SDSS J1004+4112: the case for a galaxy cluster dominated by primordial black holes

Institute for Astronomy (IfA), University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
e-mail: mrsh@roe.ac.uk

Received: 16 June 2020
Accepted: 8 September 2020

Abstract

Aims. This paper is aimed at providing a plausible explanation for the large amplitude microlensing events observed in the cluster-lensed quasar system, SDSS J1004+4112. The microlensed quasar images appear to lie well clear of the stellar population of the cluster, raising the possibility that the cluster dark matter is composed of compact bodies that are responsible for the observed microlensing.

Methods. We established the exact structure of the difference light curves attributed to microlensing from photometric monitoring programmes in the literature. We showed, based on measurements of surface brightness, that the probability of microlensing by stars in the cluster is negligible. We relaxed our initial assumption that the cluster dark matter takes the form of smoothly distributed particles and we hypothesized that it is made up of compact bodies. We then used computer simulations of the resulting magnification pattern to estimate the probability of microlensing.

Results. Our results show that for a range of values for source size and lens mass, the large microlensing amplitude that is observed is consistent with the statistics obtained from the simulations.

Conclusions. We conclude that providing the assumption of smoothly distributed dark matter is relaxed, the observed large amplitude microlensing can be accounted for by assuming that the cluster’s dark matter is in the form of compact bodies of solar mass. We further conclude that the most plausible identification of these bodies is that of primordial black holes.