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

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.

Figures (8)

• Figure 1: Images and rest frame spectroscopy of A2744-z7DLA. Top: The 2D prism spectrum extracted using the msaexp pipeline from Gabe Brammer. Bottom: The extracted 1D spectrum is shown in gray, with the $1\sigma$ error indicated by the light gray shading. Broadband photometry measurements from the literature paris_catalog are included as light green dots with error bars. The light green curve represents the best-fit power law model of the rest-frame UV continuum, spanning $1500-2600\ \mathrm{\AA}$. Important emission line wavelengths are marked with vertical dashed lines. It is evident that A2744-z7DLA exhibits a series of prominent Balmer lines and weak Ly$\alpha$ emission.
• Figure 2: GalfitS AGN–host decomposition of A2744-z7DLA. The first column displays the observed images in seven filters; the second column shows data subtracted by PSF model; the third column presents the Sérsic model; and the fourth column shows the residuals divided by the errors. We wrote the stellar mass, effective radius and Sérsic index on the lower left of the panel in first row. The residual images indicate that the model successfully reproduces the observed characteristics of A2744-z7DLA in most bands.
• Figure 3: SED fitting results from CIGALE and Bagpipes. The total model spectra from CIGALE and Bagpipes are shown as blue and cyan curve. The observed photometric fluxes for the filters F814W, F105W, F125W, F140W, F160W, F115W, F150W, F200W, F277W, F356W, F410M, and F444W are shown as white points. The observed flux and error are represented by the gray curve and shaded region, respectively. We zoom in on the UV turnover part of the spectrum in the upper-right corner of this figure. Both the CIGALE and Bagpipes models reproduce the UV and optical observations well, but neither fits the curvature at the red end of 1216$\ \mathrm{\AA}$ successfully.
• Figure 4: Fitting results of optical emission lines identified in the JWST/NIRSpec spectrum of A2744-z7DLA. The panels show $F_\lambda$ (in units of $10^{-20} \mathrm{erg}\cdot\mathrm{s}^{-1}\cdot\mathrm{cm}^{-2}\cdot\mathrm{\AA}^{-1}$) as a function of velocity offset or rest-frame wavelength. In the left panel, we showed more detailed information about the two-component Gaussian fitting of H$\alpha$ emission. Relevant measurements, such as line flux and FWHM, are displayed in this panel, using the same color as the corresponding best-fit model.All results presented here are before correction for dust extinction and lensing magnification. Emission-line measurements are summarized in Table \ref{['tab:ELflux']}. A detailed description of the fitting method and a discussion of line ratios are provided in Sections \ref{['subsect:EL']} and \ref{['subsect:ratio']}.
• Figure 5: Observed spectroscopic data and best-fit models. The rest-frame NIRSpec/prism 1D spectra and their $1\sigma$ uncertainties are shown as gray lines and shaded regions, respectively. The best-fit DLA models and their uncertainties are plotted as green lines and shaded regions. The lower limit of intrinsic Ly$\alpha$ estimated from the DLA model is shaded in sky blue. The model spectrum and nebular components from the CIGALE are plotted in orange and pink, respectively. We also mark the Balmer limit with black vertical lines. We zoomed in on the UV turnover part of the spectrum and wrote the best-fitting HI column density and Ly$\alpha$ flux in the upper-right corner of this figure. Even considering the nebular emission, it is difficult to reproduce both the UV curvature and the steep UV slope $\beta$ simultaneously, so the DLA absorption is required.