A universal minimal mass scale for present-day central black holes

Abstract

Intermediate-mass black holes (IMBHs) of mass M • ≈ 10 2 -10 5 solar masses, M , are the long-sought missing link 1 between stellar black holes, born of supernovae 2 , and massive black holes 3 , tied to galaxy evolution by the empirical M • /σ correlation 4,5 . We show that low-mass black hole seeds that accrete stars from locally dense environments in galaxies following a universal M • /σ relation 6, 7 grow over the age of the Universe to be above M 0 ≈ 3 × 10 5 M (5% lower limit), independent of the unknown seed masses and formation processes. The mass M 0 depends weakly on the uncertain formation redshift, and sets a universal minimal mass scale for present-day black holes. This can explain why no IMBHs have yet been found 3 , and it implies that present-day galaxies with σ < S 0 ≈ 40 km s −1 lack a central black hole, or formed it only recently. A dearth of IMBHs at low redshifts has observable implications for tidal disruptions 8 and gravitational wave mergers 9 . The early stages of massive black hole growth are poorly understood 10 . High-luminosity active galactic nuclei at very high redshift 11 z further imply rapid growth soon after the Big Bang. Suggested formation mechanisms typically rely on the extreme conditions found in the early Universe (very low metallicity, very high gas or star density). It is therefore plausible that these black hole seeds were formed in dense environments, at least a Hubble time ago (z > 1.8 for a look-back time of t H = 10 Gyr) 12 .β between black hole mass, M • , and stellar velocity dispersion, σ , that is observed in the local Universe over more than about three decades in massive black hole mass, correlates M • and σ on scales that are well outside the massive black hole's radius of dynamical influence 3 ,2 . Recent analyses of large heterogeneous galaxy samples find that a universal M • /σ relation holds for all galaxy types 6, 7 , although the scope of this relation and its evolution with redshift remain controversial 13 . Here we adopt the empirical fit ), where δ = 0.49 ± 0.03 is the root mean square of the intrinsic scatter. We assume that this universal M • /σ holds at all redshifts 14 , and that the black hole seeds grow in a locally (within a few r h ) dense stellar environment. By fixing r h , the M • /σ relation then imposes tight connections between the black hole and the dynamical properties of its stellar surroundings 15 , and specifically the rate at which it consumes stars (see Methods section).A central black hole grows by (1) the accretion of stars, compact remnants and dark matter particles that are deflected toward it on nearly radial orbits, and either fall whole through the event horizon or are tidally disrupted outside it, and then accreted; (2) viscosity-driven accretion of interstellar gas; and (3) mergers with other black holes. Of these growth channels, only the accretion of stars must follow from the existence of a central black hole in a stellar system. Moreover, the tidal disruption event * Department of Particle Physics & Astrophysics, W...

Tidally disrupted stars as a possible origin of both cosmic rays and neutrinos at the highest energies

“…A potential limitation of our model is the assumed evolution of the TDE rate with redshift following the IMBH number density. Recent studies suggest that black holes of intermediate and small mass might be less numerous today than in the past because they may have merged into more massive black holes 65 . This would suggest a less negative, or even positive evolution of the TDE rate with redshift for which the combined description of UHECR and PeV neutrino data rapidly becomes more challenging (see also the Supplementary Material).…”

Formation of supermassive black holes in galactic nuclei – I. Delivering seed intermediate-mass black holes in massive stellar clusters

“…We carried out direct N-body simulations of the merger of three stellar clusters to form an NSC to investigate the outcome of simulated runs containing zero, one, two and three IMBHs. If a single IMBH is delivered to the galactic nuclei, it can be retained there and can subsequently grow from gas accretion (Das et al 2021a;Natarajan 2021) or tidal capture and disruption of stars (Stone et al 2017;Alexander & Bar-Or 2017;Boekholt et al 2018;Dittmann & Miller 2020). If multiple IMBHs are delivered to the NSC, then we found that an IMBH binary is likely to form and merge due to gravitational wave (GW) radiation.…”

“…As discussed in Askar et al (2021), several studies have hypothesized that IMBHs of 10 2 − 10 4 M can form in dense stellar clusters (see Greene et al 2020, and references therein), either through runaway mergers of massive stars that may lead to the formation of a seed IMBH (Portegies Zwart & McMillan 2002;Gürkan et al 2004;Freitag et al 2006;Giersz et al 2015;Mapelli 2016;Gieles et al 2018;Reinoso et al 2018;Tagawa et al 2020;Alister Seguel et al 2020;Das et al 2021b), or through the gradual growth of stellarmass BHs and their progenitors via mergers with other BHs or stars (Miller & Hamilton 2002;Giersz et al 2015;Rizzuto et al 2021;González et al 2021;Di Carlo et al 2021). BHs of 10 2 − 10 3 M may grow to larger masses through tidal capture and disruption events in dense clusters (Stone et al 2017;Alexander & Bar-Or 2017;Sakurai et al 2019). Moreover, stellar-mass seed BHs can also grow by accretion of gas in primordial, gas-rich, massive stellar clusters (Vesperini et al 2010;Leigh et al 2013).…”

Feeding and feedback from little monsters: AGN in dwarf galaxies

“…However, independent studies performed at the same time indicated opposite results: simulations show that the stellar debris from tidal disruption events could fuel and grow seed black holes (Alexander & Bar-Or 2017;Zubovas 2019;Pfister et al 2020) whose feedback could become relevant and have significant effects on the host galaxy (Zubovas 2019). Observationally, long-slit and integral-field spectroscopy studies of two different samples of quiescent dwarf galaxies revealed that they possibly host AGN, which could be preventing the formation of stars in such galaxies (Dickey et al 2019;Penny et al 2018).…”