Andromeda IX: Properties of the Faintest M31 Dwarf Satellite Galaxy

We present WIYN observations of the recently discovered And IX dwarfspheroidal (dSph) satellite galaxy of M 31. Our data, obtained at a naturalseeing of 0.5'' and just reaching the horizontal branch level, confirm And IXas a dSph galaxy with a distance similar to M 31. A survey for carbon starsshows no evidence for an intermediate-age (1--10 Gyr) stellar population in AndIX. From the red giant branch we estimate a metallicity of roughly -2 dex.Combined with the tip of the red giant branch luminosity, this results in adistance of 735 kpc, placing And IX approximately 45 kpc from M 31. This faintdSph follows the relations between luminosity and metallicity, and luminosityand surface brightness defined by other Local Group dSph galaxies. The core andtidal radii are found to be 1.35' and 5.9', respectively. We conclude that AndIX -- despite its low luminosity -- might be an ordinary Local Group dSph anddiscuss implications for its formation from a once more massive, but strippedprogenitor or from an intrinsically low-mass seed.Comment: 5 pages, scheduled for ApJ v62