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

Qianqiao Zhou, Xin Wang, Hang Zhou, Emanuele Daddi, Luis C. Ho, Shengzhe Wang, Ruancun Li, Zuyi Chen, Cheng Cheng, Xihan Ji, Yuxuan Pang, Mengting Ju

First listed 2026-01-16 | Last updated 2026-01-16

Abstract

The Lyman-alpha (Ly$α$) 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$α$ emission and damped Ly$α$ absorption, suggesting the presence of a dense neutral hydrogen environment. Our analysis suggests that the UV continuum turnover near Ly$α$ is more likely shaped by a damped Ly$α$ 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$α$ emission line, suggesting possible AGN activity. The broad component of H$α$ 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 ($\rm Hα/Hβ$) 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.

Short digest

A2744-z7DLA is a z≈6.87 galaxy lensed by Abell 2744 that shows moderate Lyα emission together with a damped Lyα absorption trough, signaling a dense neutral H I environment. JWST/HST data reveal a compact source (re≈0.3 kpc) with a broadened Hα line (FWHM 2721±200 km s−1), implying a gas‑enshrouded broad-line AGN with MBH=2.90+2.35−1.28×10^7 M⊙ and log(MBH/Mstellar)=−1.58+0.45−0.34. The Balmer decrement yields AV≈1.15±0.23—less dust than many “little red dots”—and SED fitting indicates star formation dominates the UV–optical while the AGN emerges at longer wavelengths. Together, the DLA-shaped UV turnover and broad-line emission tie neutral-gas reservoirs to early black-hole growth near the end of reionization.

Key figures to inspect

• Figure 1: Inspect the Lyα region where weak emission sits atop a DLA trough; verify the continuum turnover near 1216 Å and the sequence of Balmer lines that anchors the systemic redshift.
• Figure 2: From the GalfitS PSF+Sérsic decomposition, check the lensing-corrected effective radius (≈0.3 kpc) and PSF fraction across filters to gauge the compact nucleus versus host contribution; look for structured residuals.
• Figure 3: Compare CIGALE versus Bagpipes SEDs—zoom into the UV turnover to see why models favor a DLA-shaped continuum and note that both place the AGN contribution primarily at longer wavelengths.
• Figure 4: Examine the two-component Hα fit separating broad and narrow emission; confirm FWHM≈2721 km s−1 used for MBH, and use Hα/Hβ plus [O III]-based ratios to assess attenuation and ionization conditions.