The IMF Long Ago and Far Away: Faint Stars in the Ursa Minor dSph Galaxy

Abstract

The dwarf spheroidal galaxy in Ursa Minor is apparently dark-matter dominated, and is of very low surface brightness, with total luminosity only equal to that of a globular cluster. Indeed its dominant stellar population is old and metal-poor, very similar to that of a classical halo globular cluster in the Milky Way Galaxy. However, the environment in which its stars formed was clearly different from that in the globular clusters in the Milky Way Galaxy -- what was the stellar IMF in this external galaxy a long time ago? The fossil record of long-lived, low-mass stars contains the luminosity function, derivable from simple star counts. This is presented here. The mass function requires a robust mass-luminosity relation, and we describe the initial results to determine this, from our survey for eclipsing low-mass binaries in old open clusters. The massive star IMF at early times is constrained by elemental abundances in low-mass stars, and we discuss the available data. All data are consistent with an invariant IMF, most probably of Salpeter slope at the massive end, with a turnover at lower masses.