Wavelength-resolved reverberation mapping of quasar CTS C30.10: Dissecting Mg II and Fe II emission regions

Context. We present the results of the reverberation monitoring of the MgII broad line and FeII pseudocontinuum for the luminous quasar CTS C30.10 (z=0.90052) with the Southern African Large Telescope in 2012-2021. Aims. We aimed at disentangling the MgII and UV FeII variability and the first measurement of UV FeII time delay for a distant quasar. Methods. We used several methods for the time-delay measurements and determined the FeII and MgII time delays. We also performed a wavelength-resolved time delay study for a combination of MgII and FeII in the 2700-2900 Å rest-frame wavelength range. Results. We obtain a time delay for MgII of 275.5-19.5+12.4 days in the rest frame, and we have two possible solutions of 270.0-25.3+13.8 days and 180.3-30.0+26.6 in the rest frame for FeII. Combining this result with the old measurement of FeII UV time delay for NGC 5548, we discuss for first time the radius-luminosity relation for UV FeII with the slope consistent with 0.5 within the uncertainties. Conclusions. Because the FeII time delay has a shorter time-delay component but the lines are narrower than MgII, we propose that the line-delay measurement is biased toward the part of the broad line region (BLR) facing the observer. The bulk of the FeII emission may arise from the more distant BLR region, however, the region that is shielded from the observer.