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 ${M}_{\mathrm{BH}}=\left({2.0}_{-0.4}^{+0.7}\right)\times {10}^{9}$ M_⊙ and an Eddington ratio of ${L}_{\mathrm{bol}}/{L}_{\mathrm{Edd}}={0.5}_{-0.2}^{+0.1}$ , 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 ${(\mathrm{Ly}\alpha +{\rm{N}}{\rm\small{V}})}_{\mathrm{rest}}=5.7\pm 0.7\,\mathring{\rm A} $ , EW ${({\rm{C}}{\rm\small{IV}})}_{\mathrm{rest}}\leqslant 5.8$ Š(3σ upper limit), and EW ${(\mathrm{Mg}{\rm\small{II}})}_{\mathrm{rest}}=8.7\pm 0.7\,\mathring{\rm A} $ . A small proximity zone size ( ${R}_{{\rm{p}}}=1.2\pm 0.4$ pMpc) indicates a lifetime of only ${t}_{{\rm{Q}}}={10}^{3.4\pm 0.7}$ years from the last quasar phase ignition. ALMA shows extended [C II] emission with a mild velocity gradient. The inferred far-infrared luminosity ( ${L}_{\mathrm{FIR}}=(1.2\pm 0.1)\times {10}^{13}\,{L}_{\odot }$ ) 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- $4900\,{M}_{\odot }\,{\mathrm{yr}}^{-1}$ , 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.