2018ApJ...856..109O

Quasars (QSOs) hosting supermassive black holes are believed to reside in massive halos harboring galaxy overdensities. However, many observations revealed average or low galaxy densities around z >= 6 QSOs. This could be partly because they measured galaxy densities in only tens of arcmin² around QSOs and might have overlooked potential larger-scale galaxy overdensities. Some previous studies also observed only Lyman break galaxies (LBGs; massive older galaxies) and missed low-mass young galaxies, like Lyα emitters (LAEs), around QSOs. Here we present observations of LAE and LBG candidates in ∼700 arcmin² around a z = 6.61 luminous QSO using the Subaru Telescope Suprime-Cam with narrowband/broadband. We compare their sky distributions, number densities, and angular correlation functions with those of LAEs/LBGs detected in the same manner and comparable data quality in our control blank field. In the QSO field, LAEs and LBGs are clustering in 4-20 comoving Mpc angular scales, but LAEs show mostly underdensity over the field while LBGs are forming 30 x 60 comoving Mpc² large-scale structure containing 3σ-7σ high-density clumps. The highest-density clump includes a bright (23.78 mag in the narrowband) extended (>=16 kpc) Lyα blob candidate, indicative of a dense environment. The QSO could be part of the structure but is not located exactly at any of the high-density peaks. Near the QSO, LAEs show underdensity while LBGs average to 4σ excess densities compared to the control field. If these environments reflect halo mass, the QSO may not be in the most massive halo but still in a moderately massive one.