What if young z>9 JWST galaxies hosted massive black holes?

TL;DR

This paper models the detectability and growth of massive black holes in high-redshift JWST galaxy candidates, exploring implications for seed black hole origins and early galaxy evolution.

Contribution

It provides a detailed analysis of the observational signatures and growth potential of massive black holes in z>9 galaxies, highlighting the detectability conditions and implications for seed black hole models.

Findings

Unobscured MBH emission is faint compared to galaxy starlight in JWST filters.

Only overmassive, high Eddington rate MBHs are detectable in these galaxies.

Growth of MBHs from ~10^5 to >3×10^7 Msun is feasible by z~6 in massive galaxies.

Abstract

JWST is discovering star forming `candidate' galaxies with photometric redshifts $z>9$ and little attenuation. We model presumptive massive black holes (MBHs) in such galaxies and find that their unobscured emission is fainter than the galaxy starlight in JWST filters, and difficult to be detected via color-color selection, and X-ray and radio observations. Only MBHs overmassive relative to expected galaxy scaling relations, accreting at high Eddington rates, would be detectable. Their discovery would point to the presence of heavy MBH seeds, but care is needed to exclude the existence of lighter seeds as only overmassive MBHs are detectable in this type of galaxies. Conversely, if no overmassive MBHs are hosted in these galaxies, either there are no heavy seeds or they are rare. The most massive/highest redshift candidate galaxies can attain stellar masses in excess of 5e10 Msun by z~6 if they grow along the SFR-mass sequence, and can nurse a MBH growing from ~1e5 Msun up to >3e7 Msun by z~6, to become hosts of some z>6 quasars. Candidate galaxies of log(M_{gal}/Msun)~8 can not grow their putative seeds fast, unless seeds are >1e6 Msun. The number density of the JWST candidate galaxies far outnumbers that of the highest-z quasar hosts and this allows for about only 1 bright z~6-7 quasar every 1000 of these galaxies.