Title: Partial dust obscuration in active galactic nuclei as a cause of broad-line profile and lag variability, and apparent accretion disc inhomogeneities,
Abstract: The profiles of the broad emission lines of active galactic nuclei (AGNs) and
the time delays in their response to changes in the ionizing continuum ("lags")
give information about the structure and kinematics of the inner regions of
AGNs. Line profiles are also our main way of estimating the masses of the
supermassive black holes (SMBHs). However, the profiles often show
ill-understood, asymmetric structure and velocity-dependent lags vary with
time. Here we show that partial obscuration of the broad-line region (BLR) by
outflowing, compact, dusty clumps produces asymmetries and velocity-dependent
lags similar to those observed. Our model explains previously inexplicable
changes in the ratios of the hydrogen lines with time and velocity, the lack of
correlation of changes in line profiles with variability of the central engine,
the velocity dependence of lags, and the change of lags with time. We propose
that changes on timescales longer than the light-crossing time do not come from
dynamical changes in the BLR, but are a natural result of the effect of
outflowing dusty clumps driven by radiation pressure acting on the dust. The
motion of these clumps offers an explanation of long-term changes in
polarization. The effects of the dust complicate the study of the structure and
kinematics of the BLR and the search for sub-parsec SMBH binaries. Partial
obscuration of the accretion disc can also provide the local fluctuations in
luminosity that can explain sizes deduced from microlensing.