Comment on "Measuring the black hole masses of high redshift quasars"

Abstract

In a recent paper, McLure & Jarvis (2002, astro-ph/0204473, v.1) reanalyze AGN broad-line reverberation measurements presented in Kaspi et al. (2000). They find the broad-line region size and the AGN luminosity at 5100 Ang can be related by R_{BLR} ~ L_{5100}^{0.5}. This differs from the result of Kaspi et al., who found R_{BLR} ~ L_{5100}^{0.7}, a departure from expectations for a constant ionization parameter. In this note I investigate the source of the discrepancy. McLure & Jarvis use quasar luminosities based on the single-epoch, multichannel-spectrophotometer measurements of Neugebauer et al. (1987), obtained in 1980. I show that the Neugebauer et al. fluxes are systematically higher, by a constant flux offset, compared with the multi-epoch CCD measurements of Kaspi et al., obtained concurrently with the echo-mapping radii. In addition, McLure & Jarvis erred when converting these fluxes to luminosities. The two effects compound to a typical factor 2 (0.3 dex) overestimate of the luminosities of the PG quasars in the sample. Since McLure & Jarvis did adopt the Seyfert luminosities in Kaspi et al. (2000), they obtained a slope that is flatter by 0.2.