# Partial dust obscuration in active galactic nuclei as a cause of broad-line profile and lag variability, and apparent accretion disc inhomogeneities

## TL;DR

This paper proposes that partial obscuration by dusty clumps in active galactic nuclei explains complex line profile asymmetries, lag variability, and other observed phenomena, impacting SMBH mass estimates and BLR structure studies.

## Contribution

It introduces a model where dusty outflows cause observed spectral and lag variability, challenging previous assumptions about BLR dynamics and structure.

## Key findings

- Dusty clumps produce asymmetries in line profiles.
- Velocity-dependent lags are explained by obscuration effects.
- Long-term lag changes are due to outflowing dust, not dynamical evolution.

## 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.