The Dwarf Spheroidal Companions to M31: WFPC2 Observations of Andromeda III

The Hubble Space Telescope WFPC2 camera has been used to image Andromeda III,a dwarf spheroidal companion (dSph) to M31. The resulting color-magnitude (c-m)diagrams reveal the morphology of the horizontal branch (HB) in this dwarfgalaxy. We find that like Andromeda I and Andromeda II, and like most of theGalactic dSph companions, the HB morphology of And III is predominantly red,redder than that of both And I and And II despite And III having a lower meanmetallicity. We interpret this red HB morphology as indicating that the bulk ofthe And III population is ~3 Gyr younger than the age of the majority ofGalactic globular clusters. Nevertheless, the And III c-m diagram does reveal afew blue HB stars, and a number of RR Lyrae variables are also evident in thedata. This indicates that And III does contain an `old' population of agecomparable to that of the Galactic globular clusters. There is no evidence,however, for any young stars in And III despite a claimed association betweenthis dSph and an HI cloud. As was the case for And II, but not And I, no radialgradient was detected in the And III HB morphology. And III is ~75 kpc from thecenter of M31, comparable to the Galactocentric distances of Sculptor andDraco. Comparison with standard globular cluster red giant branches indicates<[Fe/H]> = -1.88 +/-0.11, consistent with the absolute-magnitude - meanabundance relation followed by dSph galaxies. The same comparison yields anintrinsic abundance dispersion of sigma([Fe/H]) = 0.12, a low value compared tothe Galactic dSphs of comparable luminosity to And III. The list of candidatevariables reveals one definite and one probable Anomalous Cepheid variables.Comment: 30 pages including 1 table, 10 figures, Fig 1 as jpeg to save space. Accepted for publication in the Astronomical Journal, July 2002 issu