A Large Number of z > 6 Galaxies around a QSO at z = 6.43: Evidence for a Protocluster?

QSOs have been thought to be important for tracing highly biased regions in the early universe, from which the present-day massive galaxies and galaxy clusters formed. While overdensities of star-forming galaxies have been found around QSOs at 2<z<5, the case for excess galaxy clustering around QSOs at z>6 is less clear. Previous studies with HST have reported the detection of small excesses of faint dropout galaxies in some QSO fields, but these surveys probed a relatively small region surrounding the QSOs. To overcome this problem, we have observed the most distant QSO at z=6.4 using the large field of view of the Suprime-Cam (34’ x 27’). Newly-installed CCDs allowed us to select Lyman break galaxies (LBG) at z~6.4 more efficiently. We found seven LBGs in the QSO field, whereas only one exists in a comparison field. The significance of this apparent excess is difficult to quantify without spectroscopic confirmation and additional control fields. The Poisson probability to find seven objects when one expects four is ~10%, while the probability to find seven objects in one field and only one in the other is less than 0.4%, suggesting that the QSO field is significantly overdense relative to the control field. We find some evidence that the LBGs are distributed in a ring-like shape centered on the QSO with a radius of ~3 Mpc. There are no candidate LBGs within 2 Mpc from the QSO, i.e., galaxies are clustered around the QSO but appear to avoid the very center. These results suggest that the QSO is embedded in an overdense region when defined on a sufficiently large scale. This suggests that the QSO was indeed born in a massive halo. The central deficit of galaxies may indicate that (1) the strong UV radiation from the QSO suppressed galaxy formation in its vicinity, or (2) that star-formation closest to the QSO occurs mostly in an obscured mode that is missed by our UV selection.

💡 Research Summary

The authors investigate whether the most distant known quasar at redshift z = 6.43 (J0305‑3150) resides in a primordial overdense region that could evolve into a present‑day massive galaxy cluster. Previous work at lower redshifts (2 < z < 5) has shown that quasars often trace highly biased environments, but studies at z > 6 have been inconclusive because they have been limited to very small fields of view (a few arcminutes) with the Hubble Space Telescope. To overcome this limitation, the team used the Subaru Telescope’s Suprime‑Cam, whose newly installed CCDs provide a wide field of 34′ × 27′ (≈ 0.25 deg²) and improved sensitivity in the i′, z′, and y′ bands.

The data were reduced with a standard pipeline, and Lyman‑break galaxy (LBG) candidates at z ≈ 6.4 were selected using an “i‑dropout” technique: non‑detection or i′ − z′ > 1.5 mag, z′ − y′ < 0.5 mag, a ≥ 5σ detection in y′, and morphological criteria to reject stars and spurious sources. This colour cut isolates galaxies in the redshift window 6.3 ≲ z ≲ 6.5 with high completeness and low contamination.

In the quasar field the authors identified seven LBG candidates, whereas a control field observed with identical depth, filter set, and reduction procedures yielded only one candidate. Assuming an average expectation of four objects per field, the Poisson probability of finding seven in a single field is ≈ 10 %. However, the joint probability of obtaining seven in the quasar field and only one in the control field drops below 0.4 %, indicating a statistically significant overdensity around the quasar.

Spatial analysis reveals that none of the LBGs lie within ≈ 2 Mpc (projected) of the quasar; instead they form a roughly ring‑like distribution at a radius of ∼ 3 Mpc. The authors discuss two plausible explanations for this central deficit. First, the intense ultraviolet radiation from the quasar could ionize the surrounding intergalactic medium, suppressing gas cooling and thus star formation in its immediate vicinity. Second, star formation close to the quasar might be heavily dust‑obscured, rendering the galaxies invisible to UV‑based LBG selection but detectable at longer wavelengths (e.g., with ALMA or JWST).

The findings support the hypothesis that the quasar is embedded in a massive dark‑matter halo, consistent with models in which the earliest quasars trace the peaks of the primordial density field. Nevertheless, the study is limited by the lack of spectroscopic redshifts for the LBG candidates and by the use of a single control field. Future work should obtain spectroscopic confirmation, expand the number of comparison fields, and employ infrared/sub‑millimeter facilities to search for obscured companions. Such efforts will clarify whether the observed ring‑like overdensity is a generic feature of high‑z quasar environments and will help quantify the role of quasar feedback in shaping early galaxy formation.