Star Counts from Hst: Implications for Dark Matter

Star counts made with the Hubble Space Telescope (HST) probe four populations that are important for dark matter: disk, halo, bulge, and intergalactic. The disk mass function falls for masses M < 0.6 M⊙ in sharp contrast to the rising Salpeter function usually assumed. The amount of “observed” disk material is therefore lower than commonly believed which implies the need for disk dark matter. Halo stars contribute no more than a few percent of the dark matter. Disk and halo together contribute no more than 10% of the observed microlensing optical depth toward the Large Magellanic Cloud. The bulge luminosity function is similar to that of the disk down to MV ∼ 10. If this similarity continues to the bottom of the main sequence, the bulge microlensing events can only be explained by a large population of brown dwarfs. Intergalactic stars in the Local Group have a density lower than the local halo density by at least 10.