Bimodal Formation Time Distribution for Infall Dark Matter Halos

TL;DR

This study reveals a bimodal distribution in the formation times of infall dark matter halos, linked to different accretion phases, with implications for galaxy evolution and color bimodality.

Contribution

The paper uncovers a bimodal formation time distribution for infall halos and connects it to distinct accretion phases, a feature not seen in previous models or the overall halo population.

Findings

Infall halos show a bimodal distribution in formation times at fixed peak redshift.

Young and old populations of infall halos correspond to fast and slow accretion phases.

The bimodal distribution is absent in the overall halo population and extended Press-Schechter models.

Abstract

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.