Contribution of a disc component to single-peaked broad lines of active galactic nuclei

We study the disc emission component hidden in the single-peaked broad emission lines (BELs) of active galactic nuclei. We compare the observed broad lines from a sample of 90 Seyfert 1 spectra taken from the Sloan Digital Sky Survey with simulated line profiles. We consider a two-component broad line region model where an accretion disc and a surrounding non-disc region with isotropic cloud velocities generate the simulated BEL profiles. The analysis is mainly based on the measurements of the full widths (at 10 per cent, 20 per cent and 30 per cent of the maximum intensity) and on the asymmetries of the line profiles. Comparing these parameters for the simulated and observed H alpha broad lines, we found that the hidden disc emission may be present in BELs even if the characteristic of two-peaked-line profiles is absent. For the available sample of objects (Seyfert 1 galaxies with single-peaked BELs), our study indicates that, in the case of the hidden disc emission in single-peaked broad-line profiles, the disc inclination tends to be small (mostly i < 25 degrees) and that the contribution of the disc emission to the total flux should be smaller than the contribution of the surrounding region.