Discovery of a gas-enshrouded broad-line AGN at z $\sim$ 7

TL;DR

This paper reports the discovery of a high-redshift galaxy at z~6.87 with evidence of possible AGN activity, dense neutral hydrogen environment, and detailed physical properties analyzed through JWST and HST data.

Contribution

It presents the first detailed analysis of a z~7 galaxy showing signs of a gas-enshrouded broad-line AGN and dense neutral hydrogen during the Epoch of Reionization.

Findings

Detection of broad Hα emission indicating possible AGN activity

Presence of dense neutral hydrogen environment around the galaxy

Estimated black hole mass of approximately 3×10^7 solar masses

Abstract

The Lyman-alpha (Ly$\alpha$) absorption profile in star-forming galaxies serves as a powerful tracer of the extended, dense neutral hydrogen in their surroundings during the Epoch of Reionization (EoR). We report a unique galaxy, A2744-z7DLA, at $z\approx 6.87$ gravitationally lensed by the foreground galaxy cluster Abell 2744, which exhibits both moderate Ly$\alpha$ emission and damped Ly$\alpha$ absorption, suggesting the presence of a dense neutral hydrogen environment. Our analysis suggests that the UV continuum turnover near Ly$\alpha$ is more likely shaped by a damped Ly$\alpha$ system rather than nebular continuum from two photon process. We analyze the physical properties of A2744-z7DLA with spectroscopic and photometric data from the JWST and the HST. The galaxy shows a compact morphology ($r_e \sim 0.3\ {\rm kpc}$) and a broadened H$\alpha$ emission line, suggesting possible AGN activity. The broad component of H$\alpha$ has a FWHM of $2721 \pm 200\ {\rm km\ s^{-1}}$, corresponding to a black hole mass of $M_{\rm BH}=2.90^{+2.35}_{-1.28}\times 10^7 M_\odot$ and a black hole-to-stellar mass ratio of $\log (M_{\rm BH}/M_{\rm stellar}) = -1.58^{+0.45}_{-0.34}$. The Balmer decrement ($\rm H\alpha/H\beta$) yields a dust attenuation of $\rm A_V \approx 1.15 \pm 0.23$, indicating that this system is less dust-rich than some "little red dots". Furthermore, we perform SED fitting using both stellar and AGN models. The results show that the UV and optical wavelengths are dominated by star-forming regions, while the AGN component contributes primarily at longer wavelengths. This work provides new insights into the interplay between star formation, neutral gas, and potential AGN activity in galaxies during the EoR.