Limits on Dark Matter Compact Objects implied by Supermagnified Stars in Lensing Clusters

TL;DR

This paper uses observations of supermagnified stars in galaxy clusters to place upper limits on the abundance of dark matter compact objects, such as primordial black holes, based on microlensing effects.

Contribution

It introduces a method to constrain the fraction of dark matter in compact objects by analyzing the microlensing-induced limits on supermagnified stars in lensing clusters.

Findings

Compact objects above ~10^{-6} solar masses cannot constitute more than 2% of dark matter.

Microlensing effects create a network of critical lines that limit the maximum magnification of stars.

The analysis constrains primordial black holes as dark matter candidates based on observed supermagnified stars.

Abstract

Supermagnified stars are gravitationally lensed individual stars that are located close to a caustic of a lensing galaxy cluster, and have their flux magnified by a large enough factor (typically ~ 1000) to make them detectable with present telescopes. The maximum magnification is limited by microlensing caused by intracluster stars or other compact objects, which create a network of corrugated critical lines with an angular width proportional to the surface density of microlenses. We consider a set of 9 cases of supermagnified stars reported in the literature, and derive an upper limit on the surface density of compact objects, such as primordial black holes, that might be present as a fraction of the dark matter in addition to known intracluster stars. Any such additional compact objects would widen the corrugated critical line network and therefore the width of the distribution of supermagnified stars around the modeled critical lines of the lens. We find that any compact objects, including primordial black holes, with masses above $\sim 10^{-6}\, M_{\odot}$ (below which the microcaustics are closer together than the typical angular size of supermagnified stars) cannot account for more than ~ 2% of the dark matter.