Properties of Two New M31 Dwarf Spheroidal Companions from Keck Imaging

We have obtained Keck Low Resolution Imaging Spectrograph images in V and Iof the newly discovered Local Group dwarf galaxies Pegasus dSph and CassiopeiadSph and their surrounding field. The first stellar luminosity functions andcolor-magnitude diagrams are presented for stars with V <~ 25 and I <~ 24. Thedistances to the new dwarfs are estimated from the apparent I mag of the tip ofthe red giant branch to be: D_MW (Peg dSph) = 830 +/- 80 kpc and D_MW (CasdSph) = 760 +/- 70 kpc, consistent with their belonging to the extended M31satellite system. Both galaxies are dwarf spheroidals (dSphs) with red giantbranch morphologies indicating predominantly old stellar populations andestimated 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]>-absolutemagnitude relationship follows that of other Local Group dSphs, dwarfellipticals, and dwarf irregulars. In contrast to four out of nine Milky WaydSphs (the four that lie beyond D_MW =100 kpc), none of the six known M31 dSphsappears to have a dominant intermediate-age population.Comment: Accepted for publication in the Astrophysical Journal Letters (Dec 3, 1998) [7 pages of text including 1 table, 4 figures