Letter to the Editor

Testing the paradigm: First spectroscopic evidence of a quasar–galaxy Mpc-scale association at cosmic dawn

¹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
e-mail: roberto.decarli@inaf.it
² INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
³ Dipartimento di Fisica e Astronomia, Universitá degli Studi di Bologna, Via P. Gobetti 93/2, 40129 Bologna, Italy
⁴ Physics Department, University of Miami, Coral Gables, FL 33124, USA

Received: 30 September 2019
Accepted: 25 October 2019

Abstract

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 (≫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 ∼ 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 1.2 mm spectrum shows a 3.5σ line that, if interpreted as [C II], 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 [C II] luminosity is 3 × 10⁸ L_⊙, in line with expectations for a galaxy with a star formation rate ∼15 M_⊙ 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.