Microlensing-Based Estimate of the Mass Fraction in Compact Objects in
  Lens
  We estimate the fraction of mass that is composed of compact objects in
gravitational lens galaxies. This study is based on microlensing measurements
(obtained from the literature) of a sample of 29 quasar image pairs seen
through 20 lens galaxies. We determine the baseline for no microlensing
magnification between two images from the ratios of emission line fluxes.
Relative to this baseline, the ratio between the continua of the two images
gives the difference in microlensing magnification. The histogram of observed
microlensing events peaks close to no magnification and is concentrated below
0.6 magnitudes, although two events of high magnification, $\Delta m \sim 1.5$,
are also present. We study the likelihood of the microlensing measurements
using frequency distributions obtained from simulated microlensing
magnification maps for different values of the fraction of mass in compact
objects, $\alpha$. The concentration of microlensing measurements close to
$\Delta m \sim 0$ can be explained only by simulations corresponding to very
low values of $\alpha$ (10% or less). A maximum likelihood test yields
$\alpha=0.05_{-0.03}^{+0.09}$ (90% confidence interval) for a quasar continuum
source of intrinsic size $r_{s_0}\sim 2.6 \cdot 10^{15} \rm cm$. Regarding the
current controversy about Milky Way/LMC and M31 microlensing studies, our work
supports the hypothesis of a very low content in MACHOS (Massive Compact Halo
Objects).
