The dependence of AGN activity on stellar and halo mass in Semi-Analytic
  Models
  AGN feedback is believed to play an important role in shaping a variety of
observed galaxy properties, as well as the evolution of their stellar masses
and star formation rates. In particular, in the current theoretical paradigm of
galaxy formation, AGN feedback is believed to play a crucial role in regulating
the levels of activity in galaxies, in relatively massive halos at low
redshift. Only in recent years, however, has detailed statistical information
on the dependence of galaxy activity on stellar mass, parent halo mass and
hierarchy has become available. In this paper, we compare the fractions of
galaxies belonging to different activity classes (star-forming, AGN and radio
active) with predictions from four different and independently developed
semi-analytical models. We adopt empirical relations to convert physical
properties into observables (H_alpha emission lines, OIII line strength and
radio power). We demonstrate that all models used in this study reproduce the
overall distributions of galaxies belonging to different activity classes as a
function of stellar mass and halo mass: star forming galaxies and the strongest
radio sources are preferentially associated with low-mass and high-mass
galaxies/halos respectively. However, model predictions differ from
observational measurements in a number of ways. All models used in our study
predict that almost every >1.e12 Msun dark matter halo and/or >1.e11 Msun
galaxy should host a bright radio source, while only a small fraction of
galaxies belong to this class in the data. In addition, radio brightness is
expected to depend strongly on the mass of the parent halo mass in the models,
while strong and weak radio galaxies are found in similar environments in data.
Our results highlight that the distribution of AGN as a function of stellar
mass provides one of the most promising discriminants between different gas
accretion schemes.
