M-Dwarfs at Large Heliocentric Distances

Abstract

We present an analysis of the faint M star population seen as foreground contaminants in deep extragalactic surveys. We use space-based data to separate such stars from high redshift galaxies in a publically-available dataset, and consider the photometric properties of the resulting sample in the optical and infrared. The inferred distances place these stars well beyond the scale height of the thick disk. We find strong similarities between this faint sample (reaching i'_{AB}=25) and the brighter disk M dwarf population studied by other authors. The optical-infrared properties of the bulk of our sources spanning 6000A-4.5microns are consistent with those 5-10 magnitudes brighter. We also present deep spectroscopy of faint M dwarf stars reaching continuum limits of i'_{AB}~26, and measure absorption line strengths in the CaH2 and TiO5 bands. Both photometrically and spectroscopically, our sources are consistent with metallicities as low as a tenth solar: metal-rich compared with halo stars at similar heliocentric distances. We comment on the possible MACHO identification of M stars at faint magnitudes.