X iv : a st ro - p h / 96 08 18 9 v 1 2 8 A ug 1 99 6 The Absence of Diffuse Gas around the Dwarf Spheroidal Galaxy

We have obtained spectra of three QSO/AGNs with the GHRS aboard the Hubble Space Telescope to search for absorption from low column density gas in the halo of the dwarf spheroidal (dSph) galaxy Leo I. The probe sightlines pass 2.1, 3.7, and 8.1 kpc from the center of the galaxy, but no C IV, Si II, or Si IV absorption is found at the velocity of Leo I. We suggest that the column density of hydrogen which exists within 2− 4 kpc of the galaxy is NH ∼< 10 15−16 cm; this estimate leads to a limiting gas mass of < 10 and < 10M⊙ around Leo I, for simple models in which gas is distributed as a spherical halo, or as a shell, respectively. Although we cannot rule out the possibility that a significant fraction of hot gas exists along the lines of sight, in an ionization state higher than C IV or Si IV absorption lines would reveal, the results leave us with the impression that Leo I is a dSph galaxy which is isolated and inactive at the present epoch, a conclusion at odds with many theories of dwarf galaxy formation and evolution. Based on observations obtained with the NASA/ESA Hubble Space Telescope, obtained at STScI, which is operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Aeronautics and Space Administration, NAS5-26555. Present address: ST-ECF, ESO, Karl-Schwarzschild-Strasse 2, D-85748, Garching bei München, Germany Currently on leave of absence at the Departmento de Astronomia, Universidad de Guanajuato, Apdo. Postal 144, Guanajuato, C. P. 36000, Mexico