Long-term variability of the optical spectra of NGC4151 - II. Evolution of the broad Hα and Hβ emission-line profiles

Abstract

Aims: We present the results of a long-term monitoring (11 years, between 1996 and 2006) of Hα and Hβ line variations of the active galactic nucleus of NGC4151. Methods: High quality spectra (S/N > 50 and R ≈ 8 Å) of Hα and Hβ were investigated. During monitoring period, we analyzed line profile variations. Comparing the line profiles of Hα and Hβ, we studied different details (bumps, absorbtion features) in the line profiles. The variations in the different Hα and Hβ line profile segments were investigated. We also analyzed the Balmer decrement for entire lines and for line segments. Results: We found that the line profiles varied strongly during the monitoring period, and exhibited blue and red asymmetries. This is indicative of a complex BLR geometry inside NGC4151 with, at least, three kinematically distinct regions: one that contributes to the blue line wing, one to the line core and one to the red line wing. The variation may be caused by an accelerating outflow originating very close to the black hole, where the red part may come from a region closer to the black hole than the blue part, which originates in the region with the highest outflow velocities. Conclusions: Taking into account that the BLR of NGC 4151 has a complex geometry (probably affected by an outflow) and that a portion of the broad line emission does not seem to be produced entirely by photoionization, one may ask whether the study of the BLR using reverberation mapping would be worthwhile for this galaxy. © ESO 2010.