The undiscovered ultra-diffuse galaxies of the Local Group

TL;DR

This study predicts a significant population of isolated ultra-diffuse galaxies in the Local Group using hydrodynamic simulations, indicating many remain undiscovered and could be detected with future surveys.

Contribution

First to use hydrodynamic constrained simulations to predict the number and properties of UDGs in the Local Group, including their detectability in current and future surveys.

Findings

Approximately 12±3 isolated UDGs within 2.5 Mpc of the Local Group.

2+2−1 UDGs are detectable in SDSS footprint.

Almost all UDGs in the Local Group field could be observable in future all-sky surveys.

Abstract

Ultra-diffuse galaxies (UDGs) are attractive candidates to probe cosmological models and test theories of galaxy formation at low masses; however, they are difficult to detect because of their low surface brightness. In the Local Group a handful of UDGs have been found to date, most of which are satellites of the Milky Way and M31, and only two are isolated galaxies. It is unclear whether so few UDGs are expected. We address this by studying the population of UDGs formed in hydrodynamic constrained simulations of the Local Group from the HESTIA suite. For a Local Group with a total enclosed mass $M_{\rm LG}\!\left(< 2.5\, {\rm Mpc}\right)=8\times10^{12}\, {\rm M_\odot}$, we predict that there are $12\pm3$ isolated UDGs (68% confidence) with stellar masses $10^6 \leq M_\ast\, /\, {\rm M_\odot} < 10^9,$ and effective radii $R_{\rm e} \geq 1.5\, {\rm kpc}$, within 2.5 Mpc of the Local Group, of which $2^{+2}_{-1}$ (68% confidence) are detectable in the footprint of the Sloan Digital Sky Survey (SDSS). Accounting for survey incompleteness, we find that almost the entire population of UDGs in the Local Group field would be observable in a future all-sky survey with a depth similar to the SDSS, the Dark Energy Survey, or the Legacy Survey of Space and Time. Our results suggest that there is a population of UDGs in the Local Group awaiting discovery.