Statistical analysis of Ala III and C III] emission lines as virial black hole mass estimators in quasars

Abstract

Aims. We test the usefulness of the intermediate ionisation lines Allll/11860 and Clll]/I1909 as reliable virial mass estimators for quasars. Methods. We identified a sample of 309 quasars from the Sloan Digital Sky Survey Data Release 16 (SDSS DR16) in the redshift range 1.2 < z < 1.4 that have [On],13728 recorded on the same spectrum as Aim,11860, Sim],11892, and Cm],11909. We set the systemic quasar redshift using careful measurements of [Oil],13728. We then classified the sources as Population A (Pop. A), extreme Population A (Pop. xA), and Population B (Pop. B), and analysed the 1900 A blend using multi-component models to look for systematic line shifts of A1IIU1860 and Cm],U909 along the quasar main sequence. Results. We do not find significant shifts of the AI11UI86O line peak in Pop. B or in the vast majority of Pop. A. For Pop. xA. a small median blueshift of -250 km s"1 was observed, motivating a decomposition of the Aim line profile into a virialised component centred at rest frame and a blueshifted component for an outflow emission. For Pop. B objects, we proved the empirical necessity to fit a redshifted very broad component, clearly seen in Cm], and analysed the physical implications on a Pop. B composite spectrum using CLOUDY simulations. We find consistent black hole mass estimations using Aim and Cm] as virial estimators for the bulk of Pop. A. Conclusions. Aim (and even Cm]) is a reliable virial black hole mass estimator for Pop. A and Pop. B objects. The Pop. xA sources deserve special attention due to the significant blueshifted excess observed in the line profile of Aim, although it is not as large as the excess observed in Owl 1549.