Andromeda IX: Properties of the faintest M31 dwarf satellite galaxy

We present WIYN observations of the recently discovered And IX dwarf spheroidal (dSph) satellite galaxy of M31. Our data, obtained at a natural seeing of 0.'' 5 and just reaching the horizontal-branch level, confirmthat And IX is a dSph galaxy with a distance similar to M31. A survey for carbon stars shows no evidence for an intermediate-age ( 1 - 10 Gyr) stellar population in And IX. From the red giant branch we estimate ametallicity of roughly - 2 dex. Combined with the tip of the red giant branch luminosity, this results in a distance of 735 kpc, placing And IX approximately 45 kpc from M31. This faint dSph follows the relations between luminosity and metallicity and between luminosity and surface brightness defined by other Local Group dSph galaxies. The core and tidal radii are found to be 1.'35 and 5.'9, respectively. We conclude that, despite its low luminosity, And IX might be an ordinary Local Group dSph and discuss implications for its formation from a once more massive but stripped progenitor or from an intrinsically low-mass seed.