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

Abstract

The Hubble Space Telescope WFPC2 camera has been used to image Andromeda III, a dwarf spheroidal (dSph) companion to M31. The resulting color-magnitude (c-m) diagrams reveal for the first time the morphology of the horizontal branch (HB) in this dwarf galaxy. We find that like Andromeda I and Andromeda II, and like most of the Galactic dSph companions, the HB morphology of And III is predominantly red, redder indeed than that of both And I and And II despite And III having a lower mean metallicity. The And III HB morphology is also somewhat redder than that of the Galactic dSph Draco, which has a similar mean abundance to And III. We interpret this red HB morphology as indicating that the bulk of the And III population is ~3 Gyr younger than the age of teh majority of Galactic globular clusters. Nevertheless, the And III c-m diagram does reveal the presence of a few blue HB stars, and a number of RR Lyrac variables are also evident in the data. This indicates that And III does contain an "old" population of age comparable to that of the Galactic globular clusters. There is no evidence, however, for any young stars in And III despite a claimed association between this dSph and an HI cloud. As was the case for And II, but not And I, no radial gradient was detected in the And III horizontal branch morphology. The mean V magnitude of the horizontal branch is 25.06 ± 0.04 leading to (m - M) [subscript 0] = 24.38 ± 0.06 for this dwarf. And III is then ~75 kpc from the center of M31, comparable to the Galactocentric distances of Sculptor and draco. Comparison with standard globular cluster red giant branches indicates a mean abundance for And III of <[Fe/H]> = -1.88 ± 0.11, the lowest mean abundance of any of M31's companions. This value, however, is consistent with the absolute magnitude - mean abundance relation followed by dSph galaxies. The same comparizon yields an intrinsic abundance dispersion for And III of ơ [subscript inl] ([Fe/H]) = 0.12, a low value compared to And I and And II and to the Galactic dSphs of comparable luminosity to And III. If confirmed by future spectroscopic studies, this low value would suggest that And III retained relatively little of the enrichment products generated during its evolutionary history. The list of candidate variables reveals one definite and one probable Anomalous Cepheid variable stars in And III. Such variables are common in Galactic dSphs, so their discovery in And III is not unexpected.