Dark Matter Fraction in Lens Galaxies: New Estimates from Microlensing

TL;DR

This study estimates the stellar and dark matter fractions in lens galaxies and the size of quasar accretion disks using microlensing data, providing new insights into galaxy composition and quasar structure.

Contribution

It offers the first joint Bayesian estimates of stellar/dark matter fractions and accretion disk sizes from microlensing measurements of multiple lens systems.

Findings

Stellar mass fraction near Einstein radius is approximately 21%.

Dark matter fraction at these radii is about 79%.

Average accretion disk size is roughly 8 light days.

Abstract

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