 10 K), over-dense gas regions, of size 20-200 pc. GMCs are the ideal place for∼ the condensation of molecules, and, due to their high density, for star formation. The mechanisms driving GMCs formation and their collapse are still not well understood. The most accepted scenario is the fragmentation of the cold gas disk, and the collapse of over-densities onto the GMCs, in a top-down scenario (Elmergreen, 1975; Elmegreen et al., 1995). Other scenarios have been proposed, and this matter is still debated (see McKee and Ostriker, 2007, for a review of the GMC theoretical and observational framework). First studies on modeling of star formation were based on the direct corre- lation observed between the surface density of star formation and that of the cold gas disk, in a power law called Kennicutt-Schmidt relation (Schmidt, 1959; Kennicutt, 1998). More recently, high resolution maps of the spatial distribu- tion of star formation, atomic and molecular cold gas have become available for relatively large galaxy samples.