Probing the Nature of High-redshift Weak Emission Line Quasars: A Young Quasar with a Starburst Host Galaxy

We present the discovery of PSO J083.8371+11.8482, a weak emission line quasar with extreme star formation rate at z = 6.3401. This quasar was selected from Pan-STARRS1, UHS, and unWISE photometric data. Gemini/ GNIRS spectroscopy follow-up indicates a Mg II-based black hole mass of M = (2.0 ) ´ 10 M and an Eddington ratio of L L = 0.5 , in line with an actively accreting supermassive black hole (SMBH) at z ≥ 6. Hubble Space Telescope imaging sets strong constraint on lens boosting, showing no relevant effect on the apparent emission. The quasar is also observed as a pure point source with no additional emission component. The broad-line region (BLR) emission is intrinsically weak and not likely caused by an intervening absorber. We found rest-frame equivalent widths of EW (Lya + N V) = 5.7 ± 0.7 Å, EW (C IV) ≤ 5.8 Å (3σ upper limit), and EW (Mg II) = 8.7 ± 0.7 Å. A small proximity zone size (R = 1.2 ± 0.4 pMpc) indicates a lifetime of only t = 10 ± years from the last quasar phase ignition. ALMA shows extended [C II] emission with a mild velocity gradient. The inferred far-infrared luminosity (L = (1.2 ± 0.1) ´ 10 L ) is one of the highest among all known quasar hosts at z ≥ 6. Dust and [C II] emissions put a constraint on the star formation rate of SFR = 900–4900 M yr , similar to that of a hyperluminous infrared galaxy. Considering the observed quasar lifetime and BLR formation timescale, the weak-line profile in the quasar spectrum is most likely caused by a BLR that is not yet fully formed rather than by continuum boosting by gravitational lensing or a soft continuum due to super-Eddington accretion. BH - 0.4 e bol Edd - 0.2 rest rest rest p Q FIR ◦ ◦ + 0.7 9 + 0.1 3.4 0.7 13 -