A NEW METHOD TO OBTAIN THE BROAD LINE REGION SIZE OF HIGH REDSHIFT QUASARS

We present high signal-to-noise ratio UV spectra for eight quasars at z similar to 3 obtained with Very Large Telescope/FORS. The spectra enable us to analyze in detail the strong and weak emission features in the rest frame range 1300-2000 angstrom of each source (CIII] lambda 1909, Si III] lambda 1892, Al III lambda 1860, Si II lambda 1814, C IV lambda 1549 and blended Si IV lambda 1397+O IV] lambda 1402). The flux ratios Al III lambda 1860/Si III] lambda 1892, CIV lambda 1549/Al III lambda 1860, Si IV lambda 1397+O IV] lambda 1402/Si III lambda 1892 and Si IV lambda 1397+O IV] lambda 1402/C IV lambda 1549 strongly constrain ionizing photon flux and metallicity through the use of diagnostic maps built from CLOUDY simulations. The radius of the broad line region is then derived from the ionizing photon flux applying the definition of the ionization parameter. The rBLR estimate and the width of a virial component isolated in prominent UV lines yields an estimate of black hole mass. We compare our results with previous estimates obtained from the rBLR-luminosity correlation customarily employed to estimate the black hole masses of high redshift quasars.