The Spheroid Luminosity and Mass Functions From HST Star Counts

Abstract

We analyze 166 spheroid subdwarfs $(6.5<M_V<14.5)$ found in 53 fields observed with the Wide Field Camera on the {\it Hubble Space Telescope}. The fields cover 221 square arcmin over a wide range of directions. The spheroid luminosity function (LF) is inconsistent at about the 3 sigma level with the local spheroid LF of Dahn et al. even when the normalization of the latter is corrected to take account of the latest data on spheroid kinematics. The difference may reflect systematic errors in one of the two studies or features of the spheroid spatial distribution that are not included in the simplest models. The mass function, which shows no obvious structure, can be represented by a power law, d N/d ln M ~ M^α, with α=0.25 +/- 0.32 over the mass range 0.71 Msun > M >0.09 Msun. The spheroid therefore does not contribute significantly to microlensing unless the mass function changes slope dramatically in the substellar range. The total local mass density of spheroid stars (including remnants and unseen binary companions) is ρ~ 6.4E-5 Msun/pc^3, with an uncertainty of about 50%. The power-law indices α=0.25 for the spheroid and α=0.44 for the disk (both uncorrected for binaries) are similar to those of globular clusters of moderate to high metallicity.