Galaxy Formation in Standard and Modified Gravity

Abstract

This first part of this thesis examines the impact of changes to gravity on the formation of dark matter haloes and galaxies. We use two N-body simulations, one assuming general relativity (GR) and the other the Hu-Sawicki form of f(R) gravity, to investigate the concentration-formation time relation of dark matter haloes. At fixed mass, haloes in modified gravity are more concentrated than those in GR, especially at low masses and redshifts. We find a clear difference between halo concentrations and assembly histories in modified gravity and GR. We incorporate these changes to the properties of dark matter haloes, along with halo merger histories from a N-body simulation of f(R) gravity to build, for the first time, a partial modified gravity version of GALFORM. We concentrate on a model prediction that should, in principle, show a clear trace of modified gravity, the halo occupancy of emission line galaxies. In the second part of the thesis we present the first application of a variance-based sensitivity analysis (SA) to a galaxy formation model. We perform a multi-parameter exploration of GALFORM to compute how sensitive the present-day K-band luminosity function is to varying different model parameters. We first demonstrate the usefulness of the SA approach by varying just two model parameters, one which controls supernova feedback and the other the heating of gas by AGN. The SA analysis matches our physical intuition regarding how these parameters affect the predictions for different parts of the galaxy luminosity function. Our study marks a much needed step away from the traditional “one-at-a-time” parameter variation, often used in this area, and improves the transparency of semi-analytical models.