The capture of halo material by orbiting subhaloes

TL;DR

This study uses high-resolution simulations to evaluate the efficiency of dark matter subhaloes capturing material from their host halos, finding the process is too inefficient for detection.

Contribution

It provides the first quantitative estimate of the capture efficiency of subhaloes, showing it is too low for observational detection of initially starless subhaloes.

Findings

Captured mass is less than 0.01% of subhalo mass.

Captured material is less concentrated than original subhalo material.

Capture is insufficient for detection of starless subhaloes.

Abstract

When a dark matter halo falls into a more massive object and becomes a subhalo, it typically loses much of its mass through tidal stripping. The reverse process is also possible in principle. The subhalo may gravitationally capture material from its host. If sufficiently efficient, this process could make an initially starless subhalo visible. We use high-resolution N-body simulations to estimate the efficiency of capture. We find that after an extended period orbiting within its host, at most $\sim 10^{-4}$ of a subhalo's remaining mass has been acquired since infall. This captured material is less concentrated to subhalo centre than material retained from before infall. It is also very much less abundant than host material that is instantaneously passing through the subhalo on almost unperturbed orbits. Captured stars are not sufficiently spatially concentrated to be distinguished from the dominant background of "field" stars, and their concentration in velocity space is no greater than that of typical stellar streams in the halo. Unfortunately, stellar capture is not efficient enough to allow initially starless low-mass subhaloes to be detected.