BlackTHUNDER: Shedding light on a dormant and extreme little red dot at z=8.50

TL;DR

This study presents JWST/NIRSpec IFU observations of UNCOVER_20466, a z=8.50 Little Red Dot with an overmassive black hole. Through two-stage blue/red spectral fitting on R100 and R2700 data, the authors extract continuum and emission-line properties, deriving M_BH ≈ 10^{7.4–7.6} M⊙, L_bol ≈ 10^{44.72} erg s^{-1}, and λ_Edd ≈ 0.1–0.15, with minimal dust attenuation (A_V ≈ 0). ISM diagnostics reveal extreme conditions, including [OIII]4363/5007 ≈ 0.13 and [OII]3727/3729 ≈ 0.7, implying high densities up to n_e ≈ 10^7 cm^{-3} and a high ionization parameter (log U ≈ -1.3), alongside a low gas-phase metallicity from SFG calibrations. The galaxy sits on local M_BH–σ_* and M_BH–M_dyn relations, suggesting the BH–host coevolution may proceed toward bulge-dominated systems, while growth models favor an initial heavy seed (M_seed ≳ 10^3 M⊙) to account for the early assembly of such a massive BH. The results imply UNCOVER_20466 represents a progenitor of present-day bulges and offer a window into black hole formation in ultra-dense protogalaxies, with future ALMA and JWST observations needed to further constrain gas content and dynamics.

Abstract

Recent photometric surveys with JWST have revealed a significant population of mysterious objects with red colours, compact morphologies, frequent signs of active galactic nucleus (AGN) activity, and negligible X-ray emission. These 'Little Red Dots' (LRDs) have been explored through spectral and photometric studies, but their nature is still under debate. As part of the BlackTHUNDER survey, we have observed UNCOVER_20466, one of the most distant LRDs known (z=8.5), with the JWST/NIRSpec IFU. Previous JWST/NIRCam and JWST/NIRSpec MSA observations of this source revealed its LRD nature, as well as the presence of an AGN. Using our NIRSpec IFU data, we confirm that UNCOVER_20466 is an LRD (based on spectral slopes and compactness) that contains an overmassive black hole. However, our observed Balmer decrements do not suggest strong dust attenuation, resulting in a lower Hbeta-based bolometric luminosity and Eddington luminosity (~10%) than previously found. This source lies on local relations between M_BH-sigma_* and M_BH-M_Dyn, suggesting that this could be a progenitor of the core of a lower-redshift galaxy. We explore the possible evolution of this source, finding evidence for substantial black hole accretion in the past and a likely origin as a heavy seed at high redshift (~10^3Msol). Lyman-alpha emission is strongly detected, implying f_esc,Lya~30%. The extremely high [OIII]4363/Hgamma ratio is indicative of not only AGN photoionization and heating, but also extremely high densities (ne~10^7cm-3), suggesting that this black hole at such high redshift may be forming in an ultra-dense protogalaxy.

Figures (13)

• Figure 1: Flux map of [OIII]$\lambda5007$ in $\rm UNCOVER\_20466$, compared to adopted aperture (red circle) and PSF FWHM range of our data (white circles).
• Figure 2: Spectrum extracted from NIRSpec IFU R100 data cube (brown line) using a circular aperture of radius $0.125"$. A concatenation of the best-fit red model (using R100 data only) and blue model (using R100 and R2700 data) is shown by the blue line, where the dividing wavelength (i.e., the Balmer break) is shown by a dotted cyan line. Model residuals are shown in the lower portion of the panel (green line). The best-fit centroid wavelengths of the narrow components of each line are shown by dashed vertical black lines, whereas red dashed lines indicate undetected emission lines. The best-fit continuum underlying $\rm Ly\alpha$ is shown by a pink star. Uncertainties ($1\sigma$) for the extracted spectrum and residuals are shown by shaded regions. The wavelength ranges that were excluded from the fit to reduce contamination by faint rest-UV lines are shown by grey shaded regions.
• Figure 3: Spectrum extracted from NIRSpec IFU R2700 data cube (brown line) using a circular aperture of radius $0.125"$. The best-fit model (which was fit to the R100 and R2700 data simultaneously) is shown as blue lines. When two components or lines are overlapping, they are shown by pink lines. Model residuals are shown in the lower portion of each panel (green lines). The best-fit centroid wavelengths of the narrow components of each line are shown by dashed vertical lines. We only show the wavelength range around emission lines of interest. Uncertainties ($1\sigma$) for the extracted spectrum and residuals are shown by shaded regions.
• Figure 4: Line ratio diagnostic diagrams of mazz24, with the value for $\rm UNCOVER\_20466$ from koko23 and the limit from this work (magenta triangle). Points above each line suggest an AGN-only nature, while those below could be interpreted as AGN or star-forming galaxies (SFGs). For comparison, we include a collection of $z>6$ measurements (curt23_mirkolars23naka23lase24juod24topp24trip24uble24cull25curt25hari25ji25gnz11napo25poll25tayl25). Values from the HOMERUN models marc24_HR are coloured by hydrogen density, with star-forming models in the left column and AGN models in the right column. Models where the density, metallicity, or ionisation parameter are comparable to our derived values are shown by darker points, while the fainter points show the full model grid.
• Figure 5: Placement of $\rm UNCOVER\_20466$ on planes of $M_{\rm BH}$-$M_*$ (panel a), $M_{\rm BH}$-$\sigma_{\rm *}$ (panel b), and $M_{\rm BH}$-$M_{\rm dyn}$ (panel c, red point). For comparison, we include a sample of $z>5$ AGN studied with JWST (goul23hari23koce23koce25lars23uble23chis24furt24maio24maio25_QSO1chis24trip24REDakin25deug25juod25kiyo25naid25napo25rina25tayl25zhanJ25). These values are compared to best-fit relations from local galaxies (korm13rein15gree20). The previous estimate of $M_{\rm BH}$ from koko23 is included in the $M_{\rm BH}$-$M_*$ plot.