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

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,Lya30%. The extremely high [OIII]4363/Hgamma ratio is indicative of not only AGN photoionization and heating, but also extremely high densities (ne10^7cm-3), suggesting that this black hole at such high redshift may be forming in an ultra-dense protogalaxy.

💡 Research Summary

The BlackTHUNDER program presents the first integral‑field spectroscopic study of UNCOVER 20466, one of the most distant “Little Red Dots” (LRDs) at z = 8.5. Using JWST/NIRSpec IFU data obtained in both the low‑resolution PRISM mode (R ≈ 30–300, 0.6–5.3 µm) and the high‑resolution G395H/F290LP mode (R ≈ 2000–3700, 2.87–5.14 µm), the authors construct calibrated data cubes with 0.05″ spaxels, apply a custom reduction pipeline to mitigate 1/f noise, cosmic rays, and background residuals, and correct for the modest lensing magnification (μ = 1.33). Because the source is unresolved relative to the point‑spread function, a circular aperture capturing 60 % of the PSF at the wavelength of