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

Anthony J. Taylor, Vasily Kokorev, Dale D. Kocevski, Hollis B. Akins, Fergus Cullen, Mark Dickinson, Steven L. Finkelstein, Pablo Arrabal Haro, Volker Bromm, Mauro Giavalisco, Kohei Inayoshi, Stephanie Juneau, Gene C. K. Leung, Pablo G. Perez-Gonzalez, Rachel S. Somerville, Jonathan R. Trump, Ricardo O. Amorin, Guillermo Barro, Denis Burgarella, Madisyn Brooks, Adam Carnall, Caitlin M. Casey, Yingjie Cheng, John Chisholm, Katherine Chworowsky, Kelcey Davis, Callum T. Donnan, James S. Dunlop, Richard S. Ellis, Vital Fernandez, Seiji Fujimoto, Norman A. Grogin, Ansh R. Gupta, Nimish P. Hathi, Intae Jung, Michaela Hirschmann, Jeyhan S. Kartaltepe, Anton M. Koekemoer, Rebecca L. Larson, Ho-Hin Leung, Mario Llerena, Ray A. Lucas, Derek J. McLeod, Ross McLure, Lorenzo Napolitano, Casey Papovich, Thomas M. Stanton, Roberta Tripodi, Xin Wang, Stephen M. Wilkins, L. Y. Aaron Yung, Jorge A. Zavala

First listed 2025-05-07 | Last updated 2025-06-24

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$β$ emission line and narrow [O III]$λ\lambda4959,5007$ lines, indicative of a BLAGN. Based on the broad H$β$ line, we compute a canonical black-hole mass of $\log(M_{\textrm{BH}}/M_{\odot})=7.58\pm0.15$, 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.

Short digest

CAPERS-LRD-z9 is a PRIMER-selected little red dot spectroscopically confirmed with NIRSpec/PRISM as a z=9.288 broad-line AGN via a ~3500 km s−1 Hβ plus narrow [O III] λλ4959,5007. The broad Hβ implies a canonical black-hole mass log(MBH/M⊙)=7.58±0.15 (allowing 6.65–8.50 with systematics). A pronounced Balmer break requires a dense (~10^10 cm−3) neutral-gas cocoon around the nucleus; SED modeling (CLOUDY+bagpipes) limits the host to M*<10^9 M⊙, hinting at MBH/M* possibly >5% and a UV continuum that may be AGN-dominated. Together, these properties push early BH growth scenarios toward massive seeds or super‑Eddington episodes within ≲500 Myr of cosmic time.

Key figures to inspect

• Figure 1: Use the NIRCam/MIRI cutouts and slit overlay to verify the source’s compact, unresolved appearance and the PRISM extraction; in the 1D spectrum, note the Ly break, strong Balmer break, and the separation of broad Hβ from narrow [O III] features confirming the BLAGN nature at z=9.288.
• Figure 2: Inspect the PRISM fits to Hβ+[O III]; the necessity of a broad Hβ component (FWHM ~3500 km s−1) anchors the MBH estimate, while the narrow [O III] doublet sets the systemic redshift and highlights the BLR–NLR dichotomy at z≈9.3.
• Figure 3: Check the 2D surface-brightness modeling in F200W and F444W; point-source residuals show CAPERS-LRD-z9 is unresolved, implying pc-scale upper limits consistent with a nucleus-dominated light profile.
• Figure 4: Examine the joint CLOUDY (dense, enshrouded AGN) + bagpipes (stellar UV) fit that reproduces the red-ward continuum/Balmer break and enforces M*<10^9 M⊙; also note the models’ failure to match strong [O III] λ4363, signaling extreme gas conditions not captured by the baseline setup.