CIV Line-Width Anomalies: The Perils of Low S/N Spectra

TL;DR

This study highlights the importance of high signal-to-noise spectra for accurate CIV emission line analysis in quasars, demonstrating that proper line width measurement methods significantly improve black hole mass estimates.

Contribution

The paper shows that using line dispersion instead of FWHM in high-quality spectra reduces scatter in CIV-based black hole mass estimates, improving their reliability.

Findings

High S/N data reduces mass estimate scatter.

Line dispersion yields more consistent masses than FWHM.

Intrinsic scatter in mass residuals is about 0.2 dex.

Abstract

Comparison of six high-redshift quasar spectra obtained with the Large Binocular Telescope with previous observations from the Sloan Digital Sky Survey shows that failure to correctly identify absorption and other problems with accurate characterization of the CIV emission line profile in low S/N data can severely limit the reliability of single-epoch mass estimates based on the CIV emission line. We combine the analysis of these new high-quality data with a reanalysis of three other samples based on high S/N spectra of the CIV emission line region. We find that a large scatter between the Hbeta- and CIV-based masses remains even for this high S/N sample when using the FWHM to characterize the BLR velocity dispersion and the standard virial assumption to calculate the mass. However, we demonstrate that using high-quality data and the line dispersion to characterize the CIV line width leads to a high level of consistency between CIV- and Hbeta-based masses, with <0.3 dex of observed scatter, and an estimated ~0.2 dex intrinsic scatter, in the mass residuals.