On the origin of the C IV Baldwin effect in active galactic nuclei

Abstract

The origin of the luminosity dependence of the equivalent width (EW) of broad emission lines in active galactic nuclei (AGN) - the Baldwin effect - is not firmly established yet. We explore this question for the broad C IVλ1549 line using the Boroson & Green sample of the 87 z<= 0.5 Bright Quasar Survey (BQS) quasars. Useful UV spectra of the C IV region are available for 81 of the objects, which are used to explore the dependence of the C IV EW on various emission properties. We confirm earlier results on the strong correlations of the C IV EW with some of the emission parameters which define the Boroson & Green Eigenvector 1, and with the optical to X-ray slope α_ox. In addition, we find a strong correlation of the C IV EW with the relative accretion rate, L/L_Edd. Since L/L_Edd drives some of the Eigenvector 1 correlations, it may be the primary physical parameter which drives the Baldwin effect for C IV.