The contribution of HI-bearing ultra-diffuse galaxies to the cosmic number density of galaxies

TL;DR

This paper estimates the cosmic number density of HI-bearing ultra-diffuse galaxies (HUDs), finding they are a minor component of HI dwarfs but comparable in abundance to UDGs in groups and clusters, with implications for galaxy formation models.

Contribution

It provides the first estimate of the cosmic number density of HUDs and compares their properties and abundance to UDGs and galaxy formation models.

Findings

HUDs contribute at most 6% of HI-bearing dwarf galaxies.

Total cosmic number density of HUDs is approximately 1.5 x 10^-3 Mpc^-3.

HUDs have a steeply rising velocity width distribution towards low values.

Abstract

We estimate the cosmic number density of the recently identified class of HI-bearing ultra-diffuse sources (HUDs) based on the completeness limits of the ALFALFA survey. These objects fall in the range $8.5 < \log M_{\rm{HI}}/\rm{M_{\odot}} < 9.5$, have average $r$-band surface brightnesses fainter than 24 mag arsec$^{-2}$, half-light radii greater than 1.5 kpc, and are separated from neighbours by at least 350 kpc. We find that HUDs contribute at most 6% of the population of HI-bearing dwarfs, have a total cosmic number density of $(1.5 \pm 0.6) \times 10^{-3}$ $\rm{Mpc^{-3}}$, and an HI mass density of $(6.0 \pm 0.8) \times 10^{5}$ $\rm{M_{\odot}\,Mpc^{-3}}$. We estimate that this is similar to the total cosmic number density of UDGs in groups and clusters, and conclude that the relation between the number of UDGs hosted in a halo and the halo mass, must have a break below $M_{200} \sim 10^{12}$ $\rm{M_{\odot}}$ in order to account for the abundance of HUDs. The distribution of the velocity widths of HUDs rises steeply towards low values, indicating a preference for slow rotation rates. These objects have been absent from measurements of the galaxy stellar mass function owing to their low surface brightness. However, we estimate that due to their low number density, their inclusion would constitute a correction of less than 1%. Comparison with the Santa Cruz SAM shows that it produces HI-rich central UDGs that have similar colours to HUDs, but these are currently produced in much great a number. While previous results from this sample have favoured formation scenarios where HUDs form in high spin parameter halos, comparisons with the results of Rong et al. 2017, which invokes that formation mechanism, reveal that this model produces an order of magnitude more field UDGs than we observe in the HUD population.(Abridged)