Luminosity-variation independent location of the circum-nuclear, hot
  dust in NGC 4151
  After recent sensitivity upgrades at the Keck Interferometer (KI), systematic
interferometric 2um studies of the innermost dust in nearby Seyfert nuclei are
within observational reach. Here, we present the analysis of new
interferometric data of NGC 4151, discussed in context of the results from
recent dust reverberation, spectro-photometric and interferometric campaigns.
The complete data set gives a complex picture, in particular the measured
visibilities from now three different nights appear to be rather insensitive to
the variation of the nuclear luminosity. KI data alone indicate two scenarios:
the K-band emission is either dominated to ~90% by size scales smaller than
30mpc, which falls short of any dust reverberation measurement in NGC 4151 and
of theoretical models of circum-nuclear dust distributions. Or contrary, and
more likely, the K-band continuum emission is dominated by hot dust (>= 1300K)
at linear scales of about 50mpc. The linear size estimate varies by a few tens
of percent depending on the exact morphology observed. Our interferometric,
deprojected centro-nuclear dust radius estimate of 55+-5mpc is roughly
consistent with the earlier published expectations from circum-nuclear, dusty
radiative transfer models, and spectro-photometric modeling. However, our data
do not support the notion that the dust emission size scale follows the nuclear
variability of NGC 4151 as a R_dust \propto L_nuc^0.5 scaling relation. Instead
variable nuclear activity, lagging, and variable dust response to illumination
changes need to be combined to explain the observations.
