The $z=7.08$ quasar ULAS J1120+0641 May Never Reach a "Normal" Black Hole to Stellar Mass Ratio

TL;DR

This study uses JWST data to analyze the environment of a high-redshift quasar, revealing a dense galaxy environment and suggesting it may never reach a typical black hole to stellar mass ratio seen in local galaxies.

Contribution

It provides new insights into the growth potential of early quasars and their host galaxies by identifying surrounding galaxies and estimating future mass ratios.

Findings

The quasar's environment is strongly overdense in z~7.1 galaxies.

ULAS J1120+0641 is unlikely to reach a typical local black hole to stellar mass ratio.

The system may remain a black hole in a low-mass galaxy, escaping local detection.

Abstract

JWST observations of quasars in the Epoch of Reionization have revealed that many lie in host galaxies that are severely undermassive relative to the supermassive black holes. It is unclear how these systems will evolve to the tight local relation between stellar mass and black hole mass. We search for companions around the z=7.08 quasar ULAS J1120+0641 using JWST/NIRCam narrow, medium, and wide-band photometry to identify [O III] emitters at the quasar redshift, and explore the potential for growth of the host galaxy through future mergers. We find 22 sources near the quasar's redshift across our two 4.4 arcmin$^2$ fields, indicating that environment of ULAS J1120+0641 is strongly overdense in z~7.1 galaxies relative to the field. We estimate the potential future mass budget of the quasar host galaxy by summing the current stellar and gas masses of the quasar host and surrounding galaxies, correcting for incompleteness and selection effects. With no further black hole growth, ULAS J1120+0641 is unlikely to reach a $M_{\mathrm{BH}}/M_*$ ratio less than ~2.5% at z=0, still much higher than typical for local galaxies. However, such systems -- a quiescent black hole in a low-luminosity galaxy -- may have escaped detection locally if they are sufficiently distant.