The pre-infall bias of subhalos

TL;DR

This paper reveals a pre-infall bias in dark matter subhalos, showing they are more massive and concentrated before falling into larger halos, with a theoretical model capturing this effect.

Contribution

It introduces a new theoretical framework describing the pre-infall bias of subhalos using an extended Press-Schechter approach with a scale-factor dependent collapse barrier.

Findings

Progenitor mass functions of future subhalos are shifted toward higher masses.

Halos before infall are 10-15% more centrally concentrated.

The bias increases as infall approaches, with a specific scale-factor dependence.

Abstract

Dark matter halos destined to fall into a more massive host differ from typical field halos of the same mass even before infall. In cosmological simulations, we find that the progenitor mass functions of these "future subhalos" are systematically shifted toward higher masses, with the shift growing as infall approaches. The bias takes a compact form within extended Press-Schechter theory: the collapse barrier is multiplied by a function $\beta(D/D_\mathrm{infall},a)$, where $D$ is the linear growth factor at scale factor $a$ and $D_\mathrm{infall}$ is the growth factor at infall. We find $\beta(x,a)=(1-x)^{1.20+0.14a}$ for the $M_{200\mathrm{c}}$ mass definition and $(1-x)^{1.20+0.05a}$ for $M_{200\mathrm{m}}$; the explicit scale-factor dependence captures the late-time influence of dark energy. One consequence is that halos shortly before infall are 10-15% more centrally concentrated than typical field halos of the same mass.