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

Kavli Affiliate: Robert A. Simcoe

| First 5 Authors: Irham Taufik Andika, Knud Jahnke, Masafusa Onoue, Eduardo BaƱados, Chiara Mazzucchelli

| Summary:

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
$M_mathrm{BH}=left(2.0^{+0.7}_{-0.4}right)times10^9~M_odot$ and an
Eddington ratio of $L_mathrm{bol}/L_mathrm{Edd}=0.5^{+0.1}_{-0.2}$, in line
with actively accreting supermassive black hole (SMBH) at $zgtrsim6$. 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) $=5.7pm0.7$ Angstrom,
EW(CIV) $leq5.8$ Angstrom (3-sigma upper limit), and EW(MgII) $=8.7pm0.7$
Angstrom. A small proximity zone size ($R_mathrm{p}=1.2pm0.4$ pMpc) indicates
a lifetime of only $t_mathrm{Q}=10^{3.4pm0.7}$ years from the last quasar
phase ignition. ALMA shows extended [CII] emission with a mild velocity
gradient. The inferred far-infrared luminosity
($L_mathrm{FIR}=(1.2pm0.1)times10^{13},L_odot$) is one of the highest
among all known quasar hosts at $zgtrsim6$. Dust and [CII] emissions put a
constraint on the star formation rate of SFR
$=900-4900~M_odot,mathrm{yr^{-1}}$, 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.

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