Broad-line region in active galactic nuclei: Dusty or dustless?

TL;DR

This study investigates whether dust exists within the broad-line region of active galactic nuclei by modeling radiation reprocessing with and without dust, comparing predictions to observations of NGC 5548.

Contribution

It provides the first detailed modeling of the BLR with dust and compares the effects on emission lines, highlighting the role of dust in line formation and structure.

Findings

Dusty models match observed line equivalent widths for NGC 5548.

High ionisation lines originate in dustless inner regions.

No shielding is needed between the accretion disk and BLR.

Abstract

Context. Dust in active galactic nuclei is clearly present right outside the broad-line region (BLR) in the form of a dusty molecular torus. However, some models of the BLR predict that dust may also exist within the BLR. Aims. We study the reprocessing of radiation by the BLR with the aim of observing how the presence of dust affects the reprocessed continuum and the line properties. Methods. We calculated a range of models using the CLOUDY photoionisation code for dusty and dustless plasma. We paid particular attention to the well-studied object NGC 5548, and we compared the line equivalent width predictions with the data from observations for this object. Results. We obtained a rough agreement between the expected equivalent widths of the H$\beta$ and Mg II lines and the observed values for NGC 5548 for the line distances implied by the time-delay measurement (for H$\beta)$ and the radius-luminosity relation (for Mg II) when the medium is dusty. We found the incident radiation to be consistent with the radiation seen by the observer, so no shielding between the inner disc and the BLR is required. High ionisation lines such as He II, however, clearly form in the inner dustless region. When the additional absorber is present, the H$\beta$ emitting region moves closer to the dustless part of the accretion disc surface.