Origin of Two Distinct Populations in Dwarf Spheroidal Galaxies

We study the chemical and kinematic properties of the first galaxies which formed at a high redshift, using high resolution cosmological numerical simulations, and compared them with the recent observational results for the Sculptor dwarf spheroidal galaxy by 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 populations, 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. Subject headings: galaxies: abundances — galaxies: kinematics and dynamics —galaxies: formation —galaxies: stellar content — galaxies: individual (Sculptor)