A New (Old) Component of the Galaxy as the Origin of the Observed Large Magellanic Cloud 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, which represents less than 4% of the total dark halo mass, 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 cold dark matter (CDM) halo. We explore the distribution of proper motions expected for white dwarfs in this model and the ability of future surveys to differentiate between models. Finally, we argue that such a component is reasonable, natural, and consistent with a variety of observations, and many of the problems associated with a significant halo population of white dwarfs are ameliorated.