The impact of cooling and feedback on disc galaxies

In the standard picture of galaxy formation, mass and angular momentum are the two main parameters that determine the properties of disc galaxies. The details of how the gas inside dark matter haloes is transformed into a luminous disc, however, depend strongly on the physics of star formation, feedback and cooling, which are poorly understood. The efficiencies of these astrophysical processes are ultimately responsible for setting the galaxy mass fractions f(gal) defined as the ratio of galaxy mass (disc plus bulge) to total virial mass. Therefore, if we could somehow determine f(gal) (M-vir) from observations of the luminous component of disc galaxies, this would allow us to put stringent constraints on the efficiencies of cooling and feedback. This, however, requires estimating the total virial mass, which is a delicate problem. In this paper we use detailed, analytical models for the formation of disc galaxies to investigate the impact that cooling and feedback have on their structural properties. In particular, we investigate which observables extracted directly from the models are best suited as virial mass estimators, and to what extent they allow the recovery of the model input parameters regarding the feedback and cooling efficiencies. Contrary to naive expectations, the luminosities and circular velocities of disc galaxies are extremely poor indicators of total virial mass. Instead, we show that the product of disc scalelength and rotation velocity squared yields a much more robust virial mass estimate, which allows a fairly accurate recovery of the galaxy mass fractions as a function of the virial mass. We also show that feedback can cause a narrow correlation between f(gal) and the halo spin parameter, as found recently by van den Bosch, Burkert & Swaters from an analysis of dwarf galaxy rotation curves. Finally, we investigate the impact that cooling and feedback have on the colours, metallicities, star formation histories and Tully-Fisher relation of disc galaxies.