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

¹ Dipartimento di Fisica e AstronomiaUniversità di Firenze, via G. Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy
e-mail: susanna@arcetri.astro.it
² INAF–Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
³ APC, Astroparticules et Cosmologie, Université Paris Diderot, CNRS/IN2P3, CEA/Irfu, Observatoire de Paris, Sorbonne Paris Cité, 10 rue Alice Domon et Léonie Duquet, 75205 Paris Cedex 13, France
⁴ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China
⁵ Department of Astronomy, School of Physics, Peking University, Beijing 100871, PR China
⁶ Max-Planck Institüt für extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching bei München, Germany

Received: 25 November 2016
Accepted: 23 February 2017

Abstract

We studied the spectra of six z ~ 2.2 quasars obtained with the X-shooter spectrograph at the Very Large Telescope. The redshift of these sources and the X-shooter’s spectral coverage allow us to cover the rest of the spectral range ~1200−7000 Å 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 understand the connection between the different broad-line region line profiles generally used as virial estimators of black hole masses in quasars. The goal of this work is to compare the different emission lines for each object to check on the reliability of Hα, Mg ii and C iv with respect to Hβ. Hα and Mg ii linewidths correlate well with Hβ, while C iv shows a poorer correlation, due to the presence of strong blueshifts and asymmetries in the profile. We compared 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 ~ 1.5−1.7. We finally evaluate C iii] as a possible substitute of C iv 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 Fe ii and Fe iii emissions is performed, it is more appropriate to use this line than that of C iv if not corrected for the contamination by non-virialized components.