C iv λ1549 as an Eigenvector 1 Parameter for Active Galactic Nuclei

Abstract

We are exploring a spectroscopic unification for all types of broad-line emitting AGNs. The four-dimensional Eigenvector 1 (4DE1) parameter space organizes quasar diversity in a sequence primarily governed by Eddington ratio. This paper considers the role of C IV λ1549 measures as 4DE1 diagnostics. We use HST archival spectra for 130 sources with S/N high enough to permit reliable C IV λ1549 broad-component measures. We find a C IV λ1549 BC profile blueshift that is strongly concentrated among (largely radio-quiet [RQ]) sources with FWHM(Hβ BC ) ≲ 4000 km s -1 (which we call Population A). Narrow-line Seyfert 1 (NLSy1; with FWHM Hβ ≤ 2000 km s -1 ) sources belong to this population but do not emerge as a distinct class. The systematic blueshift, widely interpreted as arising in a disk wind/outflow, is not observed in broader line AGNs (including most radio-loud [RL] sources), which we call Population B. We find new correlations involving FWHM(C IV λ1549 BC ), C IV λ1549 line shift, and equivalent width only among Population A sources. Sulentic et al. suggested C IV λ1549 measures enhance an apparent dichotomy between sources with FWHM(Hβ BC ) less and greater than 4000 km s -1 , suggesting that it has more significance in the context of broad-line region structure than the more commonly discussed RL versus RQ dichotomy. Black hole masses computed from FWHM C IV λ1549 BC for about 80 AGNs indicate that the C IV λ1549 width is a poor virial estimator. Comparison of mass estimates derived from Hβ BC and C IV λ1549 reveals that the latter show different and nonlinear offsets for Population A and B sources. A significant number of sources also show narrow-line C IV λ1549 emission that must be removed before C IV λ1549 BC measures can be made and interpreted effectively. We present a recipe for C IV λ1549 narrow-component extraction.