Evaluating and Improving Redshift Determinations in High-z Quasars

Accuracy and precision in quasar redshifts are required to set the best velocity scale for spectral features like absorption, indicating outflows, as well as for applications like small-scale clustering and the transverse proximity effect. Different emission lines, however, can be shifted significantly relative to each other, usually with higher ionization species being significantly blueshifted on average. The low-ionization species Mg II, which has a small shift relative to systemic, can be used to about z = 2.2 for optical quasar spectra. At higher redshifts, the more problematic C III] and C IV lines are the strongest emission lines. We compare several ultraviolet-based redshifts to [O III] redshifts from the near-infrared spectra of 40 quasars with z > 2.2, and find the UV-based measurements less than ideal. Empirically, the velocity shifts relative to the [O III] redshift correlate with several properties of the C IV profile and can be significantly improved with correction factors that we derive.