The Semi-Analytic Models of Neutral and Molecular Gas in Galaxies

Interstellar gas is important baryonic components in galaxies,in which molecular gas(mainly molecular hydrogen H_2) and neutral gas(mainly atomic hydrogen HI) have great effect on the physical processes in each galaxy.In this thesis,we develop physical models about neutral gas and molecular gas in galaxy disks based on previous semi-analytic models (SAMs) of galaxy formation,and we use the model to study the role of neutral and molecular gas in galaxy formation and evolution.Our model is a modified version of the SAMs named L-Galaxies by Munich Group in Max-Planck Institute for Astrophysics.According to the methods used in the models of galaxy chemical evolution,we divide each galaxy disk into a series of concentric rings in SAMs.Then we trace the physical processes in each ring on galaxy disks,such as gas infall,conversion of atomic gas to molecular gas,star formation, metal enrichment,supernova reheating etc,and the evolution in each ring is independent.In our model,we assume that the surface density profile of infalling gas is exponential in each time step,and the newly infalling gas is directly superposed on the gas radial profile already existing on the disk.The scale radius r_d is proportional to the virial radius of the halo times its spin parameterλ.We adopt two kinds of prescriptions for molecular gas formation processes in our models:one is based on the analytic models by Krumholz et al.,in which the molecular gas fraction is a function of local gas surface density and local gas metallicity.The other one is a prescription where the H_2 fraction is determined by the pressure of interstellar medium,in which the molecular gas fraction is related to gas surface density and stellar mass surface density according to the approximation by Obreschkow et al.Since star formation processes happen in giant molecular clouds,and the observational results from Leroy et al. show that star formation rate surface density is approximately proportional to molecular gas surface density,we use H_2 surface density related star formation law in our models based on the molecular gas formation prescriptions.