PROPERTIES OF TWO NEW M31 DWARF SPHEROIDAL COMPANIONS FROM KECK IMAGING

摘要

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 lmaging Spec- trograph. 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 magnitude of the tip of the red giant branch to be D_MW(Peg dSph) = 830 ± 80 kpc and D_Mw (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 metal- licities, ([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_MW = 100 kpc), none of the six known M31 dSph's appear to have a dominant intermediate-age population.