Evidence for a very broad line region in PG 1138+222

Abstract

We present a high S/N spectrum for the quasar PG 1138+222. We detect a very broad HeII$\lambda$4686 emission line component with twice the FWHM of the conventional broad line region (BLR) as evidenced by the Balmer lines. The profile shape and centroid redshift also distinguish this HeII component from the BLR features. The large ratio of HeIIl4686 to any analogous H$\beta$ emission component is an indicator that it arises in a very high density region ($n_e \simgt 10^{11}-10^{13}$ \cm3). This Helium component is probably emitted in a Very Broad Line Region (VBLR), where the radiation field is so strong that the Stromgren depth becomes similar to the geometrical thickness of the emitting clouds. The gas could therefore be optically thin to the Lyman continuum.