The Seoul National University AGN Monitoring Project IV: H$\alpha$ reverberation mapping of 6 AGNs and the H$\alpha$ Size-Luminosity Relation

TL;DR

This study measures Hα reverberation lags in six AGNs, derives size-luminosity relations, and proposes a new virial mass estimator based on Hα, improving black hole mass estimates especially at lower masses.

Contribution

It provides new Hα reverberation measurements, refines the size-luminosity relation, and introduces a more accurate Hα-based black hole mass estimator.

Findings

Hα size-luminosity relation slopes are ~0.6, consistent with Hβ.

A linear relation exists between 5100Å continuum and Hα luminosities.

Previous mass estimates are overestimated by up to 0.7 dex at lower masses.

Abstract

The broad line region (BLR) size-luminosity relation has paramount importance for estimating the mass of black holes in active galactic nuclei (AGNs). Traditionally, the size of the H$\beta$ BLR is often estimated from the optical continuum luminosity at 5100\angstrom{} , while the size of the H$\alpha$ BLR and its correlation with the luminosity is much less constrained. As a part of the Seoul National University AGN Monitoring Project (SAMP) which provides six-year photometric and spectroscopic monitoring data, we present our measurements of the H$\alpha$ lags of 6 high-luminosity AGNs. Combined with the measurements for 42 AGNs from the literature, we derive the size-luminosity relations of H$\alpha$ BLR against broad H$\alpha$ and 5100\angstrom{} continuum luminosities. We find the slope of the relations to be $0.61\pm0.04$ and $0.59\pm0.04$, respectively, which are consistent with the \hb{} size-luminosity relation. Moreover, we find a linear relation between the 5100\angstrom{} continuum luminosity and the broad H$\alpha$ luminosity across 7 orders of magnitude. Using these results, we propose a new virial mass estimator based on the H$\alpha$ broad emission line, finding that the previous mass estimates based on the scaling relations in the literature are overestimated by up to 0.7 dex at masses lower than $10^7$~M$_{\odot}$.