Gas-rich dwarf spheroidals

We present evidence that 10 of the 21 dwarf spheroidal galaxies (dSph and dSph/dIrr) in the Local Group are associated with large reservoirs of atomic gas, reservoirs that are, in some cases, larger than the stellar mass. The gas is sometimes found at a large distance (similar to 10 kpc) from the center of a galaxy and is not necessarily centered on it. Similarly large quantities of ionized gas could be hidden in these systems as well. The properties of some of the gas reservoirs are similar to the median properties of the high-velocity clouds (HVCs); two of the H I reservoirs are cataloged HVCs. The association of the H I with the dwarf spheroidals might thus provide a link between the HVCs and stars. We show that the H I content of the Local Group dSphs and dIrrs exhibits a sharp decline if the galaxy is within 250 kpc of either the Milky Way or M31. This can be explained if both galaxies have a sufficiently massive X-ray-emitting halo that produces ram-pressure stripping if a dwarf ventures too close to either giant spiral. We also investigate tidal stripping of the dwarf galaxies and find that although it may play a role, it cannot explain the apparent total absence of neutral gas in most dSph galaxies at distances less than 250 kpc. For the derived mean density of the hot gas, n(o) = 2.5 x 10(-5) cm(-3), ram-pressure stripping is found to be more than an order of magnitude more effective in removing the gas from the dSph galaxies. The hot halo, with an inferred mass of 1 x 10(10) M., may represent a reservoir of similar to 10(3) destroyed dwarf systems-either HVCs or true dwarf galaxies similar to those we observe now.