Kavli Affiliate: Roberto Maiolino
| Summary:
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 $sim 22_-7^+12 mathringmathrmA$), 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.
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