Ω_B and Ω_0 from MACHOs and Local Group Dynamics

Abstract

We obtain restrictions on the universal baryon fraction, f_B ≡ Ω_B/Ω_0, by assuming that the observed microlensing events toward the Large Magellanic Cloud are due to baryonic MACHOs in the halo of the Galaxy and by extracting a bound to the total mass of the Milky Way from the motion of tracer galaxies in the Local Group. We find a lower bound f_B > 0.29^+0.18_-0.15. Consistency with the predictions of primordial nucleosynthesis leads to the further constraint on the total mass density, Ω_0 ~< 0.2.