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

TL;DR

This study investigates the origins of broad Hα lines in heavily obscured AGNs through multi-epoch spectroscopy, revealing multiple sources including outflows, scattering, and the classical BLR, which complicates black hole mass estimates.

Contribution

It provides the first multi-epoch spectroscopic analysis showing diverse origins of broad Hα lines in obscured AGNs, challenging the classical view.

Findings

Broad Hα lines in some AGNs originate from outflows or scattering.

Detection of flux variability confirms BLR origin in Mrk 268.

Multiple origins of broad lines introduce uncertainties in black hole mass measurements.

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$\alpha$ 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$\alpha$ 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 ($4.3\sigma$) flux variation of the broad H$\alpha$ line by using the flux ratio of the H$\alpha$ complex and the [SII]$\lambda\lambda6716$, 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$\alpha$ 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.