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The identification of a virial broadening estimator in the quasar UV restframe suitable for black hole mass computation at high redshift has become animportant issue. We compare the HI Balmer H-beta line width to the ones of twointermediate ionization lines: the Aliii 1860 doublet and the Ciii] 1909 line,over a wide interval of redshift and luminosity (0 < z < 3.5; 43 < logL < 48.5[erg a/]), for 48 sources belonging to the quasar population characterized bymid-to-high values of the Eddington ratio (Population A). The present analysisindicates that the line width of Aliii 1860 and H-beta are highly correlated,and can be considered equivalent for most Population A quasars over five ordersof magnitude in luminosity; forCiii] 1909, multiplication by a constantcorrection factor xi ~ 1.25 is sufficient to bring the FWHM of Ciii] inagreement with the one of H-betaa. The statistical concordance betweenlow-ionization and intermediate-ionization lines suggests that theypredominantly arise from the same virialized part of the broad line region.However, blueshifts of modest amplitude (few hundred km /s) with respect to thequasar rest frame and an excess (~ 1.1) Aliii broadening with respect to H-betaare found in a fraction of our sample. Scaling laws to estimate M_BH of highredshift quasar using the Aliii and the Ciii] line widths have rms scatter ~0.3 dex. The Aliii scaling law takes the form log M_BH ~ 0.58 log L1700,44 + 2log FWHM + 0.49 [solar masses].

The intermediate-ionization lines as virial broadening estimators for Population A quasars