Absence of HI in the Bootes Dwarf Spheroidal Galaxy.pdf

a r X i v : a s t r o - p h / 0 6 1 1 6 9 3 v 1 2 1 N o v 2 0 0 6 Mon. Not. R. Astron. Soc. 000, 000–000 (0000) Printed 5 February 2008 (MN LATEX style file v2.2) Absence of H I in the Boo?tes Dwarf Spheroidal Galaxy Jeremy Bailin1 Alyson Ford1,2 1Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Mail H39, PO Box 218, Hawthorn, Victoria, 3122, Australia; jbailin@astro.swin.edu.au 2Australia Telescope National Facility, CSIRO, P. O. Box 76, Epping NSW 1710, Australia Accepted 2006 November 21. Received 2006 November 14; in original form 2006 October 24 ABSTRACT Neutral hydrogen (H i) observations towards the Boo?tes dwarf spheroidal (dSph) galaxy (hereafter Boo?tes), a very low luminosity metal-poor Galactic satellite, were obtained using the Parkes Radio Telescope. We do not detect any H i in or around Boo?tes to a 3σ upper limit of 180 M⊙ within the optical half light radius and 8000 M⊙ within 1.6 kpc. Its H i mass-to-light ratio is less than 0.002 M⊙/L⊙, making Boo?tes one of the most gas-poor galaxies known. Either reionisation severely inhibited gas infall onto the proto-Boo?tes, or large amounts of gas have been removed by ram pres- sure and/or tidal stripping. Since Boo?tes 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 H i 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 ionised, or the gas in the trailing tail was removed before the current tidal disruption of the parent dSph began. Key words: galaxies: individual: Boo dSph, Sgr dSph – galaxies: dwarf – Local Group – galaxies: ISM – galaxies: evolution 1 INTRODUCTION A