The Dearth of Neutral Hydrogen in Galactic Dwarf Spheroidal Galaxies
  We present new upper limits on the neutral hydrogen (HI) content within the
stellar half-light ellipses of 15 Galactic dwarf spheroidal galaxies (dSphs),
derived from pointed observations with the Green Bank Telescope (GBT) as well
as Arecibo L-band Fast ALFA (ALFALFA) survey and Galactic All-Sky Survey (GASS)
data. All of the limits Mlim are more stringent than previously reported
values, and those from the GBT improve upon contraints in the literature by a
median factor of 23. Normalizing by V-band luminosity Lv and dynamical mass
Mdyn, we find Mlim/Lv ~ 10^{-3} Mo/Lo and Mlim/Mdyn ~ 5 x 10^{-5}, irrespective
of location in the Galactic halo. Comparing these relative HI contents to those
of the Local Group and nearby neighbor dwarfs compiled by McConnachie, we find
that the Galactic dSphs are extremely gas-poor. Our HI upper limits therefore
provide the clearest picture yet of the environmental dependence of the HI
content in Local Volume dwarfs. If ram pressure stripping explains the dearth
of HI in these systems, then orbits in a relatively massive Milky Way are
favored for the outer halo dSph Leo I, while Leo II and Canes Venatici I have
had a pericentric passage in the past. For Draco and Ursa Minor, the
interstellar medium mass that should accumulate through stellar mass loss in
between pericentric passages exceeds Mlim by a factor of ~30. In Ursa Minor,
this implies that either this material is not in the atomic phase, or that
another mechanism clears the recycled gas on shorter timescales.
