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

Abstract

The HST WFPC2 camera has been used to image Andromeda II, a dwarf spheroidal (dSph) companion to M31. The resulting color-magnitude 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 HB morphology of And II is predominantly red. Unlike And I, however, there is no evidence for a radial gradient in HB morphology in the And II data. At least 50% of the total stellar population in And II is younger than the age of the globular clusters. On the other hand, the existence of blue HB and RR Lyrae variable stars in And II argues for the existence of an additional old (age > 10 Gyr) population. Thus And II has had an extended epoch of star formation like many of the Galactic dSphs. Our derived distance confirms the association 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 160 and 230 kpc. With the current samples of dSph companions, the size of the Galaxy's and M31's dSph satellite systems are comparable, with outer radii of order 250 kpc. The And II red giant branch colors yield a mean abundance of <[Fe/H]> = -1.49 +/- 0.11 and a surprisingly large internal abundance spread, which can be characterized by sigma(int)([Fe/H]) ~ 0.36 dex, considerably larger than that derived for And I (0.21) from an identical analysis of similar data. Thus despite having very similar luminosities and mean metal abundances, these two M31 dSph companions have clearly had different chemical evolution histories. We conclude that the diversity of evolutionary histories evident among the Galactic dSph companions is now also firmly established among the dSph satellites of M31.