The hypothesis of the dust origin of the Broad Line Region in Active
  Galactic Nuclei
  Strong broad emission lines are the most important signatures of active
galactic nuclei. These lines allowed to discover the cosmological nature of
quasars, and at present these lines allow for convenient method of weighting
the black holes residing in their nuclei. However, a question remains why such
strong lines form there in the first place. Specifically, in the case of Low
Ionization Lines, there must be a mechanism which leads to an efficient rise of
the material from the surface of the accretion disk surrounding a black hole
but at the same time should not give a strong signature of the systematic
outflow, as the Balmer lines are not significantly shifted with respect to the
Narrow Line Region. We determine the effective temperature of the accretion
disk underlying the H$\beta$ line at the basis of the time delay measured from
reverberation and the simple Shakura-Sunyaev theory of accretion disks. We
obtain that this temperature is universal, and equal $995 \pm 74$ K,
independently from the black hole mass and accretion rate of the source. This
result suggests to us that the dust formation in the disk atmosphere is
responsible for the strong rise of the material. However, as the material gains
height above the disk it becomes irradiated, the dust evaporates, the radiation
pressure force suddently drops and the material fall back again at the disk.
Therefore, a failed wind forms. In the simple version of the model the disk
irradiation is neglected, but in the present paper we also discuss this
irradiation and we use the observed variation of the Broad Line Region in NGC
5548 to constrain the character of this non-local non-stationary phenomenon.
The current instruments cannot resolve the Broad Line Region but future
instrumentation may allow to test the model directly.
