Physical Society Colloquium

The baryonic mass-to-light ratio in elliptical galaxies measured from the static gravitational micro-lensing of multiply imaged quasars

Paul Schechter

MIT

Many gravitationally lensed quasars exhibit flux ratio “anomalies” that cannot be explained under the hypothesis that the lensing potential is smooth on scales smaller than one kpc. Micro-lensing by stars is a natural source of granularity in the lens potential. The character of the expected fluctuations due to micro-lensing depends sensitively on the relative surface densities of micro-lenses (stars) and smoothly distributed (dark) matter. Observations of flux ratios may in principle be used to infer the mass-to-light (M/L) ratio of stellar component along the line of sight — typically at impact parameters 1.5 times the half light radius. In practice the optical emission of quasars appears to come from regions comparable in size to the Einstein rings of the stellar microlenses, greatly complicating the M/L analysis. Fortunately, the X-ray emission from quasars appears to be point-like. Chandra observations of a dozen lensed quasars yield an M/L ratio accurate to 15%.