The Formation History of Subhalos and the Evolution of Satellite
  Galaxies
  Satellites constitute an important fraction of the overall galaxy population
and are believed to form in dark matter subhalos. Here we use the cosmological
hydrodynamic simulation TNG100 to investigate how the formation histories of
subhalos affect the properties and evolution of their host galaxies. We use a
scaled formation time ($a_{\rm nf}$) to characterize the mass assembly
histories of the subhalos before they are accreted by massive host halos. We
find that satellite galaxies in young subhalos (low $a_{\rm nf}$) are less
massive and more gas rich, and have stronger star formation and a higher
fraction of ex situ stellar mass than satellites in old subhalos (high $a_{\rm
nf}$). Furthermore, these low $a_{\rm nf}$ satellites require longer timescales
to be quenched as a population than the high $a_{\rm nf}$ counterparts. We find
very different merger histories between satellites in fast accretion (FA,
$a_{\rm nf}<1.3$) and slow accretion (SA, $a_{\rm nf}>1.3$) subhalos. For FA
satellites, the galaxy merger frequency dramatically increases just after
accretion, which enhances the star formation at accretion. While, for SA
satellites, the mergers occur smoothly and continuously across the accretion
time. Moreover, mergers with FA satellites happen mainly after accretion, while
a contrary trend is found for SA satellites. Our results provide insight into
the evolution and star formation quenching of the satellite population.
