K Band Microlensing of the Inner Galaxy

Microlensing searches toward the inner galaxy (|l|, |b| ≤ 22.5) have several major advantages. First, the event rate is strongly dominated by bulge-bulge lensing events where both the source and lens lie in the bulge. Second, these bulge-bulge events have very short time scales te ∼ 2 days and are therefore easily distinguished from the less frequent and much longer bulge-disk and disk-disk events. Third, since the optical depth is similar to that at higher impact parameters, while the events are shorter, the event rate is high Γ ∼ 3 × 10−7day−1. Fourth, because the Einstein rings are small, re ∼ 5× 1012 cm, and the source stars are large rs > ∼ 1012 cm, the lens will transit the face of the source for a significant fraction (∼ 20%) of events. For these transit events it will often be possible to measure a second lens parameter, the angular Einstein radius (or proper motion). In addition to the bulge-bulge events, the optical depth of the disk is ∼ 7 times larger toward the inner Galaxy than toward Baade’s Window. A microlensing search toward the inner Galaxy can be carried out by making frequent (∼ 4 day−1) K band images of a large area ∼ 0.5 deg2 to a depth of K ∼ 16, and hence requires either a 10242 infrared array on a dedicated 2m telescope or four such arrays on a 1m telescope. Subject Headings: dark matter – Galaxy: stellar content – gravitational lensing – stars: low-mass submitted to The Astrophysical Journal Letters: December 20, 1994 Preprint: OSU-TA-27/94 ⋆ Alfred P. Sloan Foundation Fellow