2022ApJ...927..141M

We quantify galaxy overdensities around three high-redshift quasars with known [C II]_158 µm_ companions: PJ231-20 (z = 6.59), PJ308-21 (z = 6.24), and J0305-3150 (z = 6.61). Recent SCUBA2 imaging revealed the presence of 17 submillimeter galaxies (SMGs) with sky separations 0.'7 < θ < 2.'4 from these three quasars. We present ALMA Band 6 follow-up observations of these SCUBA2-selected SMGs to confirm their nature and redshift. We also search for continuum-undetected [C II]_158 µm_ emitters in the ALMA pointings and make use of archival MUSE observations to search for Lyα emitters (LAEs) associated with the quasars. While most of the SCUBA2-selected sources are detected with ALMA in the continuum, no [C II]_158 µm_ line emission could be detected, indicating that they are not at the quasar redshifts. Based on the serendipitous detection of CO 7-6 and [C I]_809 µm_ emission lines, we find that four SMGs in the field of PJ231-20 are at z ∼ 2.4, which is coincident with the redshift of an Mg II absorber in the quasar rest-frame UV spectrum. We report the discovery of two LAEs within <0.6 cMpc of PJ231-20 at the same redshift, indicating an LAE overdensity around this quasar. Taken together, these observations provide new constraints on the large-scale excess of Lyα- and [C II]_158 µm_-emitting galaxies around z > 6 quasars and suggest that only wide-field observations, such as MUSE, ALMA, or JWST mosaics, can reveal a comprehensive picture of large-scale structure around quasars in the first billion years of the universe.