An unambiguous AGN and a Balmer break in an Ultraluminous Little Red Dot at z=4.47 from Ultradeep UNCOVER and All the Little Things Spectroscopy

TL;DR

This study analyzes a highly luminous z=4.47 Little Red Dot behind Abell 2744 using deep JWST UNCOVER and ALT data to confirm a broad-line AGN and a Balmer-break feature. Through multi-wavelength spectroscopy and forward-modeled SED fitting, the authors disentangle AGN and stellar contributions, revealing a compact, massive stellar core (~8×10¹⁰ M_⊙) coexisting with a supermassive black hole (~10⁹ M_⊙) and a Balmer-break consistent with an evolved population. The work highlights extreme BLR emission, strong UV Fe II, and unusual nitrogen lines, while acknowledging model degeneracies and the need for higher-resolution spectroscopy to robustly separate stellar absorption from gas emission. These findings advance our understanding of AGN-host coevolution in the early universe and demonstrate the power of JWST in resolving the inner structure and spectral components of the most distant, luminous little red dots.

Abstract

We present a detailed exploration of the most optically-luminous Little Red Dot ($L_{Hα}=10^{44}$erg/s, $L_V=10^{45}$erg/s, F444W=22AB) found to date. Located in the Abell 2744 field, source A744-45924 was observed by NIRSpec/PRISM with ultradeep spectroscopy reaching SNR$\sim$100pix$^{-1}$, high-resolution 3-4 micron NIRCam/Grism spectroscopy, and NIRCam Medium Band imaging. The NIRCam spectra reveal high rest-frame EW $W_{Hα,0,broad}>800$Å, broad H$α$ emission (FWHM$\sim$4500 km/s), on top of narrow, complex absorption. NIRSpec data show exceptionally strong rest-frame UV to NIR Fe II emission ($W_{FeII-UV,0}\sim$340Å), N IV]$λλ$1483,1486 and N III]$λ$1750, and broad NIR O I $λ$8446 emission. The spectra unambiguously demonstrate a broad-line region associated with an inferred $M_{BH}\sim10^9M_\odot$ supermassive black hole embedded in dense gas, which might explain a non-detection in ultradeep Chandra X-ray data (>$10\times$ underluminous relative to broad $L_{Hα}$). Strong UV Nitrogen lines suggest supersolar N/O ratios due to rapid star formation or intense radiation near the AGN. The continuum shows a clear Balmer break at rest-frame 3650Å, which cannot be accounted for by an AGN power-law alone. A stellar population model produces an excellent fit with a reddened Balmer break and implying a massive ($M_*\sim8\times10^{10}M_\odot$), old $\sim$500 Myr, compact stellar core, among the densest stellar systems known ($ρ\sim3\times10^6M_\odot$/pc$^2$ for $R_{e,opt}=70\pm10$ pc), and AGN emission with extreme intrinsic EW $W_{Hα,0}\gg$1000Å. However, although high $M_*$ and $M_{BH}$ are supported by evidence of an overdensity containing 40 galaxies at $z=4.41-4.51$, deep high-resolution spectroscopy is required to confirm stellar absorption and rule out that dense gas around the AGN causes the Balmer break instead.

Figures (6)

• Figure 1: Top: broadband NIRCam images of A2744$-$45924 in the short wavelength channel filters F070W, F090W, F115W, F150W, F200W at 20 mas pixel size and long wavelength channel filters F277W, F356W, F444W at 40 mas pixel size. The images are $3\hbox{$^{\prime\prime}$}$ on a side. North is up, East is left. Also shown are color composite images and an overlay of the micro-shutter array locations of the NIRSpec PRISM observations. Middle: the 2D NIRSpec/PRISM spectrum used for analysis. Bottom: observed NIRSpec/PRISM spectrum (black) and error array (grey). Positions of key spectroscopic features are indicated. Top axis is rest-frame wavelength in micron.
• Figure 2: Results of fitting two-component models to the F070W, F200W, and F300M images of A2744$-$45924 using pysersic, orientation has North up as above in Figure \ref{['fig:thedata']}. We show zoomed in cutouts of the three bands modeling alongside the joint posterior distribution of the Sersic index and effective radius for the bright compact component. We find that A2744$-$45924 is marginally resolved in F070W and F200W, but point-like at longer wavelengths. We adopt the F200W size, as it is most likely to be dominated by evolved stars § \ref{['sec:contfit']}.
• Figure 3: Rest-frame far-UV fit to A2744$-$45924. We see prominent N$\;$]$\lambda 1483$+N$\;$]$\lambda 1487$ Å, CIV $\lambda\lambda$1548,1551, HeII $\lambda$1640, OIII] $\lambda$1661,1666, N$\;$] $\lambda 1750$, and [CIII]$\lambda$1907+CIII]$\lambda$1909. Note that SiIII] and CIII] are blended, as are HeII and OIII]. Also needed to explain the spectrum is the blend of Al$\;$$\lambda 1860$, [Ne$\;$]+Si$\;$$\lambda 1815$ and Fe$\;$$\lambda 1786$, which are lines also found in the Shen:2019 z$\gtrsim$5.7 quasar composite. This broad-line fit is marginally preferred over fits constrained to narrower FWHM. The CIV profile drives this preference, which may be because of the blending in most other lines or because CIV alone is broad. We show the broad fit because of this fit preference and the evidence from Fe II of broad-line emission in the rest-frame UV.
• Figure 4: Model fits to the rest-frame near-UV. Shown are the observations (black solid line), the local AGN powerlaw continuum (purple dashed), strong broad-line Fe$\;$ pseudo-continuum emission (brown dashed) and the broad Mg ii$\lambda2798$ (yellow), broad He ii$\lambda3203$ (red), and narrow [Ne$\;$]$\lambda3426$ (green) emission lines.
• Figure 5: The total H$\alpha$+[N$\;$] emission line profile as seen by the JWST/F356W GrismR with best-fit line emission and absorption complex. The profile is centered on the central velocity of the broad H$\alpha$ component. Green dashed lines mark the location of the [N$\;$] doublet. The top panels shows the two-dimensional grism spectrum from each roll angle with a $\gamma=0.5$ power-law normalisation to highlight the dynamic range in the data. The bottom panel shows the residuals of the best fit, normalised by the noise level. In pink and purple we show the residuals from individual roll angles, while in blue we show the residuals from the average of both roll angles.