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 Hβ BLR is often estimated from the optical
continuum luminosity at 5100 Å, while the size of the Hα BLR and its correlation with the luminosity is much less
constrained. As a part of the Seoul National University AGN Monitoring Project, which provides 6 yr photometric
and spectroscopic monitoring data, we present our measurements of the Hα lags of high-luminosity AGNs.
Combined with the measurements for 42 AGNs from the literature, we derive the size–luminosity relations of the
Hα BLR against the broad Hα and 5100 Å continuum luminosities. We find the slope of the relations to be
0.61 ± 0.04 and 0.59 ± 0.04, respectively, which are consistent with the Hβ size–luminosity relation. Moreover,
we find a linear relation between the 5100 Å continuum luminosity and the broad Hα luminosity across 7 orders of
magnitude. Using these results, we propose a new virial mass estimator based on the Hα broad emission line,
finding that the previous mass estimates based on scaling relations in the literature are overestimated by up to 0.7
dex at masses lower than 107 Me.
