Sagittarius: the Nearest Dwarf Galaxy

We have discovered a new Galactic satellite galaxy in the constellation of Sagittar-ius. The Sagittarius dwarf galaxy subtends an angle of 10 degrees on the sky, lies at a distance of 24 kpc and is comparable in size and luminosity to the largest dwarf spheroidal, Fornax. The new galaxy has many features in common with the other eight Galactic dwarf spheroidal systems: including an extended low density spatial structure; a well populated red horizontal branch with a blue extension; and a substantial carbon star population. In terms of stellar populations it most closely resembles the Fornax dwarf, having a strong intermediate age stellar component and evidence of a metallicity spread. Sagittarius is the nearest galaxy known and currently lies only 16 kpc from the centre of the Milky Way. Isodensity maps show it to be markedly elongated along a direction pointing towards the Galactic centre and suggest that it has been tidally distorted. The close proximity to the Galactic centre, the morphological appearance and the radial velocity of 140 km/s indicate that this system must have undergone at most very few close orbital encounters with the Milky Way. It is currently undergoing strong tidal disruption prior to being integrated into the Galaxy. We nd that at least some of the four globular clusters, M54, Arp 2, Ter 7 and Ter 8, are associated with the Sagittarius dwarf galaxy, and will probably share the fate of their progenitor. The Sagittarius dwarf galaxy was found serendipitously using a combination of UK Schmidt Telescope sky survey plates, the APM automatic plate measuring facility and the Anglo Australian Telescope multiibre spectrograph, AUTOFIB. 1 INTRODUCTION Galaxies of the dwarf spheroidal class (dSph) are among the smallest and faintest known. Eleven such objects have been discovered so far near the local group spirals: three are companions to M31, and a further eight surround the Milky Way. The rst seven of the Milky Way satellite galaxies were discovered by visual inspection of photographic plates; whilst the eighth was found from machine measurements of a photographic plate (Irwin et al. 1990). Their surface brightness is so low that all have been discovered from the concentration of resolved stars over and above that of the foreground component of our own galaxy. These dwarfs, also known as Sculptor-type systems, or dwarf elliptical galaxies, range in diameter from 1-5 kpc, subtend angles of up to several degrees on the sky, have …