Prescriptions for Correcting Ultraviolet-based Redshifts for Luminous Quasars at High Redshift

Abstract

High-redshift quasars typically have their redshift determined from rest-frame ultraviolet (UV) emission lines. However, these lines, and more specifically the prominent C iv lambda 1549 emission line, are typically blueshifted yielding highly uncertain redshift estimates compared to redshifts determined from rest-frame optical emission lines. We present near-infrared spectroscopy of 18 luminous quasars at 2.15 z < 3.70 that allows us to obtain reliable systemic redshifts for these sources. Together with near-infrared spectroscopy of an archival sample of 44 quasars with comparable luminosities and redshifts, we provide prescriptions for correcting UV-based redshifts. Our prescriptions reduce velocity offsets with respect to the systemic redshifts by similar to 140 km s(-1) and reduce the uncertainty on the UV-based redshift by similar to 25% with respect to the best method currently used for determining such values. We also find that the redshifts determined from the Sloan Digital Sky Survey Pipeline for our sources suffer from significant uncertainties, which cannot be easily mitigated. We discuss the potential of our prescriptions to improve UV-based redshift corrections given a much larger sample of high-redshift quasars with near-infrared spectra.