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

Shoichiro Mizukoshi, Takeo Minezaki, Subaru Ubukata, Kazuya Matsubayashi, Hiroaki Sameshima, Mitsuru Kokubo, Takashi Horiuchi, Hirofumi Noda, Satoshi Yamada, Bovornpratch Vijarnwannaluk, Chian-Chou Chen

First listed 2026-01-13 | Last updated 2026-01-13

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.

Short digest

Multi-epoch optical monitoring (Seimei/KOOLS-IFU, 2023–2025) of three heavily obscured AV > 50 mag AGNs with reported broad Hα—MCG -3-34-64, UGC 5101, and Mrk 268—tests whether the broad component is true BLR emission. Using the Hα-complex to [S II] λλ6716,6731 flux ratio, they see no significant broad-Hα variability in MCG -3-34-64 or UGC 5101, but detect a 4.3σ change in Mrk 268. Spectral decompositions point to strong ionized-outflow contribution in MCG -3-34-64 and either outflow or polar-scattered light in UGC 5101, while Mrk 268’s broad line is direct BLR emission with a mildly obscured sightline. The mixed origins imply potential biases in single-epoch Hα black-hole masses, especially for dust-reddened JWST-selected AGNs.

Key figures to inspect

• Figure 1: Check how the KOOLS-IFU footprints sit on the Pan-STARRS hosts—does the IFU fully cover the nucleus and circumnuclear regions where extended outflows or scattering could broaden Hα and dilute variability?
• Figure 2: Inspect each target’s placement in AV versus NH relative to Galactic/SMC relations to verify the extreme obscuration and any gas-to-dust mismatch supporting the obscured-line-of-sight picture.
• Figure 3: Examine the epoch-5 spectral fits around Hα and [S II]—the narrow/broad decompositions, asymmetries, and residuals that motivate outflow-broadened components in MCG -3-34-64 and a possible polar-scattered contribution in UGC 5101; note the integration bands used for flux ratios.
• Figure 4: Follow the multi-epoch Hα/[S II] ratio curves—flat for MCG -3-34-64 and UGC 5101 versus a significant excursion for Mrk 268—alongside the aligned spectra to connect the variability (or lack thereof) with the BLR versus extended/scattered origins and the stable optical continuum in Mrk 268.