2020ApJ...903...34A

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_BH=(2.0_–0.4^+0.7)×10⁹M_☉ and an Eddington ratio of L_bol/L_Edd=0.5_–0.2^+0.1, 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 (Lyα+NV)_rest=5.7±0.7Å, EW (CIV)_rest≤ 5.8 Å (3σ upper limit), and EW (MgII)_rest=8.7±0.7Å. A small proximity zone size (R_p=1.2±0.4 pMpc) indicates a lifetime of only t_Q=10^3.4±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 (L_FIR=(1.2±0.1)×10¹³L_☉) 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 = 900-4900M_☉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.