Constraints on the abundance of supermassive primordial black holes from lensing of compact radio sources

TL;DR

This study uses high-resolution radio observations to set upper limits on the abundance of supermassive primordial black holes, constraining their contribution to dark matter in the mass range of 10^6 to 10^8 solar masses.

Contribution

It provides new constraints on supermassive primordial black holes using non-detections of milli-lensing in a large radio source sample.

Findings

Less than 1.48% of dark matter in supermassive PBHs (10^6-10^8 M_sun) at 95% confidence.

No milli-lensed images detected in 543 sources, constraining PBH abundance.

Projected improvements could reduce the upper limit to 0.06% with more data.

Abstract

The possibility that primordial black holes (PBHs) form a part of dark matter has been considered over a wide mass range from the Planck mass ($10^{-5}~\rm g$) to the level of the supermassive black hole in the center of the galaxy. Primordial origin might be one of the most important formation channel of supermassive black holes. We use the non-detection of lensing effect of very long baseline interferometer observations of compact radio sources with extremely high angular resolution as a promising probe to constrain the abundance of intergalactic PBHs in the mass range $\sim10^4$-$10^9~M_{\odot}$. For a sample of well-measured 543 flat-spectrum compact radio sources, no milli-lensed images are found with angular separations between $1.5$ milli-arcseconds and $50$ milli-arcseconds. From this null search result, we derive that the fraction of dark matter made up of supermassive PBHs in the mass range $\sim10^6$-$10^8~M_{\odot}$ is $\lesssim1.48\%$ at $95\%$ confidence level. This constraints would be significantly improved due to the rapid increase of the number of measured compact radio sources. For instance, on the basis of none confirmed milli-lensing candidate in the latest $\sim14000$ sources, we derive the abundance of supermassive PBHs and obtain that it is $\lesssim0.06\%$ at $95\%$ confidence level.