BlackTHUNDER : a non-stellar Balmer break in a black hole-dominated little red dot at z=7.04

Recent observations from James Webb Space Telescope (JWST) have revealed an abundant population of active galactic nuclei (AGNs) and the so-called 'Little Red Dots' (LRDs) at 2 <= z <= 11, many of which are characterized by V-shaped UV-to-optical continua with turnovers around the Balmer limit. The physical nature of these LRDs is unclear, and it remains debated whether the peculiar spectral shape originates from AGN, compact galaxies, or both. We present the analysis of new NIRSpec-IFU data from the BlackTHUNDER JWST Large Programme and archival NIRSpec-MSA data of a lensed LRD at z = 7 . 04. The spectra confirm the presence of a smooth Balmer break and a broad H,B tracing the Broad Line Region (BLR) of an AGN. The small velocity dispersion of the H,B narrow component indicates a small dynamical mass of the host galaxy of Mdyn < 4 x 108 M (R), which implies that the stellar population cannot contribute more than 10 per cent to the optical continuum. We show that the Balmer break can be well described by an AGN continuum absorbed by very dense (nH <^> 1010 cm-3) and nearly dust-free gas along our line of sight (possibly gas in the BLR or its surrounding). The same gas is expected to produce H,B absorption, at a level consistent with a tentative detection (3 Q ) in the high-resolution spectrum. Such a non-stellar origin of the Balmer break may apply to other LRDs, and would alleviate the issue of extremely high stellar mass surface densities inferred in the case of a stellar interpretation of the Balmer break. We note that this is a rare case of a black hole that is overmassive relative to both the host galaxy stellar and dynamical masses. We finally report indications of variability and the first attempt of AGN reverberation mapping at such an early epoch.