BlackTHUNDER strikes twice: rest-frame Balmer-line absorption and high Eddington accretion rate in a Little Red Dot at $z=7.04$

Francesco D'Eugenio, Roberto Maiolino, Michele Perna, Hannah Uebler, Xihan Ji, William McClymont, Sophie Koudmani, Debora Sijacki, Ignas Juodžbalis, Jan Scholtz, Jake Bennett, Andrew J. Bunker, Stefano Carniani, Stéphane Charlot, Giovanni Cresci, Emma Curtis-Lake, Elena Dalla Bontà, Kohei Inayoshi, Gareth C. Jones, Jianwei Lyu, Alessandro Marconi, Giovanni Mazzolari, Erica J. Nelson, Eleonora Parlanti, Brant E. Robertson, Raffaella Schneider, Charlotte Simmonds, Sandro Tacchella, Giacomo Venturi, Chris Willott, Joris Witstok, Callum Witten

First listed 2025-03-14 | Last updated 2026-04-10

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 $\sim 22_{-7}^{+12} \mathring{\mathrm{A}}$), 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 $σ_{\rm abs}=110_{-10}^{+20}$ 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($M_{\rm BH}/{\rm M_\odot}$)=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 $σ_{\rm n}=22_{-6}^{+5}$ km s$^{-1}$, which places a stringent constraint on the black-hole-to-dynamical-mass ratio of this system to be $M_{\rm BH}/M_{\rm dyn}$=0.15-1.2, confirming the overmassive nature of the black hole and potentially leaving little room for a host galaxy.

Short digest

BlackTHUNDER re-analyzes JWST/NIRSpec-IFS G395H data for the Little Red Dot Abell2744-QSO1 at z=7.04, focusing on the Hα region. The Hα profile is distinctly non-Gaussian, requiring at least two broad Gaussians plus a narrow component, and shows strong rest-frame Balmer-line absorption (EW ≈22 Å) centered at the systemic velocity of the broad line; the absorber’s dispersion (σ_abs ≈110 km s⁻¹) matches that inferred from the Balmer break, arguing for a long‑lived, non-flowing structure. An Hα-based virial mass of log(MBH/M⊙) ≈7.2 and a deconvolved narrow-line width σ_n ≈22 km s⁻¹ yield MBH/Mdyn = 0.15–1.2, confirming an overmassive black hole with limited room for a host galaxy. The revised mass implies high accretion, λ_Edd ≈0.7–1.6 (potentially super‑Eddington at the low‑mass end), tying Balmer-break strength directly to Balmer-line absorption in LRDs.

Key figures to inspect

• Hα spectral decomposition with two broad Gaussians + narrow component + absorption trough: inspect non-Gaussian wings, absorption EW (~22 Å), and the fit residuals near line center.
• Velocity alignment plot of the Hα absorber vs. broad-line centroid: check that the absorption is at Δv ≈ 0 km s⁻¹, ruling out classic inflow/outflow interpretations.
• Line-width constraints panel: posterior on σ_abs (~110 km s⁻¹) from the absorption and σ_n (~22 km s⁻¹) after LSF deconvolution from G395H (R≈3700); compare to micro-turbulent widths required by the Balmer break.
• Black-hole mass and accretion plot from Hα virial estimator: show log(MBH/M⊙) ≈7.2 and inferred λ_Edd ≈0.7–1.6, contrasted with previous Hβ-based estimates.
• Dynamical-mass constraint diagram using the narrow-line dispersion: visualize allowed MBH/Mdyn = 0.15–1.2 and the resulting minimal room for a stellar host.