Absence of HI in the Bootes Dwarf Spheroidal Galaxy

Abstract

Neutral hydrogen observations towards the Bootes dwarf spheroidal galaxy, a very low luminosity metal-poor Galactic satellite, were obtained using the Parkes Radio Telescope. We do not detect any HI in or around Bootes to a 3sigma upper limit of 180 Msun within the optical half light radius and 8000 Msun within 1.6 kpc. Its HI mass-to-light ratio is less than 0.002 Msun/Lsun, making Bootes one of the most gas-poor galaxies known. Either reionisation severely inhibited gas infall onto the proto-Bootes, or large amounts of gas have been removed by ram pressure and/or tidal stripping. Since Bootes 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 3sigma column density limit of 2 x 10^17 cm^-2. This suggests that either the leading gaseous tail is ionised, or the gas in the trailing tail was removed before the current tidal disruption of the parent dSph began.