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

TL;DR

This paper investigates the duration that outlier black holes, formed from heavy seeds, remain above typical mass ratios in early universe galaxies, using simulations to inform future JWST and X-ray observations.

Contribution

It provides the first detailed analysis of the lifespan of outlier black holes in the early universe, highlighting their persistent high mass ratios until galaxy mergers.

Findings

Outlier black holes maintain high mass ratios from formation at z≈15 to z≈8.

These black holes are spatially resolvable from neighboring galaxies until mergers.

Future telescopes can detect these outliers to test heavy-seed formation models.

Abstract

The existence of $10^9\ {\rm M_\odot}$ 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. Several suggested formation pathways for these SMBHs lead to a heavy seed, with an initial black hole mass of $10^4-10^6~{\rm M_\odot}$. This can lead to an overly massive BH galaxy (OMBG), whose nuclear black hole's mass is comparable to or even greater than the surrounding stellar mass: the black hole to stellar mass ratio is $M_{\rm bh}/M_* \gg 10^{-3}$, well in excess of the typical values at lower redshift. We investigate how long these newborn BHs remain outliers in the $M_{\rm bh}-M_{*}$ relation, by exploring the subsequent evolution of two OMBGs previously identified in the \texttt{Renaissance} simulations. We find that both OMBGs have $M_{\rm bh}/M_* > 1$ during their entire life, from their birth at $z\approx 15$ until they merge with much more massive haloes at $z\approx 8$. We find that the OMBGs are spatially resolvable from their more massive, $10^{11}~{\rm M_\odot}$, neighboring haloes until their mergers are complete at $z\approx 8$. This affords a window for future observations with {\it JWST} and sensitive X-ray telescopes to diagnose the heavy-seed scenario, by detecting similar OMBGs and establishing their uniquely high black hole-to-stellar mass ratio.