On the Role of Cold Gas in Galaxy Evolution

My PhD thesis is focused on the role of cold gas in galaxy evolution. The main tools I have used for my research are state-of-the- art semi-analytic models (SAMs), coupled to high-resolution N-body simulations of cosmological volumes. In the first part of my thesis, I analyze results from six different SAMs, all based on the same cosmological simulation, against the latest observational data on the neutral hydrogen (HI) content of galaxies in the local Universe. Only one of the models considered includes a self-consistent partition of the cold gas into HI and molecular hydrogen (H2), and a star formation based on the H2 surface density. In the others, I partition the cold gas in post-processing, following Obreschkow et al.(2009). All models reproduce the observed HI mass function and the HI-stellar mass relation, with the exception of one model, that has low cold gas in high mass galaxies, due to a too efficient feedback. I calculate the 2-point correlation function (2PCF) of model galaxies in different HI mass bins, and compare model predictions with data, finding a good agreement for the HI-rich galaxies. Satellites play a fundamental role in the 2PCF determination, and represent the largest fraction of HI poor galaxies. I show that the HI content of these galaxies is influenced by hot gas stripping, but also by the self-regulation between star formation and stellar feedback. Finally, I demonstrate that although the HI-halo mass relation depends on the specific model considered, the HI-spin parameter relation is similar for all models, with HI-rich galaxies settled preferentially in high spin halos. This is due to the dependence of the cold gas disk radius on the halo spin. In the second part of my thesis, I use the SAM developed in Trieste to study the dynamical properties of galaxies. I include a treatment for gas dissipation during mergers to obtain realistic bulge sizes. I calculate the half-mass radius (R50) and the specific angular momentum (j) of the galactic stellar components, mimicking observational methods. The R50-mass and j-mass relations show a good agreement with data. For the latter, I find a dependence on the cold gas content, with high cold gas corresponding to high specific angular momenta. I demonstrate that this is due to the fact that gas-rich galaxies grow their stars gradually, acquiring j from the cold gas until recent times. I evaluate how these relations are affected by different prescriptions for (i) cold gas accretion and (ii) stellar feedback. I find that the former has little influence on the local relation because it is more effective at high redshift and effectively washed out by subsequent evolution. A different stellar feedback scheme can instead influence the relation significantly. For the work carried out in the first part of my thesis, I have developed a light-cone software that I am now using to produce dedicated mocks.