Mergers and the outside-in formation of dwarf spheroidals

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved
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
Our collaboration has been supported by DFG grants GO 563/21-1 and GO 563/24-1 as well as by CONICET. ABL, JFN and MGA aknowledge support from ANPCyT grant PICT2012- 1137. GY acknowledges support from he Spanish MINECO under research grants AYA2012-31101 and FPA2012-34694. YH has been partially supported by the Israel Science Foundation (1013/12)