DECARLI, ROBERTOROBERTODECARLIMIGNOLI, MarcoMarcoMIGNOLIGILLI, RobertoRobertoGILLIBALMAVERDE, BarbaraBarbaraBALMAVERDEBrusa, MarcellaMarcellaBrusaCAPPELLUTI, NicoNicoCAPPELLUTICOMASTRI, AndreaAndreaCOMASTRINANNI, RICCARDORICCARDONANNIPECA, ALESSANDROALESSANDROPECAPENSABENE, ANTONIOANTONIOPENSABENEVANZELLA, ErosErosVANZELLAVIGNALI, CRISTIANCRISTIANVIGNALI2021-01-192021-01-1920190004-6361http://hdl.handle.net/20.500.12386/29851State-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 ($\gg$1 Mpc) 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 $z\sim6$ 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 $z\sim6.3$ dropout identification of the galaxy. The NOEMA 1.2mm spectrum shows a 3.5$\sigma$ 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 $3\times10^8$ L$_\odot$, in line with expectations for a galaxy with a star formation rate $\sim15$ M$_\odot$ yr$^{-1}$, 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.STAMPAenArticle10.1051/0004-6361/2019368132-s2.0-85077306520000499098500001http://arxiv.org/abs/1910.12887v1https://www.aanda.org/articles/aa/full_html/2019/11/aa36813-19/aa36813-19.html2019A&A...631L..10DFIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation