Are Gas-rich Ultra-diffuse Galaxies and Field Dwarfs Distinct?

TL;DR

This study compares gas-rich ultra-diffuse galaxies and classical dwarfs through observations and simulations, finding they are not fundamentally different populations in terms of gas content, size, or star formation.

Contribution

It provides a comprehensive comparison of observed and simulated gas-rich UDGs and dwarfs, showing they are similar in key properties and challenging the idea they are distinct classes.

Findings

Gas richness correlates with effective radius in both populations.

Simulated and observed properties of UDGs and dwarfs are consistent.

Gas-rich UDGs and dwarfs are not distinct populations.

Abstract

We explore the differences in gas-rich field Ultra-Diffuse Galaxies (UDGs) and classical dwarf galaxies using an extensive atomic gas (HI) follow-up survey of optically-selected UDG candidates from the Systematically Measuring Ultra-Diffuse Galaxies (SMUDGes) catalogue. We also compare the SMUDGes-HI observations with two state-of-the-art cosmological hydrodynamical simulations: Numerical Investigation of a Hundred Astrophysical Objects (NIHAO), where UDGs form through a series of bursty star formation episodes and Romulus25, in which UDGs occupy dark matter halos with high spins as a result of major mergers. Although the suggested formation scenarios for UDGs within these simulations are different, the present-day HI masses $M_{HI}$, stellar masses $M_*$, and star formation rates $SFR$ of simulated galaxies are qualitatively and quantitatively consistent with each other and with the observed SMUDGes-HI sample. We find that when controlling for $M_*$, there is a positive correlation between the gas richness $M_{HI}$/$M_*$ and the effective optical radius $R_{eff}$ , and that this trend is not different between the UDG and dwarf populations, within the measured scatter. Taken together, our results suggest that gas-rich, star-forming UDGs and dwarfs are not distinct galaxy populations, either observationally or in simulations.