The Astrophysical Journal (Jan 2023)

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

  • Brandon M. Matthews,
  • Cooper Dix,
  • Ohad Shemmer,
  • Michael S. Brotherton,
  • Adam D. Myers,
  • I. Andruchow,
  • W. N. Brandt,
  • S. C. Gallagher,
  • Richard Green,
  • Paulina Lira,
  • Jacob N. McLane,
  • Richard M. Plotkin,
  • Gordon T. Richards,
  • Jessie C. Runnoe,
  • Donald P. Schneider,
  • Michael A. Strauss

DOI
https://doi.org/10.3847/1538-4357/acd04c
Journal volume & issue
Vol. 950, no. 2
p. 95

Abstract

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Quasars at z ≳ 1 most often have redshifts measured from rest-frame ultraviolet emission lines. One of the most common such lines, C iv λ 1549, shows blueshifts up to ≈5000 km 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 ] λ 5007 feature. We present spectroscopic measurements for 260 sources at 1.55 ≲ z ≲ 3.50 having −28.0 ≲ M _i ≲ − 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 ] λ 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 Å. Applying these corrections can improve the accuracy and the precision in the C iv -based redshift by up to ∼850 km s ^−1 and ∼150 km s ^−1 , respectively, which correspond to ∼8.5 and ∼1.5 Mpc in comoving distance at z = 2.5. Our prescriptions also improve the accuracy of the best available multifeature redshift determination algorithm by ∼100 km 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.

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