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

TL;DR

This study reports the spectroscopic confirmation of CAPERS-LRD-z9 as the highest-redshift broad-line AGN at z=9.288, featuring a broad Hβ line atop narrow [O III] emission. The authors model the rest-frame optical emission with a dense, gas-enshrouded AGN (Cloudy) plus a host galaxy component (BAGPIPES), deriving a canonical black-hole mass of log(M_BH/M⊙)=7.58±0.15 and a host stellar mass upper limit of <10^9 M⊙, yielding a potentially extreme BH-to-stellar mass ratio. They demonstrate that such an SMBH by z≈9.3 requires either a heavy seed or super-Eddington growth for a stellar seed, and they argue for a dense gas origin of the Balmer-break features that differentiates LRDs from typical star-forming galaxies. The UV continuum may have a significant non-stellar, AGN-related component, consistent with a scenario where LRDs represent an early AGN phase enshrouded by dense gas, with implications for black hole seeding and early galaxy evolution. Overall, CAPERS-LRD-z9 provides pivotal constraints on SMBH formation channels and the physical conditions in the immediate AGN environment at the dawn of cosmic time.

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.

Figures (11)

• Figure 1: Top: $5" \times 5"$ NIRCam and MIRI cutouts of CAPERS-LRD-z9 in the PRIMER-COSMOS NIRCam and COSMOS-3D MIRI images. We also show RGB images generated for both the short wavelength (SW) and long wavelength (LW) NIRCam detectors. We overplot the NIRSpec MSA slitlet. Bottom: We plot the 2D and 1D photometrically calibrated spectra of CAPERS-LRD-z9 in $f_{\lambda}$. We mark strongly detected emission features with red dashed lines and overplot the NIRCam photometry with blue squares. Despite being near the corner of the central shutter in the slitlet, CAPERS-LRD-z9 exhibits a detectable Ly$\alpha$ break, a strong Balmer break, and clear rest-optical emission line detections--including broadened H$\beta$ and H$\gamma$, and narrow [O iii]$\lambda\lambda 4959,5007$. The combination of narrow forbidden lines and a broad H$\beta$ clearly indicates that CAPERS-LRD-z9 is a BLAGN at $z=9.288$.
• Figure 2: The observed NIRSpec/PRISM spectrum of CAPERS-LRD-z9 is shown as the fine black curve with uncertainties in gray. Left: Line fits and 1$\sigma$ uncertainties on H$\gamma$ (blue curves and shaded regions), [O iii]$\lambda4363$ (orange curve and shaded region) and the combined fit (thich black curve and shaded region). We de-blend these lines using the high confidence line widths and redshift solution measured from H$\beta$+[O iii]$\lambda\lambda4959,5007$. We show the region of masked spurious pixels with gray shading. Right: Line fits to H$\beta$+[O iii]$\lambda\lambda4959,5007$. A broad (FWHM$=3521\pm502$ km s$^{-1}$, blue curve) component is clearly necessary to reproduce the H$\beta$. We also fit narrow emission (light blue) and absorption (dashed red) components to H$\beta$ to better model the double peak structure of the line, though their fluxes are degenerate at PRISM resolution. The [O iii]$\lambda\lambda4959,5007$ doublet is also detected at ${\rm S/N}\sim4$. The combination of narrow [O iii]$\lambda\lambda4959,5007$ and broad H$\beta$ are clear indicators of a broad-line AGN.
• Figure 3: Properties of CAPERS-LRD-z9
• Figure 4: Results of two-dimensional surface brightness profile fitting. We show $2^{\prime\prime}\times2^{\prime\prime}$ cutouts in F200W and F444W around CAPERS-LRD-z9 in the left column. Our best-fit, point-source models in each band are shown in the middle column, while the residuals (data-model) are shown in the rightmost column. CAPERS-LRD-z9 is unresolved in all bands, with $r_{h} < 0\farcs04$ and $0\farcs08$ in F200W and F444W, respectively, corresponding to respective physical sizes of $\lesssim 175$ pc and $\lesssim 350$ pc.
• Figure 5: Cloudy parameter grid & fitted values