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

The Hubble Space Telescope WFPC2 camera has been used to image Andromeda II , a dwarf spheroidal (dSph) companion to M31. The resulting color-magnitude (c-m) diagrams reveal the morphology of the horizontal branch (HB) in this dwarf galaxy. We find that like Andromeda I, and like most of the Galactic dSph companions, the HE morphology of And II is predominantly red. Unlike And I, however, there is no evidence for a radial gradient in HE morphology in the And II data. Based on a comparison with a combination of standard G alactic globular cluster c-m diagrams scaled to reproduce the And II mean a bundance and abundance dispersion, we interpret the observed HE morphology of And II as indicating that at least 50% of the total stellar population i s younger than the age of the globular clusters. This inference is strength ened by the small number of confirmed upper-AGE carbon stars in And II. The relatively faint luminosities (M-bol approximate to -4.1) of these stars, however, suggest an age or ages nearer 6-9 Gyr, rather than 1-3 Gyr, for th is population. On the other hand, the existence of blue HE and RR Lyrae var iable stars in And II argues for the existence of an additional old (age > 10 Gyr) population in this dSph. Thus, And II has had an extended epoch of star formation like many of the Galactic dSphs. The mean magnitude of the b lue HE in And II suggests (m-M)(0) = 24.17 +/- 0.06 and that And II is 125 +/- 60 kpc closer than M31 along the line of sight. This confirms the assoc iation of And II with M31, rather than with M33 to which And II lies closer on the sky. The true distance of And II from the center of M31 is between similar to 160 and similar to 230 kpc, comparable to the Galactocentric dis tances of Fornax and of the Leo dSphs. With the current samples of dSph com panions, the size of the Galaxy's and M31's dSph satellite systems are comp arable, with outer radii of order 250 kpc. The And II red giant branch colo rs yield a mean abundance of <[Fe/H]> = -1.49 +/- 0.11 and a surprisingly l arge internal abundance spread, which can be characterised by sigma(int)([F e/H]) approximate to 0.36 dex. Both these values are in good agreement with the recent ground-based spectroscopic study of Cati et al. The And II abun dance dispersion found here is considerably larger than that derived for An d I from an identical analysis of similar data (sigma(int)([Fe/H])= 0.21 de x). Thus, despite having very similar luminosities and mean metal abundance s, these two M31 dSph companions have clearly had different chemical evolut ion histories. We find that we cannot model the abundance distribution in A nd II with single component simple chemical enrichment models. However, we can reproduce the form of the distribution if we assume two components, eac h with a simple model abundance distribution. The "metal-poor" component ha s mean abundance log (< z >/z(sun))= -1.6, while the "metal-rich" one has m ean abundance log (< z >/z(sun))= -0.95 and is outnumbered by the metal-poo r population by a ratio of similar to 2.3 to 1. We end by concluding that t he diversity of evolutionary histories evident among the Galactic dSph comp anions is now also firmly established among the dSph satellites of M31. An appendix discusses minor revisions to our earlier And I results that arise from the calibration and analysis techniques adopted in this paper. In part icular, our comparisons with ground-based photometry indicate that the zero point for the WFPC2 F450W to B transformation should be modified, by 0.055 mag, to produce fainter B magnitudes and thus redder B-V colors.