2021ApJ...920...74J

We measure a strong excess in the galaxy number density around PG 1630+377, an extremely massive (M_BH ≃ 10^9.7M_☉) quasar at z = 1.475, using near-infrared narrowband imaging. We identify 79 narrow H-band excess objects in a 525 arcmin² area including the vicinity and surroundings of the quasar. These sources are likely Hα line emitting, star-forming galaxies at z ≃ 1.47. We detect a δ = 6.6 ± 2.7 overdensity of narrow H-band excess objects located at a projected distance ≃2.1 Mpc northeast of the quasar, which is the densest region in the target area. The overdensity is present in BzK color-selected galaxies, while a previously reported overdensity in the immediate vicinity of PG 1630+377 is not, and yet appears as a group-like structure. These megaparsec-scale environments are estimated to merge into a ≃10^14.7M_☉ cluster at present. Our results support the view that extremely massive black holes form and grow in group-scale environments and later incorporate into a galaxy cluster.