Microlensing halo models with abundant brown dwarfs

All previous attempts to understand the microlensing results toward the Lar ge Magellanic Cloud (LMC) have assumed homogeneous present-day mass functio ns (PDMFs) for the lensing populations. Here, we present an investigation i nto the microlensing characteristics of halos with spatially varying PDMFs and anisotropic velocity dispersion tensors, One attractive possibility-sug gested by baryonic dark cluster formation in pregalactic and protogalactic cooling flows-is that the inner halo is dominated by stellar mass objects, whereas low-mass brown dwarfs become more prevalent on moving outward. The contribution to the microlensing rate must be dominated by dark remnants (s imilar to 0.5 M.) to recover the observed timescales of the microlensing ex periments. But, even though stellar remnants control the rate, they do not dominate the mass of the baryonic halo, and so the well-known enrichment an d mass budget problems are much less severe. Using a simple Ansatz for the spatial variation of the PDMF models are constructed in which the contribut ion of brown dwarfs to the mass of the baryonic halo is similar to 55% and to the total halo is similar to 30%. An unusual property of the models is t hat they predict that the average timescale of events toward M31 is shorter than the average timescale toward the LMC. This is because the longer line of sight toward M31 probes more of the far halo in which brown dwarfs are the most common constituent.