Kavli Affiliate: Kohei Inayoshi
| First 5 Authors: Anthony J. Taylor, Vasily Kokorev, Dale D. Kocevski, Hollis B. Akins, Fergus Cullen
| Summary:
We present CAPERS-LRD-z9, a little red dot (LRD) which we confirm to be a
$z=9.288$ broad-line AGN (BLAGN). First identified as a high-redshift LRD
candidate from PRIMER NIRCam photometry, follow-up NIRSpec/PRISM spectroscopy
of CAPERS-LRD-z9 from the CANDELS-Area Prism Epoch of Reionization Survey
(CAPERS) has revealed a broad $3500$ km s$^{-1}$ H$beta$ emission line and
narrow [O III]$lambdalambda4959,5007$ lines, indicative of a BLAGN. Based on
the broad H$beta$ line, we compute a canonical black-hole mass of
$log(M_{textrm{BH}}/M_{odot})=7.58pm0.15$, although full consideration of
systematic uncertainties yields a conservative range of
$6.65<log(M_{textrm{BH}}/M_{odot})<8.50$. These observations suggest that
either a massive black hole seed, or a lighter stellar remnant seed undergoing
periods of super-Eddington accretion, is necessary to grow such a massive black
hole in $lesssim500$ Myr of cosmic time. CAPERS-LRD-z9 exhibits a strong
Balmer break, consistent with a central AGN surrounded by dense ($sim
10^{10}textrm{ cm}^{-3}$) neutral gas. We model CAPERS-LRD-z9 using CLOUDY to
fit the emission red-ward of the Balmer break with a dense gas-enshrouded AGN,
and bagpipes to fit the rest-ultraviolet emission as a host-galaxy stellar
population. This upper limit on the stellar mass of the host galaxy
($<10^9,{rm M_odot}$) implies that the black-hole to stellar mass ratio may
be extremely large, possibly $>5%$ (although systematic uncertainties on the
black-hole mass prevent strong conclusions). However, the shape of the UV
continuum differs from typical high-redshift star-forming galaxies, indicating
that this UV emission may also be of AGN origin, and hence the true stellar
mass of the host may be still lower.
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