Microlensing of Quasar Broad Emission Lines: Constraints on Broad Line
  Region Size
  We measure the differential microlensing of the broad emission lines between
18 quasar image pairs in 16 gravitational lenses. We find that high ionization
lines such as CIV are more strongly microlensed than low ionization lines,
indicating that the high ionization line emission regions are more compact. If
we statistically model the distribution of microlensing magnifications, we
obtain estimates for the broad line region radius of 24 (-15/+22) and 55
(-35/+150) light-days (90% confidence) for the high and low ionization lines,
respectively. When the sample is divided attending to quasar luminosity, we
find that the line emission regions of more luminous quasars are larger, with a
slope consistent with the expected scaling from photoionization models. Our
estimates also agree well with the results from local reveberation mapping
studies.
