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

• Published in: The Astrophysical journal Vol. 950; no. 2; pp. 95 - 109
• Main Authors: Matthews, Brandon M., Dix, Cooper, Shemmer, Ohad, Brotherton, Michael S., Myers, Adam D., Andruchow, I., Brandt, W. N., Gallagher, S. C., Green, Richard, Lira, Paulina, McLane, Jacob N., Plotkin, Richard M., Richards, Gordon T., Runnoe, Jessie C., Schneider, Donald P., Strauss, Michael A.
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01-06-2023; IOP Publishing
• Subjects: Active galactic nuclei; Algorithms; Astrophysics; Blue shift; Emission lines; Emission measurements; Luminosity; Near infrared radiation; Prescriptions; Quasars; Red shift; Regression analysis; Surveys; Ultraviolet emission
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/acd04c

Abstract
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.