How long do high-redshift massive black hole seeds remain outliers in black hole vs. host galaxy relations?

Abstract

The existence of 10 9 M supermassive black holes (SMBHs) within the first billion years of the universe remains a puzzle in our conventional understanding of black hole formation and growth. The so-called direct-collapse scenario suggests that the formation of supermassive stars (SMSs) can yield the massive seeds of early SMBHs. This scenario leads to an overly massive BH galaxy (OMBG), whose nuclear black hole's mass is comparable to or even greater than the surrounding stellar mass: a 10 4 − 10 6 M seed black hole is born in a dark matter halo with a mass as low as 10 7 − 10 8 M . The black hole to stellar mass ratio is 𝑀 bh /𝑀 * 10 −3 , well in excess of the typical values at lower redshift. We investigate how long these newborn BHs remain outliers in the 𝑀 bh − 𝑀 * relation, by exploring the subsequent evolution of two OMBGs previously identified in the Renaissance simulations. We find that both OMBGs have 𝑀 bh /𝑀 * > 1 during their entire life, from their birth at 𝑧 ≈ 15 until they merge with much more massive haloes at 𝑧 ≈ 8. We find that the OMBGs are spatially resolvable from their more massive, 10 11 M , neighboring haloes until their mergers are complete at 𝑧 ≈ 8. This affords a window for future observations with JWST and sensitive X-ray telescopes to diagnose the direct-collapse scenario, by detecting similar OMBGs and establishing their uniquely high black hole-to-stellar mass ratio.

Supercritical Growth Pathway to Overmassive Black Holes at Cosmic Dawn: Coevolution with Massive Quasar Hosts

“…In conclusion, it is plausible but dependent sensitively on the value of  that seed BHs can be observable even at z  13, where the rapid growth of the DM halo facilitates BH growth even in the presence of outflows. The expected detection number of such accreting seed BHs is 1 in 10 NIRCam fields of view at the depth of 10 ks exposures (Inayoshi et al 2022a) with the help of photometric selection for seed BH candidates (e.g., Pacucci et al 2016;Natarajan et al 2017;Valiante et al 2018;Inayoshi et al 2022b) as well as the measurement of the BH-to-galaxy mass ratio (Scoggins et al 2022). Inayoshi et al (2022b) recently found that seed BHs growing at super-Eddington rates produce extremely strong Balmer lines (the rest-frame Hα equivalent width is ;7 times larger than the typical value for low-z quasars) because of efficient collisional excitation of hydrogen to higher levels (n 3) in the dense disk.…”

A Simple Condition for Sustained Super-Eddington Black Hole Growth

“…Accompanying these are more detailed multidimensional simulations that confirm the key physics at play in these flows, in particular photon trapping (Begelman 1978b), that allow for super-Eddington accretion to be sustained (e.g., Ohsuga et al 2005;Jiang et al 2014;Sądowski & Narayan 2016). While this work collectively supports the possibility of early supermassive BH growth by super-Eddington accretion, it remains for this theory to be tested decisively against observations (e.g., Pognan et al 2020) and it has only recently been incorporated into large-scale cosmological simulations (e.g., Mayer 2019;Scoggins et al 2022).…”

“…This implies that, given a sufficiently large gas supply, a BH accreting in this regime can grow to supermassive BH scales in well under the time limits implied by the existence of z 6 quasars hosting 10 9 M e supermassive BHs. Recent cosmological simulations provide support for this possibility, showing that a gas supply sufficient to fuel sustained super-Eddington accretion may allow BHs to grow from 10 5 to 10 9 M e in rapidly assembling galaxies at z  6 (Scoggins et al 2022). Support is also provided by recent observational results suggesting that the gas density in high-z galaxies increases rapidly with redshift out to z  7.5 (Gilli et al 2022), implying that there may be an ample supply of dense gas for sustained super-Eddington accretion going back to early epochs of BH growth (see also Venemans et al 2017;Trinca et al 2022).…”

“…Simulations of super-Eddington BH growth in isolated galaxies (e.g., Sassano et al 2022) and over relatively brief periods following initial BH formation (e.g., Regan et al 2019) have shown that feedback effects, including those from supernovae and BH jets, can drive the gas out from the vicinity of the BH, precluding sustained episodes of super-Eddington growth. Sustained super-Eddington accretion at rates above the limit given by Equation (10) has, however, been realized in full cosmological simulations in which there is an ever-increasing gas supply in a rapidly growing halo (Scoggins et al 2022). We may therefore expect that sustained super-Eddington growth of BHs is most likely to be realized in gas-rich galaxies that are rapidly growing in the early universe.…”