The dwarf spheroidal companions to M31: WFPC2 observations of Andromeda I

Images have been obtained with the Hubble Space Telescope WFPC2 camera of Andromeda I, a dwarf spheroidal (dSph) galaxy that lies in the outer halo of M31. The resulting color-magnitude diagrams reveal for the first time the morphology of the horizontal branch in this system. We find that, in a similar fashion to many of the galactic dSph companions, the horizontal branch (HB) of And I is predominantly red. Combined with the metal abundance of this dSph, this red HB morphology indicates that And I can be classified as a ''second parameter'' system in the outer halo of M31. This result then supports the hypothesis that the outer halo of M31 formed in the same extended chaotic manner as is postulated for the outer halo of the Galaxy. In addition to the red HB stars, blue HB and RR Lyrae variable stars are also found in the And I color-magnitude diagram. The presence of these stars indicates that And I contains a minority population whose age is comparable to that of the galactic globular clusters. We estimate, however, that the bulk of the stellar population in And I is similar to 10 Gyr old. Thus, again like many of the galactic dSphs, there is clear evidence for an extended epoch of star formation in And I. A radial gradient in the And I HB morphology has also been discovered in the sense that there are relatively more blue HE stars beyond the galaxy's core radius. This may be evidence for more centrally concentrated star formation after the initial episode. Similar HB morphology gradients have also been identified in two of three galactic dSphs studied. The mean magnitude of the blue HB stars suggests that And I lies along the line-of-sight at the same distance as M31 to within similar to+/-70 kpc. Consequently, the true distance of And I from the center of M31 is between similar to 45 and similar to 85 kpc, with the higher estimates being more likely. Such distances are comparable to the galactocentric distances of the nearer Milky Way dSph companions Ursa Minor, Draco, and Sculptor. From the mean color of the lower giant branch, the mean metal abundance of And I is estimated as [Fe/H] = -1.45 +/- 0.2 dex, while the presence of an internal abundance spread with total range of similar to 0.6 dex is suggested by the intrinsic color width of the upper giant branch. A small population of faint blue stars, which we identify as blue stragglers, is also present. (C) 1996 American Astronomical Society.