The virial mass distribution of ultra-diffuse galaxies in clusters and groups

TL;DR

This study constrains the distribution of halo masses hosting ultra-diffuse galaxies in clusters and groups, revealing most UDGs reside in low-mass haloes and challenging models with narrow mass distributions.

Contribution

It provides the first constraints on the virial mass distribution of satellite UDGs using observed abundances and DM subhalo functions, highlighting a broad mass distribution.

Findings

Most UDGs are hosted by low-mass haloes.

A broad UDG mass distribution is required to match observed abundances.

Satellite UDG abundance may deviate in low-mass hosts.

Abstract

We use the observed abundances of ultra-diffuse galaxies (UDGs) in clusters and groups and \Lambda CDM subhalo mass functions to put constraints on the distribution {of present-day halo masses of satellite} UDGs. If all of the most massive subhaloes in the cluster host a UDG, UDGs occupy all subhaloes with \log M_{sub}/M_\odot\gtrsim11. For a model in which the efficiency of UDG formation is higher around some characteristic halo mass, higher fractions of massive UDGs require larger spreads in the UDG mass distribution. In a cluster with a virial mass of 10^{15}M_\odot, the 90% upper limit for the fraction of UDGs with \log M_{sub}/M_\odot>12 is 7%, occupying 70% of all cluster subhaloes above the same mass. To reproduce the observed abundances, however, the mass distribution of satellite UDGs has to be broad, with >30% having \log M_{sub}/M_\odot<10.9. This strongly supports that UDGs are part of a continuous distribution in which a majority are hosted by low mass haloes. The abundance of satellite UDGs may fall short of the linear relation with the cluster/group mass M_{host} in low-mass hosts, \log M_{host}/M_\odot\sim 12. Characterising these deviations -- or the lack thereof -- will allow for stringent constraints on the UDG virial mass distribution.