Holes in the BH$^\star$? AGN signatures in the FUV spectrum of a black-hole dominated Little Red Dot at $z=7.04$

TL;DR

This study uses JWST FUV spectroscopy of a high-redshift LRD to investigate black-hole signatures, revealing complex emission features that challenge the fully-covered black-hole star model.

Contribution

It provides new observational evidence of FUV emission lines in a $z=7.04$ LRD, suggesting a clumpy or holey absorbing medium around the black hole.

Findings

Detection of broad Lyα emission with FWHM ~1000 km/s.

Evidence of spatially extended low-velocity Lyα emission.

Indications of a clumpy or holey absorbing medium around the black hole.

Abstract

It has been suggested that "Little Red Dots" (LRDs) might be accreting black holes enshrouded by dense gas in a nearly closed geometry, which completely covers the central black hole, leading to an atmosphere-like structure known as the "black-hole star" ($\rm BH^\star$). We test this scenario by analysing new JWST spectroscopy in the far ultraviolet (FUV, rest-frame) of the prototypical LRD Abell2744-QSO1, at $z=7.04$. We found the presence of broad Ly$\alpha$ emission with an FWHM of $\sim 1000$ km/s, and detections of OI, CIV, and/or FeII emission lines. The NIRCam imaging and NIRSpec slit images indicate that the low-velocity component ($v\lesssim 200$ km/s) of Ly$\alpha$ is likely spatially extended, but the high-velocity component ($v\gtrsim 200$ km/s) of Ly$\alpha$ remains unresolved. Based on the multi-component kinematics and flux of Ly$\alpha$ relative to Balmer lines, we conclude that the observed line profile is unlikely to be broadened by subsequent resonant scattering through the interstellar medium. This suggests that the high-velocity component of Ly$\alpha$ originates in the broad-line region, although resonant scattering in the dense gas likely makes Ly$\alpha$ broader than H$\alpha$ as observed. The nebular features of this LRD indicate that there is at least one relatively optically thin direction where Ly$\alpha$ can escape from the broad-line region (BLR). We also found indications that photons from the BLR are powering fluorescence of FeII and OI on a larger physical scale. The FUV features thus challenge the fully-covered geometry interpretation and suggest that there are "holes" in the $\rm BH^\star$, or the absorbing medium is simply clumpy.