Properties of Two New M31 dSph Companions from Keck Imaging

We have obtained Keck Low Resolution Imaging Spectrograph images in V and I of the newly discovered Local Group dwarf galaxies PegasusdSph and Cassiopeia dSph and their surrounding field. The first stellar luminosity functions and color-magnitude diagrams are presented for stars with V ∼< 25 and I ∼< 24. The distances to the new dwarfs are estimated from the apparent I mag of the tip of the red giant branch to be: DMW(Peg dSph) = 830± 80 kpc and DMW(Cas dSph) = 760 ± 70 kpc, consistent with their belonging to the extended M31 satellite system. Both galaxies are dwarf spheroidals (dSphs) with red giant branch morphologies indicating predominantly old stellar populations and estimated mean metallicities, 〈[Fe/H]〉, of −1.3 ± 0.3 for PegdSph and −1.4 ± 0.3 for Cas dSph. Their central surface brightness–〈[Fe/H]〉–absolute magnitude relationship follows that of other Local Group dSphs, dwarf ellipticals, and dwarf irregulars. In contrast to four out of nine Milky Way dSphs (the four that lie beyond DMW = 100 kpc), none of the six known M31 dSphs appears to have a dominant intermediate-age population. Subject headings: Local Group – galaxies: individual [Peg dSph (AndVI), Cas dSph (AndVII), AndV, M31] – galaxies: stellar content – galaxies: luminosity function – galaxies: photometry – galaxies: structure