BlackTHUNDER strikes twice: Balmer-line absorption in an overmassive Little Red Dot at z = 7.04

Abstract JWST has revealed a population of ‘Little Red Dots’ (LRDs): compact, red objects at redshifts z = 2–9 with ‘v’-shaped spectral energy distributions, broad permitted lines, and, often, hydrogen Balmer absorption. We use NIRSpec/IFS data from the BlackTHUNDER survey to study the H α line in the LRD Abell2744-QSO1 at z = 7.04, which is a confirmed AGN due to time-variable equivalent width (EW) in its broad emission lines. The H α spectral profile is non-Gaussian, requiring at least two Gaussian components. We also detect a narrow-line Gaussian component, and strong H α absorption (EW relative to the continuum $\sim 22_{+12}^{-7}\mathring{\rm A}$), confirming a connection between the strong Balmer break and line absorption. The absorber is at rest with respect to broad H α, suggesting that the gas cannot be interpreted as an inflow or outflow, forming instead a long-lived structure. Its velocity dispersion is $\sigma _abs = 110^{+20}_{-10}$ km s−1, consistent with the value inferred from the analysis of the Balmer break. Based on H α, we infer a black hole mass of log (M•/M⊙) = 7.2, smaller but close to the previous estimates based on H β. The Eddington ratio is 0.09. Combining the high signal-to-noise ratio of the narrow H α line with the spectral resolution R = 3, 700 of the G395H grating, we infer a narrow-line intrinsic dispersion $\sigma _\mathrm{n}=22_{-6}^{+5}$ km s−1, which places a stringent constraint on the black-hole-to-dynamical-mass ratio of this system to be M•/Mdyn = 0.15–1.2, confirming the overmassive nature of the black hole and potentially leaving little room for a host galaxy.