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

Abstract

Bridging the gap between the approximately ten solar mass 'stellar mass' black holes and the 'supermassive' black holes of millions to billions of solar masses are the elusive 'intermediatemass' black holes. Their discovery is key to understanding whether supermassive black holes can grow from stellar-mass black holes or whether a more exotic process accelerated their growth soon after the Big Bang. Currently, tentative evidence suggests that the progenitors of supermassive black holes were formed as B10 4 -10 5 M } black holes via the direct collapse of gas. Ongoing searches for intermediate-mass black holes at galaxy centres will help shed light on this formation mechanism.O ver the last decade we have come to understand that supermassive black holes, with masses of millions to billions of times the mass of the Sun, are very common in the centres of massive galaxies 1 . We would like to understand when and how they formed and grew.We cannot yet watch the first supermassive black holes form. They did so soon after the Big Bang, and light from those distant events is beyond the reach of today's telescopes. However, we do have two very interesting limits on the formation of the first black holes. The first comes from observations of the most distant known black holes: light is emitted by material falling into the deep gravitational potential of the black hole. These monsters are so bright that they must be powered by at least billion solar mass black holes. They had very little time to grow, as we see them only a few hundred million years after the Big Bang 2,3 . Whatever process formed and grew the first supermassive black holes had to be very efficient.At the other extreme, we can study the lowest-mass black holes in galaxy nuclei near to us, the left-over seeds that for some reason never grew to be a billion suns. As we describe below, conditions were best to make supermassive black hole seeds soon after the Big Bang. Therefore, black holes found in small galaxies today, with black hole masses of B10 4 -10 6 M } , likely formed early and have not grown significantly since. These black holes have masses between those of 'stellar-mass' black holes that form as the end-product of the life of a massive star and supermassive black holes. We will refer to black holes in this mass range as 'low-mass' black holes, as they are at the low-mass end of supermassive black holes. If we assume that black holes form in a similar way in all galaxies, then the numbers and masses of black holes in small galaxies today contain clues about the formation of the first black holes 4 . The sheer number of left-overs will indicate how commonly black hole seeds were formed, as well as inform future gravitational wave experiments that expect to see a large number of paired low-mass black holes as they spiral together and coalesce 5 . Studying the energy output from low-mass black holes could tell us whether growing black holes at early times were important in shaping early star formation in the galaxies around them 6 .

Dark-ages reionization and galaxy formation simulation – X. The small contribution of quasars to reionization

“…Our choice of black hole seed mass, 1000 h −1 M lies between the light seed (∼10 2 − 10 3 M ) from a remnant Pop III star and the massive seed (∼10 3 − 10 5 M ) from the direct collapse of a gas cloud at early times (Greene 2012). The massive seed mass is frequently used to initialize massive haloes ( 10 6 − 10 12 M ) in hydrodynamic simulations (Springel et al 2005;Vogelsberger et al 2014;Schaye et al 2014;Feng et al 2016) while the ∼10 3 M seeds are also often adopted in semi-analytic models (Somerville et al 2008;Bonoli et al 2009).…”

AGNs and Their Host Galaxies in the Local Universe: Two Mass-independent Eddington Ratio Distribution Functions Characterize Black Hole Growth

“…At high redshift the occupation fraction might however be lower than 100% (Treister et al 2013;Weigel et al 2015;Trakhtenbrot et al 2016). Furthermore, depending on the black hole seed formation mechanism, the black hole occupation fraction in dwarf galaxies might be lower than in more massive galaxies (Volonteri et al 2008;Natarajan 2011;Greene 2012;Reines et al 2013;Moran et al 2014;Sartori et al 2015). If this possible mass dependence is not taken into account, the black hole occupation fraction can be defined in the following way using the stellar mass and black hole mass function:…”

Blossoms from black hole seeds: properties and early growth regulated by supernova feedback

“…BHs are also observed in low-mass galaxies (Greene 2012;Reines, Greene & Geha 2013;Miller et al 2015). However we also observe galaxies without any signatures of BHs (e.g., M33, NGC 205, Gebhardt et al 2001;Valluri et al 2005).…”