Local Dwarf Spheroidal Galaxies and their Histories

Abstract

In the first part of this review, an assessment of the possible contribution of disrupted dwarf Spheroidal (dSph) galaxies to the formation the Galactic halo is presented. The discovery of a major difference between the [α/Fe] ratios for red giants in the Draco, Ursa Minor and Sextans dSphs and for field halo stars is highlighted. This difference suggests that the disruption of dSphs similar to those studied did not contribute significantly to the halo. The lack of intermediate-age stars in the halo field also argues against a significant contribution from the disruption of dSph systems similar to Fornax, except at the earliest times. The properties of the M31 dSph companions are then presented and compared with those of the Milky Way. Considerable similarities are found but an apparent real difference exists in the comparative lack of ~1-6 Gyr intermediate-age populations in the M31 systems. In the third part, recent results for dSphs beyond the Local Group are discussed. The contribution then concludes with speculation as to whether any underlying trends can be discerned from these data that might aid in explaining the surprising diversity of star formation histories among these supposedly simple systems.