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

Gareth C. Jones, Hannah Übler, Roberto Maiolino, Xihan Ji, Alessandro Marconi, Francesco D'Eugenio, Santiago Arribas, Andrew J. Bunker, Stefano Carniani, Stéphane Charlot, Giovanni Cresci, Kohei Inayoshi, Yuki Isobe, Ignas Juodžbalis, Giovanni Mazzolari, Pablo G. Pérez-González, Michele Perna, Raffaella Schneider, Jan Scholtz, Sandro Tacchella

First listed 2025-09-24 | Last updated 2026-01-14

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.

Short digest

BlackTHUNDER presents JWST/NIRSpec IFU spectroscopy of UNCOVER_20466 (z=8.50), confirming its LRD nature and an overmassive central black hole. Balmer decrements indicate little dust, yielding an Hβ-based bolometric output and Eddington ratio only ~10% of earlier estimates—consistent with a currently subdued/dormant phase. Strong Lyα (f_esc ≈ 30%) and an extreme [O III]4363/Hγ imply AGN photoionization with ultra-dense gas (n_e ~ 10^7 cm^-3). The source sits on local M_BH–σ_* and M_BH–M_dyn relations and is compatible with growth from a heavy seed (~10^3 M⊙), marking it as a likely progenitor core of a later galaxy.

Key figures to inspect

• Figure 1 (R100 IFU spectrum): Inspect the V‑shaped continuum and Balmer break that define the LRD slopes, the Balmer decrement used for the low‑dust inference, and the placement/strength of Lyα within the full PRISM coverage.
• Figure 2 (R2700 zooms): Check narrow Hβ/Hγ and [O III]λ5007,4959,4363 profiles, the absence of a BLR component in [O III], and the measured [O III]4363/Hγ that drives the ultra‑high density estimate.
• Figure 3 (diagnostic planes): Compare the object’s position (new limit vs. prior value) against the Mazzolari et al. AGN/SFG boundaries and density‑colored HOMERUN grids to see why an AGN interpretation is favored.
• Figure 4 (BH–host scaling): Read off M_BH versus σ_* and M_dyn to see that UNCOVER_20466 aligns with local relations and how this contrasts with earlier mass/λ_Edd estimates.