Limiting the Abundance of LIGO/Virgo Black Holes with Microlensing Observations of Quasars of Finite Size

TL;DR

This paper uses quasar microlensing observations to strongly constrain the abundance of primordial black holes with masses similar to those detected by LIGO/Virgo, suggesting they cannot be the main dark matter component.

Contribution

It introduces a general argument that leverages quasar microlensing data, accounting for finite source size, to limit the presence of massive black holes as dark matter candidates.

Findings

Massive black holes would cause frequent high flux magnifications not observed.

Quasar microlensing is sensitive to lens mass and source size.

Results exclude primordial black holes in LIGO/Virgo mass range as dominant dark matter.

Abstract

We present a simple but general argument that strongly limits the abundance of Primordial Black Holes (PBHs) (or other unknown population of compact objects) with masses similar to those determined by LIGO/Virgo from BH binary mergers. We show that quasar microlensing can be very sensitive to the mass of the lenses, and that it is able to distinguish between stars and BHs of high mass, when the finite size of the source is taken into account. A significant presence of massive BHs would produce frequent high flux magnifications (except for unrealistically large sources) which have been very rarely observed. On the contrary, a typical stellar population would induce flux magnifications consistent with the observations. This result excludes PBHs (or any type of compact object) in the mass range determined by LIGO/Virgo as the main dark matter constituents in the lens galaxies.