Properties of Two New M31 Dwarf Spheroidal Companions from Keck Imaging

Abstract

We have obtained images in V and I of the newly discovered Local Group dwarf galaxies Pegasus dSph and Cassiopeia dSph and their surrounding field using the Keck 10 m telescope and Low-Resolution Imaging Spectrograph. The first stellar luminosity functions and color-magnitude diagrams are presented for stars with V25 and I24. The distances to the new dwarfs are estimated from the apparent I magnitude of the tip of the red giant branch to be DMW(Peg dSph) = 830?80 kpc and DMW(Cas dSph) = 760?70 kpc, which are consistent with their belonging to the extended M31 satellite system. Both galaxies are dwarf spheroidals (dSph's) with red giant branch morphologies indicating predominantly old stellar populations and estimated mean metallicities, [Fe/H], of -1.3?0.3 for Peg dSph and -1.4?0.3 for Cas dSph. Their central surface brightness-[Fe/H]-absolute magnitude relationship follows that of other Local Group dSph, dwarf elliptical, and dwarf irregular galaxies. In contrast to four out of nine Milky Way dSph's (the four that lie beyond D=100 kpc), none of the six known M31 dSph's appear to have a dominant intermediate-age population.