Gemini Near Infrared Spectrograph -- Distant Quasar Survey: Augmented Spectroscopic Catalog and a Prescription for Correcting UV-Based Quasar Redshifts

Abstract

Quasars at $z\gtrsim 1$ most often have redshifts measured from rest-frame ultraviolet emission lines. One of the most common such lines, C IV $\hat{I}»1549$, shows blueshifts up to $\approx 5000\mathrm{km}{\mathrm{s}}^{-1}$, and in rare cases even higher. This blueshifting results in highly uncertain redshifts when compared to redshift determinations from rest-frame optical emission lines, e.g., from the narrow [O III] $\hat{I}»5007$ feature. We present spectroscopic measurements for 260 sources at $1.55\lesssim z\lesssim 3.50$ having $-28.0\lesssim M_i\lesssim -30.0$ mag from the Gemini Near Infrared Spectrograph - Distant Quasar Survey (GNIRS-DQS) catalog, augmenting the previous iteration which contained 226 of the 260 sources whose measurements are improved upon in this work. We obtain reliable systemic redshifts based on [O III] $\hat{I}»5007$ for a subset of 121 sources which we use to calibrate prescriptions for correcting UV-based redshifts. These prescriptions are based on a regression analysis involving C IV full-width-at-half-maximum intensity and equivalent width, along with the UV continuum luminosity at a rest-frame wavelength of 1350 A. Applying these corrections can improve the accuracy and the precision in the C IV-based redshift by up to $\sim 850\mathrm{km}{\mathrm{s}}^{-1}$ and $\sim 150\mathrm{km}{\mathrm{s}}^{-1}$, respectively, which correspond to $\sim 8.5$ Mpc and $\sim 1.5$ Mpc in comoving distance at $z=2.5$. Our prescriptions also improve the accuracy of the best available multi-feature redshift determination algorithm by $\sim 100\mathrm{km}{\mathrm{s}}^{-1}$, indicating that the spectroscopic properties of the C IV emission line can provide robust redshift estimates for high-redshift quasars. We discuss the prospects of our prescriptions for cosmological and quasar studies utilizing upcoming large spectroscopic surveys.

20 pages (AASTeX 6.3.1), 8 figures, accepted for publication in ApJ