Merger history of central galaxies in Semi-Analytic Models of galaxy
  formation
  We investigate the dynamical evolution of galaxies in groups with different
formation epochs. Galaxy groups have been selected to be in different dynamical
states, namely dynamically old and dynamically young, which reflect their early
and late formation times, respectively, based on their halo mass assembly.
Brightest galaxies in dynamically young groups have suffered their last major
galaxy merger typically $\sim 2$ Gyr more recently than their counterparts in
dynamically old groups. Furthermore, we study the evolution of velocity
dispersion in these two classes and compare them with the analytic models of
isolated halos. The velocity dispersion of dwarf galaxies in high mass,
dynamically young groups increases slowly in time, while the analogous
dispersion in dynamically old high-mass groups is constant. In contrast, the
velocity dispersion of giant galaxies in low mass groups decreases rapidly at
late times. This increasing velocity bias is caused by dynamical friction, and
starts much earlier in the dynamically old groups. The recent {\sc Radio-SAGE}
model of galaxy formation suggests that radio luminosities of central galaxies,
considered to be tracers of AGN activity, are enhanced in halos that assembled
more recently, independent of the time since the last major merger.
