Analyzing temporal variations of AGN emission line profiles in the
  context of (dusty) cloud structure formation in the broad line region
  The formation processes and the exact appearance of the dust torus and broad
line region (BLR) of active galactic nuclei (AGN) are under debate. Theoretical
studies show a possible connection between the dust torus and BLR through a
common origin in the accretion disk. However observationally the dust torus and
BLR are typically studied separately. NGC~4151 is possibly one of the best
suited Seyfert~1 galaxies for simultaneous examinations because of its high
number of both photometric and spectroscopic observations in the past. Here we
compare changes of the dust radius to shape variations of broad emission lines
(BEL). While the radius of the dust torus decreased by almost a factor of two
from 2004 to 2006 shape variations can be seen in the red wing of BELs of
NGC~4151. These simultaneous changes are discussed in a dust and BEL formation
scheme. We also use the BEL shape variations to assess possible cloud
distributions, especially in azimuthal direction, which could be responsible
for the observed variations. Our findings can best be explained in the
framework of a dust inflated accretion disk. The changes in the BELs suggest
that this dusty cloud formation does not happen continuously, and over the
whole accretion disk, but on the contrary in spatially confined areas over
rather short amount of times. We derive limits to the azimuthal extension of
the observed localized BEL flux enhancement event.
