DARK MATTER MASS FRACTION IN LENS GALAXIES: NEW ESTIMATES FROM MICROLENSING

Abstract

We present a joint estimate of the stellar/dark matter mass fraction in lens\ngalaxies and the average size of the accretion disk of lensed quasars from\nmicrolensing measurements of 27 quasar image pairs seen through 19 lens\ngalaxies. The Bayesian estimate for the fraction of the surface mass density in\nthe form of stars is $\\alpha=0.21\\pm0.14$ near the Einstein radius of the\nlenses ($\\sim 1 - 2$ effective radii). The estimate for the average accretion\ndisk size is $R_{1/2}=7.9^{+3.8}_{-2.6}\\sqrt{M/0.3M_\\sun}$ light days. The\nfraction of mass in stars at these radii is significantly larger than previous\nestimates from microlensing studies assuming quasars were point-like. The\ncorresponding local dark matter fraction of 79\\% is in good agreement with\nother estimates based on strong lensing or kinematics. The size of the\naccretion disk inferred in the present study is slightly larger than previous\nestimates.\n