Low-mass black holes as the remnants of primordial black hole formation

TL;DR

This paper reviews the search for intermediate-mass black holes, focusing on low-mass black holes in galaxy centers, and discusses evidence supporting their role as progenitors of supermassive black holes formed by direct gas collapse.

Contribution

It provides a comprehensive review of current searches for low-mass black holes and presents tentative evidence linking them to the origins of supermassive black holes.

Findings

Evidence of low-mass black holes in small galaxies

Support for direct gas collapse as a formation mechanism

Implications for supermassive black hole origins

Abstract

This article documents our ongoing search for the elusive "intermediate-mass" black holes. These would bridge the gap between the approximately ten solar mass "stellar-mass" black holes that are the end-product of the life of a massive star, and the "supermassive" black holes with masses of millions to billions of solar masses found at the centers of massive galaxies. The discovery of black holes with intermediate mass is the key to understanding whether supermassive black holes can grow from stellar-mass black holes, or whether a more exotic process accelerated their growth only hundreds of millions of years after the Big Bang. Here we focus on searches for black holes with masses of 10^4-10^6 solar masses that are found at galaxy centers. We will refer to black holes in this mass range as "low-mass" black holes, since they are at the low-mass end of supermassive black holes. We review the searches for low-mass black holes to date and show tentative evidence, from the number of low-mass black holes that are discovered today in small galaxies, that the progenitors of supermassive black holes were formed as ten thousand to one-hundred thousand solar mass black holes via the direct collapse of gas.