THE DARK-MATTER FRACTION IN THE ELLIPTICAL GALAXY LENSING THE QUASAR PG 1115+080

Abstract

We determine the most likely dark-matter fraction in the elliptical galaxy quadruply lensing the quasar PG 1115+080 based on analyses of the X-ray fluxes of the individual images in 2000 and 2008. Between the two epochs, the A[subscript 2] image of PG 1115+080 brightened relative to the other images by a factor of 6 in X-rays. We argue that the A[subscript 2] image had been highly demagnified in 2000 by stellar microlensing in the intervening galaxy and has recently crossed a caustic, thereby creating a new pair of microimages and brightening in the process. Over the same period, the A[subscript 2] image has brightened by a factor of only 1.2 in the optical. The most likely ratio of smooth material (dark matter) to clumpy material (stars) in the lensing galaxy to explain the observations is ~90% of the matter in a smooth dark-matter component and ~10% in stars.