Program: GS-2005B-Q-64

One of the foremost outstanding problems in the physical sciences is the nature and distribution of the "dark matter'' that is the gravitationally dominant component of mass in all galaxies, including the Milky Way. A previous experiment to search for the transient brightening of background stars due to the gravitational lensing by foreground MACHO's has produced a peculiar result: while the detected rate of lensing events indicates that MACHOs comprise at most 20% of the dark matter halo, the number of events far exceeds that expected from known stellar populations. The nature of these excess lensing objects remains a mystery. We intend to determine the nature of this lensing population, which may outweigh all other known components of the Galaxy, by conducting a search with at least a tenfold improvement in the event detection rate. Our approved NOAO Survey will discover microlensing events in Oct - Dec of 2001 through 2005. The major source of contamination of microlensing events in the LMC is supernovae which occur in background galaxies. We propose to use Gemini GMOS-S to obtain spectra to separate the microlensing events from SNe and other transient "contaminants" and obtain important information on the SNe discovered (type, redshift, and age).