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

Meredith A. Stone, George H. Rieke, Jianwei Lyu, Michael K. Florian, Kevin N. Hainline, Yang Sun, Yongda Zhu

First listed 2025-07-17 | Last updated 2025-08-15

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.

Short digest

JWST/NIRCam narrow–medium–wide imaging selects [O III] 5007 emitters at z≈7.08 around ULAS J1120+0641 across two 4.4 arcmin² fields, revisiting a site previously deemed underdense. The team finds 22 candidates near the quasar redshift, demonstrating a strong overdensity relative to field expectations, and sums the stellar+gas masses of the host plus neighbors with incompleteness corrections to estimate the merger-fed growth budget. Even if the black hole ceases accretion, the system is projected to retain an elevated MBH/M* ≳2.5% by z=0, far above the local relation. This implies some descendants could hide as quiescent black holes in faint galaxies at low redshift.

Key figures to inspect

• Figure 2: Sky map in F410M with candidates and the quasar marked—use it to gauge the angular clustering, proximity to the QSO, and the distribution across the two modules, including the medium-confidence sources.
• Figure 4: Number-counts comparison (JWST quasar field vs adjacent field vs deep HST fields) to quantify the overdensity at z≈7.1 and see where JWST completeness drops at the faint end.
• Figure 1: Visual vetting examples showing how detector artifacts and obvious low-z morphologies were culled; assess contamination control for the [O III]-selected sample.
• Figure 3: SEDs of the two HST-selected LBGs overlapping the field—check why one fails the F405N selection and why the other likely lies off the quasar redshift despite its brightness in longer bands.