A measure of cosmological distance using the C IV Baldwin effect in quasars

Abstract

We use the anticorrelation between the equivalent width (EW) of the C IV 1549 Å emission line and the continuum luminosity in the quasars rest frame (Baldwin effect) to measure their luminosity distance as well as estimate cosmological parameters. We obtain a sample of 471 Type I quasars with the UV-optical spectra and EW (C IV) measurements in the redshift range of 2.3 < z < 7.1 including 25 objects at 5 < z < 7.1, which can be used to investigate the C IV Baldwin effect and determine cosmological luminosity distance. The relation EW(C IV) ∝ (λLλ)γ can be applied to check the inverse correlation between the C IV EW and Lλ of quasars and give their distance, and the data suggest that the EW of C IV is inversely correlated with continuum monochromatic luminosities. On the other hand, we also consider dividing the Type I quasar sample into various redshift bins, which can be used to check if the C IV EW–luminosity relation depends on the redshift. Finally, we apply a combination of Type I quasars and Type Ia supernovae (SNIa) of the Pantheon sample to test the property of dark energy concerning whether or not its density deviates from the constant, and give the statistical results.