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

Xihan Ji, Roberto Maiolino, Hannah Übler, Jan Scholtz, Francesco D'Eugenio, Fengwu Sun, Michele Perna, Hannah Turner, Stefano Carniani, Santiago Arribas, Jake S. Bennett, Andrew Bunker, Stéphane Charlot, Giovanni Cresci, Mirko Curti, Eiichi Egami, Andy Fabian, Kohei Inayoshi, Yuki Isobe, Gareth Jones, Ignas Juodžbalis, Nimisha Kumari, Jianwei Lyu, Giovanni Mazzolari, Eleonora Parlanti, Brant Robertson, Bruno Rodríguez Del Pino, Raffaella Schneider, Debora Sijacki, Sandro Tacchella, Alessandro Trinca, Rosa Valiante, Giacomo Venturi, Marta Volonteri, Chris Willott, Callum Witten, Joris Witstok

First listed 2025-01-22 | Last updated 2025-10-28

Abstract

Recent observations from JWST have revealed an abundant population of active galactic nuclei (AGN) and so-called ``Little Red Dots'' (LRDs) at $2\lesssim z \lesssim 11$, 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$β$ tracing the Broad Line Region (BLR) of an AGN. The small velocity dispersion of the H$β$ narrow component indicates a small dynamical mass of the host galaxy of $M_{\rm dyn}<4 \times 10^8~M_{\odot}$, which implies that the stellar population cannot contribute more than 10% to the optical continuum. We show that the Balmer break can be well described by an AGN continuum absorbed by very dense ($n_{\rm H}\sim 10^{10}~{\rm cm^{-3}}$) 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$β$ absorption, at a level consistent with a tentative detection ($3σ$) 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.

Short digest

BlackTHUNDER analyzes JWST NIRSpec-IFU plus archival MSA data of the triply imaged lensed LRD Abell2744-QSO1 at z=7.04, confirming a smooth Balmer break and a broad Hβ from the BLR. The narrow Hβ dispersion gives M_dyn < 4×10^8 Msun, capping the stellar contribution to <10% of the optical continuum and favoring a non-stellar Balmer break produced by an AGN continuum filtered by very dense (n_H ~ 10^10 cm^-3), nearly dust-free gas likely in/around the BLR; a tentative 3σ Hβ absorption appears in the R2700 spectrum. The black hole is overmassive relative to both stellar and dynamical masses, with hints of variability and an initial reverberation-mapping attempt. This picture alleviates the extreme stellar-density problem for LRDs and may generalize to similar V-shaped continua.

Key figures to inspect

• Figure 1: Use the IFU maps and PRISM/grating spectra of the three lensed images to verify the smooth Balmer break, the presence of broad vs narrow Hβ, and the spatial compactness; in the R2700 panel, check the improved residuals when a narrow Hβ absorption component (∼3σ) is included.
• Figure 2: Inspect the stellar–dynamical mass comparison to see why a purely stellar Balmer-break interpretation is unphysical for Abell2744-QSO1 (stellar mass would exceed M_dyn), and note the dynamical upper limit that constrains the stellar light fraction.
• Figure 3: Examine the M_BH–M_* and M_BH–M_dyn planes to appreciate how Abell2744-QSO1 sits well above local scaling relations, underscoring a black hole-dominated system at early times.
• Figure 4: Look at the Cloudy slab models showing how a dust-free, very dense screen imprints Balmer-line absorption and a Balmer break on an AGN continuum; adding a modest dust screen reproduces the rising optical slope typical of LRDs.