The Dark-Matter Fraction in the Elliptical Galaxy Lensing the Quasar PG1115+080

TL;DR

This study estimates that approximately 90% of the mass in the elliptical galaxy lensing quasar PG1115+080 is dark matter, based on X-ray flux variations and microlensing analysis over multiple epochs.

Contribution

It provides a novel estimate of the dark-matter fraction in a lensing galaxy using multi-epoch X-ray flux data and microlensing effects.

Findings

Dark matter constitutes about 90% of the galaxy's mass.

X-ray flux variations indicate microlensing effects.

The galaxy's mass distribution includes a small stellar component.

Abstract

We determine the most likely dark-matter fraction in the elliptical galaxy quadruply lensing the quasar PG1115+080 based on analyses of the X-ray fluxes of the individual images in 2000 and 2008. Between the two epochs, the A2 image of PG1115+080 brightened relative to the other images by a factor of six in X-rays. We argue that the A2 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 micro-images and brightening in the process. Over the same period, the A2 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.