HIDES. I. The Population and Diversity of H I -rich Faint Dwarf Galaxies in the Hestia and Auriga Simulations

Abstract We present our investigation of H I -rich Faint Dwarf Galaxies in Simulations (HIDES), specifically using the hestia and Auriga simulations in this work. We select galaxies that are faint ( M g > −10) and contain sufficient H I ( M HI > 10 5 M ⊙ ), and identify 89 such objects, one of which is completely starless. Their demographics generally converge across simulations of different resolution, with median masses within the virial radius of M 200 ∼ 10 9.5 M ⊙ , M gas ∼ 10 7.4 M ⊙ , M HI ∼ 10 6.5 M ⊙ , and M * ∼ 10 5.6 M ⊙ ; low gas metallicity; little or no current star formation; a mean stellar age of ∼11 Gyr; a broadly Navarro–Frenk–White-shaped dark matter profile; and some of them can survive in dense environments as close as ∼300 kpc from a Milky Way–mass neighbor. We find a large scatter in their H I density profiles and M HI − M * relation, which cannot be fully modelled by their current halo mass or concentration as in the literature, but can be attributed to ram pressure stripping in dense environments, past mergers, or stellar feedback. In particular, close encounters with massive halos and dense environments can reshape the H I content, which may explain the asymmetric H I map of an intriguing observed analogue, Cloud-9. An empirical fit to their number density profile extended to 3.7 Mpc in constrained local volume simulations, n = 0.25 d MW / 1 Mpc − 1.4 Mpc − 3 , is also provided to aid observational forecasts. We conclude that both the mass assembly history and environmental history play a crucial role in the formation and subsequent diversity of these galaxies.