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

Abstract

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 MgII-based black hole mass of $M_{\mathrm{BH}}=\left(2.0_{-0.4}^{+0.7}\right)\times 10^9{M}_{\odot }$ and an Eddington ratio of $L_{\mathrm{bol}}/L_{\mathrm{Edd}}=0.5_{-0.2}^{+0.1}$, in line with actively accreting supermassive black hole (SMBH) at $z\gtrsim 6$. HST 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(Ly$\alpha$+NV) $=5.7\pm 0.7$ Angstrom, EW(CIV) $\le 5.8$ Angstrom (3-sigma upper limit), and EW(MgII) $=8.7\pm 0.7$ Angstrom. A small proximity zone size ($R_{\mathrm{p}}=1.2\pm 0.4$ pMpc) indicates a lifetime of only $t_{\mathrm{Q}}=10^{3.4\pm 0.7}$ years from the last quasar phase ignition. ALMA shows extended [CII] 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 $z\gtrsim 6$. Dust and [CII] emissions put a constraint on the star formation rate of SFR $=900-4900M_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$, similar to that of hyper-luminous 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 which is not yet fully formed rather than continuum boosting by gravitational lensing or a soft continuum due to super-Eddington accretion.

28 pages, 16 figures, 3 tables, accepted for publication in ApJ

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