H I in Local Group Dwarf Galaxies and Stripping by the Galactic Halo

We examine the H I content and environment of all of the Local Group dwarf galaxies (M _tot < 10¹⁰ M _⊙), including the numerous newly discovered satellites of the Milky Way and M31. All of the new dwarfs, with the exception of Leo T, have no detected H I. The majority of dwarf galaxies within ~270 kpc of the Milky Way or Andromeda are undetected in H I (<10⁴ M _⊙ for Milky Way dwarfs), while those further than ~270 kpc are predominantly detected with masses ~10⁵ to 10⁸ M _⊙. Analytical ram-pressure arguments combined with velocities obtained via proper motion studies allow for an estimate of the halo density of the Milky Way at several distances. This halo density is constrained to be greater than 2× 10^-4-3 × 10^-4 cm^-3 out to distances of at least 70 kpc. This is broadly consistent with theoretical models of the diffuse gas in a Milky Way-like halo and is consistent with this component hosting a large fraction of a galaxy's baryons. Accounting for completeness in the dwarf galaxy count, gasless dwarf galaxies could have provided at most 2.1 × 10⁸ M _⊙ of H I gas to the Milky Way, which suggests that most of our Galaxy's star formation fuel does not come from accreted small satellites in the current era.