  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.
