Observational estimate of the partial covering probability of quasar emission regions by distant H2 absorption clouds

We have searched high spectral resolution spectra of quasars known to exhibit high redshift (z > 1.7) intervening H-2-bearing damped Lyman-alpha (DLA) systems for partial coverage of the quasar emission by intervening H-2 clouds. Partial coverage manifests itself by the presence of non-zero residual flux in the core of saturated H-2 absorption lines. The residual flux can be observed either only at the bottom of absorption lines redshifted on top of quasar emission lines, in which case part of the broad line region (BLR) is not covered, or in all absorption lines, in case some continuum source is not covered. Among 35 H-2 absorption clouds in 14 quasar spectra obtained with the VLT-UVES or Keck-HIRES spectrographs, we detect partial coverage of the BLR for 13 clouds. This result suggests that the probability of partial coverage of the QSO BLR by a distant H-2 absorption cloud is about 40 per cent. For four systems towards Q0013-0029, Q0405-4418, Q0812+3208, and J 2100-0641, partial coverage is detected for the first time. We determine the theoretical probability of partial coverage of the BLR by a distant H-2 cloud as a function of the ratio between the cloud and the BLR sizes. Using this model, we obtain an estimate of the characteristic BLR radius of 50(-23)(+19) light days. This is similar to the estimate of the BLR size obtained by reverberation-mapping analysis (similar to 100 light days).