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 MBH = (2.0-+0.40.7) ´ 109 Me and an Eddington ratio of Lbol LEdd = 0.5-+0.20.1, 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)rest = 5.7 ± 0.7 Å, EW (C IV)rest ≤ 5.8 Å (3σ upper limit), and EW (Mg II)rest = 8.7 ± 0.7 Å. A small proximity zone size (Rp = 1.2 ± 0.4 pMpc) indicates a lifetime of only tQ = 103.4±0.7 years from the last quasar phase ignition. ALMA shows extended [C II] emission with a mild velocity gradient. The inferred far-infrared luminosity (LFIR = (1.2 ± 0.1) ´ 1013 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-1, 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.