The environment and H? content of galaxies

In this thesis we use both observations and modelling to explore the gas content of galaxies. We use the L-Galaxies semi-analytic model to simultaneously match the H? and stellar mass properties of model galaxies to observations using Markov Chain Monte Carlo methods. We add the observed H? mass function as an extra model constraint and successfully match the H? and stellar mass functions. However, the fit to the star formation properties has been weakened compared to without the H? constraint. We suggest that this problem may be partially resolved by forming stars out of only H2 gas instead of the total cold gas. The environment in which a galaxy resides can affect its evolution. We use the counts in a fixed size cylinder method to estimate 3 environment measures for the GAMA survey. We use density and edge corrections to allow us to calculate estimates for every galaxy out to z = 0.4 in our flux limited sample. We then use these estimates to examine the effect of environment on the luminosity and stellar mass functions. Using H? observations of the groups and galaxies in the ALFALFA and GAMA surveys we calculate H? masses using the stacking technique. The use of the stacking technique has allowed us to exploit survey data that would not otherwise be possible. We stack galaxies in halo mass bins and calculate the H? to halo mass fraction as a function of halo mass. We see a steady decline in the H? fraction as we move to higher mass halos. These are the highest density environments where there is less cold gas. Combining this fraction with the halo mass function we are able to calculate a lower limit value for OH? of 1.8 ± 0.39 x 10-4h-¹.