The Seoul National University AGN Monitoring Project. IV. H & alpha; Reverberation Mapping of Six AGNs and the H & alpha; Size-Luminosity Relation

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 & ANGS;, 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, which provides 6 yr photometric and spectroscopic monitoring data, we present our measurements of the H & alpha; lags of high-luminosity AGNs. Combined with the measurements for 42 AGNs from the literature, we derive the size-luminosity relations of the H & alpha; BLR against the broad H & alpha; and 5100 & ANGS; continuum luminosities. We find the slope of the relations to be 0.61 & PLUSMN; 0.04 and 0.59 & PLUSMN; 0.04, respectively, which are consistent with the H & beta; size-luminosity relation. Moreover, we find a linear relation between the 5100 & ANGS; 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 scaling relations in the literature are overestimated by up to 0.7 dex at masses lower than 10(7) M (& ODOT;).