The C iv linewidth distribution for quasars and its implications for broad-line region dynamics and virial mass estimation

Abstract

We perform an extensive analysis of the C iv λ1549 line in three large spectroscopic surveys of quasars. Differing approaches for fitting the C iv line can be found in the literature, and we compare the most common methods to highlight the relative systematics associated with each. We choose the line‐fitting procedure that results in a symmetric profile for the C iv line and gives accurate fits to local emission features around the line, and use this approach to measure the width of the C iv line in spectra from the Sloan Digital Sky Survey (SDSS) and 2dF QSO Redshift (2QZ) and 2dF SDSS LRG And QSO (2SLAQ) surveys. The results are compared with a previous study of the Mg ii λ2799 line in the same sample. We find the C iv line tends to be broader than the Mg ii line in spectra that have both lines, and the average ratio between the lines is consistent with a simplistic model for a photoionized, virialized and stratified broad‐line region. There exists a statistically significant correlation between the widths of the C iv and Mg ii lines. However, the correlation is weak, and the scatter around a best fit is only marginally less than the full dynamic range of linewidths. Motivated by previous work on the Mg ii line, we examine the dispersion in the distribution of C iv linewidths. We find that the dispersion in C iv linewidths is essentially independent of both redshift and luminosity. This result is in stark contrast to the Mg ii line, which shows a strong luminosity dependence. Furthermore, we demonstrate that the low level of dispersion in C iv linewidth (∼0.08 dex) is inconsistent with a pure‐disc model for the emitting region and use our data to constrain simple models for the broad‐line region. Finally, we consider our results in terms of their implications for the virial technique for estimating black hole masses. The inconsistency between Mg ii and C iv linewidths in single spectra, combined with the differing behaviour of the Mg ii and C iv linewidth distributions as a whole, indicates that there must be an inconsistency between Mg ii and C iv virial mass estimators. Furthermore, the level of intrinsic dispersion in Mg ii and C iv linewidths contributes less dynamic range to virial mass estimates than the error associated with the estimates. The indication is that the linewidth term in these UV virial mass estimators may be essentially irrelevant with respect to the typical uncertainty on a mass estimate.

Extending the Calibration of C iv-based Single-epoch Black Hole Mass Estimators for Active Galactic Nuclei*

“…Noisy data can lead to biases in line width measurements especially when the line profile has very extended wings as typical of C IV lines ; see also Fine et al 2010).…”

Mid-Infrared-Selected Quasars. I. Virial Black Hole Mass and Eddington Ratios

“…We did not subtract a narrow C iv from the line profile because it is still debated as to whether a narrow C iv component is present (Wills et al 1993;Marziani et al 1996;Sulentic et al 2007), and to be comparable with other studies (e.g.,VP06, S11, Assef et al 2011;Park et al 2013). For the same reason, we also did not fit the 1600 Å feature (Laor et al 1994;Fine et al 2010). It is common (>70%) that more than one Gaussian component is required (Table 6) to fit the C iv BEL profile: 48/61 for SDSS, 56/75 for MMT09, and 5/7 for MMT05b.…”

Active galactic nuclei atz∼ 1.5 – II. Black hole mass estimation by means of broad emission lines

“…Greene & Ho 2005;Vestergaard & Peterson 2006;Fine et al 2008;Wang et al 2009;Fine et al 2010;Xiao et al 2011;Shen & Liu 2012;Trakhtenbrot & Netzer 2012;Marziani et al 2013a;Park et al 2013;Runnoe et al 2013;Zuo et al 2015). Trakhtenbrot & Netzer (2012, hereafter TN12) combined Sloan digital sky survey archival data (SDSS; Abazajian et al 2009) with smaller surveys and samples to improve earlier Mg II-based MBH prescriptions (e.g., McLure & Jarvis 2002;McLure & Dunlop 2004;Wang et al 2009), by assuming virialization of the Mg II emitting clouds.…”