Bimodal Formation Time Distribution for Infall Dark Matter Halos
  We use a 200 $h^{-1}Mpc$ a side N-body simulation to study the mass accretion
history (MAH) of dark matter halos to be accreted by larger halos, which we
call infall halos. We define a quantity $a_{\rm nf}\equiv (1+z_{\rm
f})/(1+z_{\rm peak})$ to characterize the MAH of infall halos, where $z_{\rm
peak}$ and $z_{\rm f}$ are the accretion and formation redshifts, respectively.
We find that, at given $z_{\rm peak}$, their MAH is bimodal. Infall halos are
dominated by a young population at high redshift and by an old population at
low redshift. For the young population, the $a_{\rm nf}$ distribution is narrow
and peaks at about $1.2$, independent of $z_{\rm peak}$, while for the old
population, the peak position and width of the $a_{\rm nf}$ distribution both
increases with decreasing $z_{\rm peak}$ and are both larger than those of the
young population. This bimodal distribution is found to be closely connected to
the two phases in the MAHs of halos. While members of the young population are
still in the fast accretion phase at $z_{\rm peak}$, those of the old
population have already entered the slow accretion phase at $z_{\rm peak}$.
This bimodal distribution is not found for the whole halo population, nor is it
seen in halo merger trees generated with the extended Press-Schechter
formalism. The infall halo population at $z_{\rm peak}$ are, on average,
younger than the whole halo population of similar masses identified at the same
redshift. We discuss the implications of our findings in connection to the
bimodal color distribution of observed galaxies and to the link between central
and satellite galaxies.
