Estimating Black Hole Masses of AGNs using Ultraviolet Emission Line Properties

Abstract

Based on measuredbroad line region sizes in the reverberation-mappingAGN sam- ple, two new empirical relations are introduced to estimate the central black hole masses of radio-loud high-redshift ( z> 0.5) AGNs. First, using the archival IUE /HST spectroscopy data at UV band for the reverberation-mapping objects, we obtained two new empirical re- lations between the BLR size and Mg II/C IV emission line luminosity. Secondly, using the newly determined black hole masses of the reverberation-mappingsample as calibration, we found two new relationships for determining the black hole mass with the full width at half maximum and the luminosity of Mg II/C IV line. We then apply the relations to estimate the black hole masses of the AGNs in the Large Bright Quasar Survey and a sample of radio-loud quasars. For the objects with small radio-loudness, the black hole mass estimated using the RBLR-LMg II/ CI Vrelation is consistent with that from the RBLR-L 3000 ˚ A/1350 ˚ A relation. For radio-loud AGNs, however, the mass estimated from the R BLR-LMg II/ CI V relation is sys- tematically lower than that from the continuumluminosity L 3000 ˚ A/1350 ˚ A. Because jets could have significant contributions to the UV/optical continuum luminosity of radio-loud AGNs, we emphasize once again that for radio-loud AGNs, the emission line luminosity may be a better tracer of the ionizing luminosity than the continuum luminosity, so that the relations between the BLR size and UV emission line luminosities should be used to estimate the black hole masses of high redshift radio-loud AGNs.