Diagnosing the massive-seed pathway to high-redshift black holes: statistics of the evolving black hole to host galaxy mass ratio

TL;DR

This paper investigates the evolution of the black hole to host galaxy mass ratio in early massive seed black holes, suggesting that many remain outliers for hundreds of millions of years, which can be detected by JWST and X-ray telescopes.

Contribution

It introduces a statistical analysis of the black hole-host galaxy mass ratio evolution for protogalaxies with massive seeds using Monte-Carlo merger trees, highlighting observable outliers at high redshift.

Findings

Up to 60% of massive seed host galaxies remain outliers for hundreds of Myr.

Predicted detection rate of these outliers is 0.1-1 per JWST field at z>6.

Detected SMBHs with low stellar mass hosts may be these outliers.

Abstract

Supermassive black holes (SMBHs) with masses of $\sim 10^9 {\rm M_\odot}$ within the first billion year of the universe challenge our conventional understanding of black hole formation and growth. One pathway to these SMBHs proposes that supermassive stars (SMSs) born in pristine atomic cooling haloes (ACHs) yield massive seed BHs evolving to these early SMBHs. This scenario leads to an overly massive BH galaxy (OMBG), in which the BH to stellar mass ratio is initially $M_{\rm bh}/M_* \geq 1$, well in excess of the typical values of $\sim 10^{-3}$ at low redshifts. Previously, we have investigated two massive seed BH candidates from the \texttt{Renaissance} simulation and found that they remain outliers on the $M_{\rm bh}-M_{*}$ relation until the OMBG merges with a much more massive halo at $z{=}8$. In this work, we use Monte-Carlo merger trees to investigate the evolution of the $M_{\rm bh}-M_{*}$ relation for $50,000$ protogalaxies hosting massive BH seeds, across $10,000$ trees that merge into a $10^{12} {\rm M_\odot}$ halo at $z{=}6$. We find that up to $60\%$ (depending on growth parameters) of these OMBGs remain strong outliers for several 100 Myr, down to redshifts detectable with {\it JWST} and with sensitive X-ray telescopes. This represents a way to diagnose the massive-seed formation pathway for early SMBHs. We expect to find ${\sim} 0.1{-}1$ of these objects per {\it JWST} NIRCam field per unit redshift at $z\gtrsim 6$. Recently detected SMBHs with masses of $\sim 10^7~{\rm M_\odot}$ and low inferred stellar-mass hosts may be examples of this population.