The origin of the broad line region in active galactic nuclei

Aims. Although broad emission lines are the most reliable signature of the nuclear activity of a galaxy and the location of the emitting material is well measured by the reverberation method, the physical cause of the formation of the broad line region remains unclear. We attempt to place some constraints on its origin. Methods. We study the properties of the accretion disk underlying the broad line region. Results. We find that the effective temperature at the disk radius corresponding to the location of the broad line region, as inferred from the H beta line, is universal in all monitored sources and equal to 1000 K. This value is close to the limiting value that permits the existence of the dust. Conclusions. The likely origin of the low ionization part of the broad line region is the strong local dusty wind from the disk. This wind becomes exposed to the irradiation by the central regions when moving higher above the disk surface and subsequently behaves like a failed wind, thus leading to a local mixture of inflow and outflow. This may provide the physical explanation of the turbulence needed both to smooth the line profiles as well as provide additional mechanical heating.