Mergers and the outside-in formation of dwarf spheroidals
  We use a cosmological simulation of the formation of the Local Group to
explore the origin of age and metallicity gradients in dwarf spheroidal
galaxies. We find that a number of simulated dwarfs form "outside-in", with an
old, metal-poor population that surrounds a younger, more concentrated
metal-rich component, reminiscent of dwarf spheroidals like Sculptor or
Sextans. We focus on a few examples where stars form in two populations
distinct in age in order to elucidate the origin of these gradients. The
spatial distributions of the two components reflect their diverse origin; the
old stellar component is assembled through mergers, but the young population
forms largely in situ. The older component results from a first episode of star
formation that begins early but is quickly shut off by the combined effects of
stellar feedback and reionization. The younger component forms when a late
accretion event adds gas and reignites star formation. The effect of mergers is
to disperse the old stellar population, increasing their radius and decreasing
their central density relative to the young population. We argue that
dwarf-dwarf mergers offer a plausible scenario for the formation of systems
with multiple distinct populations and, more generally, for the origin of age
and metallicity gradients in dwarf spheroidals.
