Gas-rich Ultra-diffuse Galaxies Are Originated from High Specific Angular Momentum

TL;DR

This study investigates the formation of gas-rich ultra-diffuse galaxies, revealing that high specific angular momentum in gas plays a key role in their unique properties and distinguishing them from other galaxy types.

Contribution

It introduces a semi-analytic model showing that high gas specific angular momentum explains the distinct features of ultra-diffuse galaxies, advancing understanding of their formation mechanisms.

Findings

Gas-rich ultra-diffuse galaxies are not failed $L^{ ext{star}}$ galaxies or dark matter deficient.

These galaxies have higher baryonic mass fraction and specific angular momentum compared to dwarf galaxies.

High gas specific angular momentum explains their large sizes and low stellar densities.

Abstract

Ultra-diffuse galaxies, characterized by comparable effective radii to the Milky Way but possessing 100-1,000 times fewer stars, offer a unique opportunity to garner novel insights into the mechanisms governing galaxy formation. Nevertheless, the existing corpus of observational and simulation studies has not yet yielded a definitive constraint or comprehensive consensus on the formation mechanisms underlying ultra-diffuse galaxies. In this study, we delve into the properties of ultra-diffuse galaxies enriched with neutral hydrogen using a semi-analytic method, with the explicit aim of constraining existing ultra-diffuse galaxy formation models. We find that the gas-rich ultra-diffuse galaxies are statistically not failed $L^{\star}$ galaxies nor dark matter deficient galaxies. In statistical terms, these ultra-diffuse galaxies exhibit comparable halo concentration, but higher baryonic mass fraction, as well as higher stellar and gas specific angular momentum, in comparison to typical dwarf galaxy counterparts. Our analysis unveils that higher gas specific angular momentum serves as the underlying factor elucidating the observed heightened baryonic mass fractions, diminished star formation efficiency, expanded stellar disk sizes, and reduced stellar densities in ultra-diffuse galaxies. Our findings make significant contributions to advancing our knowledge of ultra-diffuse galaxy formation and shed light on the intricate interplay between gas dynamics and the evolution of galaxies.