Identifying Direct Collapse Black Hole Seeds through their Small Host Galaxies

TL;DR

This paper proposes a method to identify direct collapse black hole seeds in small host galaxies by their high black hole to halo mass ratios and spatial separation from larger halos, aiding early universe SMBH formation studies.

Contribution

It demonstrates that early DCBHs have distinct high black hole to halo mass ratios and specific orbital characteristics, enabling their detection with future X-ray and infrared telescopes.

Findings

DCBHs have higher black hole to halo mass ratios than Pop III remnants.

Merging halos with DCBHs often orbit larger halos at separations detectable by future telescopes.

High mass ratios and orbital features can distinguish DCBHs from other black hole seeds.

Abstract

Observations of high-redshift quasars indicate that super massive black holes (SMBHs) with masses greater than ${\sim}10^9~M_\odot$ were assembled within the first billion years after the Big Bang. It is unclear how such massive black holes formed so early. One possible explanation is that these SMBHs were seeded by `heavy' direct collapse black holes (DCBHs) with masses of $M_{\rm BH}\approx10^5~M_\odot$, but observations have not yet confirmed or refuted this scenario. In this Letter, we utilize a cosmological N-body simulation to demonstrate that before they grow roughly an order of magnitude in mass, DCBHs will have black hole mass to halo mass ratios much higher than expected for black hole remnants of Population III (Pop III) stars which have grown to the same mass (${\sim}10^6~M_\odot$). We also show that when $T_{\rm vir}\approx 10^4~{\rm K}$ halos (the potential sites of DCBH formation) merge with much larger nearby halos ($M_{\rm h} > 10^{10}~M_\odot$), they almost always orbit their larger host halos with a separation of a few kpc, which is sufficient to be spatially resolved with future X-ray and infrared telescopes. Thus, we propose that a future X-ray mission such as Lynx combined with infrared observations will be able to distinguish high-redshift DCBHs from smaller black hole seeds due to the unusually high black hole mass to stellar mass ratios of the faintest observed quasars, with inferred BH masses below ${\sim} 10^6 M_\odot$.