2019A&A...631L..10D

State-of-the-art models of massive black hole formation postulate that quasars at z>6 reside in extreme peaks of the cosmic density structure in the early universe. Even so, direct observational evidence of these overdensities is elusive, especially on large scales (≫1Mpc) as the spectroscopic follow-up of z>6 galaxies is observationally expensive. Here we present Keck/DEIMOS optical and IRAM/NOEMA millimeter spectroscopy of a z∼6 Lyman-break galaxy candidate originally discovered via broadband selection, at a projected separation of 4.65 physical Mpc (13.94 arcmin) from the luminous z=6.308 quasar J1030+0524. This well-studied field presents the strongest indication to date of a large-scale overdensity around a z>6 quasar. The Keck observations suggest a z∼6.3 dropout identification of the galaxy. The NOEMA 1.2mm spectrum shows a 3.5σ line that, if interpreted as [CII], would place the galaxy at z=6.318 (i.e., at a line-of-sight separation of 3.9 comoving Mpc assuming that relative proper motion is negligible). The measured [CII] luminosity is 3x10⁸L_☉, in line with expectations for a galaxy with a star formation rate ∼15M_☉/yr, as inferred from the rest-frame UV photometry. Our combined observations place the galaxy at the same redshift as the quasar, thus strengthening the overdensity scenario for this z>6 quasar. This pilot experiment demonstrates the power of millimeter-wavelength observations in the characterization of the environment of early quasars.