Neutron stars and black holes as MACHOs

We consider the contribution of neutron stars and black holes to the dynami cal mass of galactic halos. In particular, we show that if these compact ob jects were produced by an early generation of stars with initial metallicit y less than or similar to 10(-4) Z(.), they can contribute at most 30%-40% of the Galactic halo mass without creating supersolar levels of enrichment. We show that the case for halo neutron stars and black holes cannot be rej ected on metal overproduction arguments alone because of the critical facto r of the choice of progenitor metallicity in determining the yields. We sho w that this scenario satisfies observational constraints, similar to but no more severe than those faced by halo white dwarfs. We also discuss the rec ent results on halo microlensing, the presence of enriched hot gas in clust ers and groups of galaxies, and other observations. If there are halo neutr on stars and black holes, they will be detected in the future as longer tim escale events by microlensing experiments.