2022MNRAS.515.3729K

We use observations of 66 reverberation-measured Mg II quasars (QSOs) in the redshift range 0.36 ≤ z ≤ 1.686 - a subset of the 78 QSOs we previously studied that also have ${\cal R}_{\rm {Fe\, {\small II}}}$ (flux ratio parameter of UV Fe II to Mg II that is used as an accretion-rate proxy) measurements - to simultaneously constrain cosmological model parameters and QSO two-parameter and three-parameter radius-luminosity (R-L) relation parameters in six different cosmological models. We find that these QSO R-L relation parameters are independent of the assumed cosmological model and so these QSOs are standardizable through the R-L relations. Also: (1) With the two-parameter R-L relation, we find that the low-${\cal R}_{\rm {Fe\, {\small II}}}$ and high-${\cal R}_{\rm {Fe\, {\small II}}}$ data subsets obey the same R-L relation within the error bars. (2) Extending the two-parameter R-L relation to a three-parameter one does not result in the hoped-for reduction in the intrinsic dispersion of the R-L relation. (3) Neither of the three-parameter R-L relations provide a significantly better fit to the measurements than does the two-parameter R-L relation. These are promising results for the ongoing development of Mg II cosmological probes. The first and third of these results differ significantly from those we found elsewhere from analyses of reverberation-measured H β QSOs.