Baryonic Dark Matter: Limits from HST and ISO

Recent HST and ISO observations provide very severe limits on any compact baryonic contributions to galactic (dark) halos. When combined with Milky Way Galaxy microlensing results, almost the entire plausible range of massive compact baryonic objects is excluded by direct observation. Deep direct imaging at 7mu and 15mu with ISOCAM on the ISO spacecraft directly excludes hydrogen-burning stars of any mass above the hydrogen-burning limit, and of any chemical abundance, from being the predominant explanation of the dark halos of external spiral galaxies. In the Milky Way Galaxy, HST has provided luminosity functions to the hydrogen-burning limit in several globular clusters. The resulting mass functions do not provide any support for dominance by very low-mass stars. This is consistent with field surveys for sub-stellar mass brown dwarfs, which show such objects to be relatively rare. These results are complemented by very deep HST luminosity functions in the Large Magellanic Cloud, providing strong support for the (near)-universality of the stellar mass function. Very recent HST results are available for the nearby dSph galaxy UMi. This galaxy, the most dark-matter dominated object known on kpc scales, has a normal stellar mass function at low masses. The prospects are bright for dark elementary particles.