Deconstructing dwarf galaxies: a Suprime-Cam survey of Andromeda II

We present deep, subhorizontal-branch, multicolour photometry of the Andromeda II dwarf spheroidal (And II dSph) taken with the Subaru Suprime-Cam wide-field camera. We identify a red clump population in this galaxy, the first time this feature has been detected in an M31 dSph, which are normally characterized as having no significant intermediate-age populations. We construct radial profiles for the various stellar populations and show that the horizontal branch (HB) has a nearly constant density spatial distribution out to large radius, whereas the reddest red giant branch stars are centrally concentrated in an exponential profile. We argue that these populations trace two distinct structural components in And II, and show that this assumption provides a good match to the overall radial profile of this galaxy. The extended component dominates the stellar populations at large radius, whereas the exponential component dominates the inner few arcminutes. By examining colour-magnitude diagrams in these regions, we show that the two components have very different stellar populations; the exponential component has an average age of similar to 7-10 Gyr, is relatively metal-rich ([Fe/H] similar to -1) but with a significant tail to low metallicities, and possesses a red clump. The extended component, on the other hand, is ancient (similar to 13 Gyr), metal-poor ([Fe/H] similar to -1.5) with a narrower dispersion sigma([Fe/H]) similar or equal to 0.28, and has a well-developed blue HB. The extended component contains approximately three-quarters of the light of And II and its unusual density profile is unique in Local Group dwarf galaxies. This suggests that its formation and/or evolution may have been quite different from other dwarf galaxies. The obvious chemodynamical complexity of And II lends further support to the accumulating body of evidence which shows that the evolutionary histories of faint dSph galaxies can be every bit as complicated as their brighter and more massive counterparts.