A microlensing measurement of dark matter fractions in three lensing galaxies

TL;DR

This study uses gravitational microlensing observations and simulations to measure dark matter fractions in three lensing galaxies, revealing varying smooth matter percentages from nearly zero to about 80%.

Contribution

It introduces a novel application of microlensing to directly estimate dark matter fractions at specific galaxy locations, complementing traditional methods.

Findings

MG 0414+0534 has ~50% smooth matter.

SDSS J0924+0219 has ~80% smooth matter.

Q2237+0305 has near zero smooth matter.

Abstract

Direct measurements of dark matter distributions in galaxies are currently only possible through the use of gravitational lensing observations. Combinations of lens modelling and stellar velocity dispersion measurements provide the best constraints on dark matter distributions in individual galaxies, however they can be quite complex. In this paper, we use observations and simulations of gravitational microlensing to measure the smooth (dark) matter mass fraction at the position of lensed images in three lens galaxies: MG 0414+0534, SDSS J0924+0219 and Q2237+0305. The first two systems consist of early-type lens galaxies, and both display a flux ratio anomaly in their close image pair. Anomalies such as these suggest a high smooth matter percentage is likely, and indeed we prefer ~50 per cent smooth matter in MG 0414+0534, and ~80 per cent in SDSS J0924+0219 at the projected locations of the lensed images. Q2237+0305 differs somewhat in that its lensed images lie in the central kiloparsec of the barred spiral lens galaxy, where we expect stars to dominate the mass distribution. In this system, we find a smooth matter percentage that is consistent with zero.