CAPERS-LRD-z9: A Gas Enshrouded Little Red Dot Hosting a Broad-line AGN at z=9.288

Abstract

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${\hat{I}}^2$ emission line and narrow [O III]$\hat{I}»\lambda 4959,5007$ lines, indicative of a BLAGN. Based on the broad H${\hat{I}}^2$ line, we compute a canonical black-hole mass of $\log (M_{\text{BH}}/M_{\odot })=7.58\pm 0.15$, although full consideration of systematic uncertainties yields a conservative range of $6.65<\log (M_{\text{BH}}/M_{\odot })<8.50$. 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 $\lesssim 500$ Myr of cosmic time. CAPERS-LRD-z9 exhibits a strong Balmer break, consistent with a central AGN surrounded by dense ($\sim 10^{10}{\text{ cm}}^{-3}$) 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 ($<10^9{\mathrm{M}}_{\odot }$) 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.

25 pages, 8 figures, 3 tables, 1 appendix. Accepted to ApJL. V2: Updated acknowledgements. V3: Fixed plotting error in Figure 7 and updated name of "MoM-150135" to "MoM-BH*-1". V4: Accepted to ApJL, updated to respond to referee comments including an appendix containing an alternate flexible dust law fit