Poster abstract details

RR Lyrae stars are the best, unambiguous tracers of old (> 10 Gyr) stellar populations. They were in place at early times when most of the halo mass assembly occurred. Therefore, they provide a direct observable to trace the early star formation of the host galaxy, and constraints into galaxy formation and evolution models under the Cold Dark Matter scenario. This poster presents a series of results focused on the properties of RR Lyrae stars in Local Group galaxies, as tracers of the early evolution of the Milky Way and the M31 systems. On one side, the MW and M31 halo contain a population of relatively metal-rich RRL stars that is not observed in dwarf galaxies, with the exception of the most massive ones. This occurrence sets strong constraints on the building blocks of both halos, which were most likely formed by accretion of massive dwarf galaxies. On the other side, the pulsation properties of the RR Lyrae stars can be used to obtain individual metallicities, and derive the metallicity distribution of the purely old population, thus the early chemical evolution of the host galaxy. Using the Sculptor dwarf spheroidal galaxy as a test case, we compared the metallicity distribution obtained with RR Lyrae and RGB stars, and we found that i) the star formation in the center of this galaxy lasted substantially longer than in the outer parts, thus constraining the timescales for the outside-in evolution of this galaxy; ii) the RR Lyrae population has an intrinsic metallicity spread and presents a clear spatial gradient, which therefore was in place at a very early epoch. Finally, a similar analysis is applied to several Local Group galaxies, to show the potential of this approach.