Calibrating emission lines as quasar bolometers

Abstract

Historically, emission lines have been considered a valuable tool for estimating the bolometric thermal luminosity of the accretion flow in active galactic nucleus, Lbol.We study the reliability of this method by comparing line strengths to the optical/ultraviolet continuum luminosity of Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7) radio-quiet quasars with 0.4 < z < 0.8.We find formulae for Lbol as a function of single line strengths for the broad components of Hβ and MgII, as well as the narrow lines of [O III] and [O II].We determine the standard errors of the formulae that are fitted to the data. Our new estimators are shown to be more accurate than archival line strength estimations in the literature. It is demonstrated that broad lines are superior estimators of the continuum luminosity (and Lbol) with Hβ being the most reliable. The fidelity of each of the estimators is determined in the context of the SDSS DR7 radio-loud quasars as an illustrative application of our results. In general, individual researchers can use our results as a tool to decide if a particular line strength provides an adequate estimate of Lbol for their purposes. Finally, it is shown that considering all four line strengths, simultaneously, can yield information on both Lbol and the radio jet power. © 2011 The Authors.