Accretion disk wind as explanation for the broad-line region structure in NGC 5548

Abstract

Context. Supermassive black holes in the centers of active galactic nuclei (AGN) are surrounded by broad-line regions (BLRs). The broad emission lines seen in the AGN spectra are emitted in this spatially unresolved region.
Aims: We intend to obtain information on the structure and geometry of this BLR based on observed line profiles.
Methods: We modeled the rotational and turbulent velocities in the line-emitting region on the basis of the line-width FWHM and line dispersion σ_line of the variable broad emission lines in NGC 5548. Based on these velocities we estimated the height of the line-emitting regions above the midplane in the context of their distances from the center.
Results: The broad emission lines originate at distances of 2 to 27 light days from the center. Higher ionized lines originate in the inner region (≤ 13 light days) in specific filamentary structures 1 to 14 light days above the midplane. In contrast, the Hβ line is emitted in an outer (6-26 light days), more flattened configuration at heights of 0.7 to 4 light days only above the midplane.
Conclusions: The derived geometry of the line-emitting region in NGC 5548 is consistent with an outflowing wind launched from an accretion disk.