Origin of two distinct populations in dwarf spheroidal galaxies

We study the chemical and kinematic properties of the first galaxies that formed at high redshift, using high-resolution cosmological numerical simulations, and compare them with the recent observational results for the Sculptor dwarf spheroidal galaxy of Tolstoy et al., who found two distinct stellar populations: the lower metallicity stars are more spatially extended and possess a higher velocity dispersion than the higher metallicity stars. Our calculations reproduce these observations as the result of a steep metallicity gradient within a single population, induced by dissipative collapse of the gas component. We also predict strong [N/O] enhancements in the lowest metallicity stars in dwarf spheroidals, due to the preferential retention of ejected gas from intermediate-mass stars, compared to Type II supernovae.