The absence of HI in the Boötes dwarf spheroidal galaxy

Neutral hydrogen (HI) observations towards the Boötes dwarf spheroidal (dSph) galaxy (hereafter Boötes), a very low-luminosity metal-poor Galactic satellite, have been obtained using the Parkes Radio Telescope. We do not detect any HI in or around Boötes to a 3σ upper limit of 180 M⊙ within the optical half-light radius and 8000 M⊙ within 1.6 kpc. Its HI mass-to-light ratio is less than 0.002 M⊙/L⊙, making Boötes one of the most gas-poor galaxies known. Either reionization severely inhibited gas infall on to the proto-Boötes, or large amounts of gas have been removed by ram pressure and/or tidal stripping. Since Boötes lies on the mass-metallicity fundamental line, this relation and the inefficiency of star formation at the faintest end of the galaxy luminosity function must be partly driven, or at least not disrupted, by extreme gas loss in such low-luminosity galaxies. We also do not detect any HI associated with the leading tidal tail of the Sagittarius dSph galaxy, which fortuitously passes through the observed field, to a 3σ column density limit of 2 × 10¹⁷ cm^-2. This suggests that either the leading gaseous tail is ionized, or the gas in the trailing tail was removed before the current tidal disruption of the parent dSph began.