Diverse Origins of Broad H$α$ Lines in Heavily Obscured AGNs Revealed by Multi-epoch Spectroscopy

TL;DR

This work tests whether broad H$\alpha$ detections in heavily obscured AGNs come from the BLR or from alternative structures such as ionized outflows or scattering by polar material. Through multi-epoch optical spectroscopy of three local, highly extinguished AGNs and a variability-focused analysis of the H$\alpha$ complex relative to [S II], the authors diagnose the origin of the broad component. They find that Mrk 268 shows BLR-origin variability, while MCG-3-34-64 is better explained by ionized outflows and UGC 5101 by scattering, indicating multiple origins for broad H$\alpha$ in obscured AGNs. These results highlight potential uncertainties in single-epoch BH mass estimates for obscured AGNs, with implications for JWST-era studies of dusty, distant AGNs and the need for multi-component interpretation.

Abstract

According to the classical AGN model, broad emission lines originate from the broad-line region (BLR) and are observable only when the attenuation by the dusty torus is small. However, we recently found several heavily-obscured ($A_V > 50$ mag) AGNs with broad H$α$ detections: MCG -3-34-64, UGC 5101, and Mrk 268. To investigate the origin of the observed broad line in these AGNs, we performed multi-epoch optical spectroscopic observations to search for flux variability of the broad H$α$ line. For MCG -3-34-64 and UGC 5101, no significant variability was detected, suggesting that the broad line of these AGNs may arise from sources other than the BLR. Spectral fitting analysis suggests possible large contribution of ionized outflows to the observed broad component of MCG -3-34-64, while both the outflow and scattering by polar material can explain that of UGC 5101. For Mrk 268, we detected a significant ($4.3σ$) flux variation of the broad H$α$ line by using the flux ratio of the H$α$ complex and the [SII]$λ\lambda6716$, 6731 doublet, indicating that the broad line originates directly from the BLR. The lack of significant flux variation in the optical continuum implies that the line of sight to the nucleus of Mrk 268 is mildly obscured. Our results demonstrate that the observed broad H$α$ lines in obscured AGNs likely have multiple origins. Such complexity may introduce additional uncertainties in black hole mass measurements of distant AGNs revealed by e.g., JWST.

Figures (14)

• Figure 1: Optical $giy$ composite images of our targets obtained by Pan-STARRS Chambers16. These images are displayed in logarithmic brightness scaling with adjusted color balance. The KOOLS-IFU fiber arrays are overlaid ($8.0\arcsec\times8.4\arcsec$).
• Figure 2: Comparison between dust extinction $A_V$ and neutral gas column density ${N_{\mathrm{H}}}$ for our targets. The orange triangle, circle, and square represents the samples of this study: MCG -3-34-64, UGC 5101, and Mrk 268, respectively. Gray markers represent the entire AGN samples in BASS DR2 catalog for which $A_V$ was measured in Mizukoshi24. The black marker in the upper right corner shows the typical uncertainty of $A_V$ for these two markers. Dark yellow band represents the typical relation for the Galactic ISM Predehl95Nowak12, and dark yellow dotted line and dot-dashed line represent the same as the dark yellow band, but for SMC bar and wing, respectively Gordon03.
• Figure 3: (Left) Spectra of MCG -3-34-64 (upper panel), UGC 5101 (middle panel), and Mrk 268 (bottom panel) obtained in the epoch 5 and the best-fit result of the spectral fitting analysis. The black solid line and surrounding gray band represent the observed spectrum and its $\pm1\sigma$ uncertainty. The orange line represents the best-fit result, the blue lines represent the best-fit results for narrow-line fitting, and the green dashed lines represent those for broad-line fitting. The gray dotted line with error bars represents the residual between the data and the fitting model. The pale orange bands represent the wavelength range used for flux calculation of the H$\alpha$ complex and [S ii] doublet. (Right) Zoom-in spectra of H$\alpha$ complex and [S ii] doublet. The colors are used in the same manner as left panels.
• Figure 4: (Left) Normalized continuum-subtracted spectra around H$\alpha$ complex and [S ii] doublet for each observational epoch for each target. The spectra are vertically shifted by an interval of 0.15, and the blue, green, yellow, red, and magenta spectra represent the data obtained in epoch 1 (2023 March), 2 (2023 April), 3 (2023 December), 4 (2024 May), and 5 (2025 May), respectively. The pale orange bands represent the same as in Figure\ref{['fig:spectra']}. (Right) The time variation of the normalized flux ratio of the H$\alpha$ complex and [S ii] doublet for each target. The colored diamonds connected with black dotted lines represent observational data and each color corresponds to those of the spectra in the left panel. The gray band represents the $\pm1\sigma$ range of each data point.
• Figure 5: Structure function of the H$\alpha_{\mathrm{com.}}$/[S ii] ratio in this study. The left, middle, and right panels show the results of MCG -3-34-64, UGC 5101, and Mrk 268, respectively. The orange diamonds represent the observed SF data of the H$\alpha_{\mathrm{com.}}$/[S ii] ratio for each time separation. The darkred line with pale red band represents the SF of the H$\alpha_{\mathrm{com.}}$/[S ii] ratio based on the DRW model of typical quasars (see Section \ref{['subsec: method of DRW model calculation']}) and its $\pm1\sigma$ uncertainty.