A New (Old) Component of the Galaxy as the Origin of the Observed LMC Microlensing Events

Abstract

We suggest a new component of the Milky Way galaxy that can account for both the optical depth and the event durations obtained by the MACHO microlensing survey toward the Large Magellanic Cloud. This component is consistent with recent evidence for a significant population of faint white dwarf stars, detected in a proper motion study of the Hubble Deep Field, which cannot be accounted for by stars in the disk or spheroid. This new component consists of (mostly) old white dwarf stars distributed in a highly extended (very thick) disk configuration. It extends beyond the traditional thin and thick disks, but well within the dark, roughly spherical CDM halo. The total mass in this component is $\sim 7-9 \times 10^{10} M_\odot$. We argue that such a component is reasonable, natural, consistent with a variety of observations, and many of the problems associated with a significant halo population of white dwarfs are ameliorated.