Relativistic redshifts in quasar broad lines

TL;DR

This study analyzes relativistic effects on broad emission lines in quasars, finding that line profiles are consistent with emission from a Keplerian disk viewed at certain inclinations, impacting our understanding of quasar dynamics.

Contribution

It provides the first large-scale observational evidence linking relativistic redshifts in quasar broad lines to a Keplerian disk model with specific inclination limits.

Findings

Redshift variations match relativistic predictions for a Keplerian disk.

Line-of-sight inflow/outflow velocities are much less than Keplerian velocities.

Results support simple AGN unification models.

Abstract

The broad emission lines commonly seen in quasar spectra have velocity widths of a few per cent of the speed of light, so special- and general-relativistic effects have a significant influence on the line profile. We have determined the redshift of the broad H-beta line in the quasar rest frame (determined from the core component of the [OIII] line) for over 20,000 quasars from the Sloan Digital Sky Survey DR7 quasar catalog. The mean redshift as a function of line width is approximately consistent with the relativistic redshift that is expected if the line originates in a randomly oriented Keplerian disk that is obscured when the inclination of the disk to the line of sight exceeds ~30-45 degrees, consistent with simple AGN unification schemes. This result also implies that the net line-of-sight inflow/outflow velocities in the broad-line region are much less than the Keplerian velocity when averaged over a large sample of quasars with a given line width.