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-BH = (2.0-(+0.7)(0.4)) x 10(9) M-circle dot and an Eddington ratio of = L-bol/E-dd =0.5(-0.2)(+0.1), in line with an actively accreting supermassive black hole (SMBH) at z greater than or similar to 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 (Mg II)(rest) = 8.7 +/- 0.7 angstrom. 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) x 10(13) L-circle dot) is one of the highest among all known quasar hosts at z greater than or similar to 6. Dust and [C II] emissions put a constraint on the star formation rate of SFR = 900-4900 M-circle dot 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.