Simultaneous detection and analysis of optical and ultraviolet broad emission lines in quasars at z ~ 2.2

Abstract

We studied the spectra of six $z \sim 2.2$ quasars obtained with the X-shooter spectrograph at the Very Large Telescope. The redshift of these sources and X-shooter's spectral coverage allow us to cover the rest spectral range $\sim1200 - 7000$\AA\ for the simultaneous detection of optical and ultraviolet lines emitted by the Broad Line Region. Simultaneous measurements, avoiding issues related to quasars variability, help us understanding the connection between different Broad Line Region line profiles generally used as virial estimators of Black Holes masses in quasars. The goal of this work is comparing the emission lines from the same object to check on the reliability of H$\alpha$, MgII and CIV with respect to H$\beta$. H$\alpha$ and MgII linewidths correlate well with H$\beta$, while CIV shows a poorer correlation, due to the presence of strong blueshifts and asymmetries in the profile. We compare our sample with the only other two whose spectra were taken with the same instrument and for all examined lines our results are in agreement with the ones obtained with X-shooter at $z \sim 1.5 - 1.7$. We finally evaluate CIII] as a possible substitute of CIV in the same spectral range and find that its behaviour is more coherent with those of the other lines: we believe that, when a high quality spectrum such as the ones we present is available and a proper modelization with the FeII and FeIII emissions is performed, the use of this line is more appropriate than that of CIV if not corrected for the contamination by non-virialized components.