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

ABSTRACT Recent photometric surveys with James Webb Space Telescope (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 Integral Field Unit (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 ${\rm H}\,\beta$-based bolometric luminosity and $\lambda _{\rm Edd}$ ($\sim 10 {{\ \rm per\ cent}}$) than previously found. This source lies on local relations between $M_{\rm BH}-\sigma _{*}$ and $M_{\rm BH}-M_{\rm dyn}$, 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 ($\sim 10^3\, {\rm M}_{\odot }$). ${\rm Ly}\,\alpha$ emission is strongly detected, implying $f_{\rm esc}^{\rm Ly\,\alpha }\sim 30 {{\ \rm per\ cent}}$. The extremely high ${\rm [O\,{\small III}}]\lambda 4363$/${\rm H}\,\gamma$ ratio is indicative of not only AGN photoionization and heating, but also extremely high densities ($n_{\rm e}\sim 10^7\, \rm cm^{-3}$), suggesting that this black hole at such high redshift may be forming in an ultra-dense protogalaxy.