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

¹ Center for Theoretical Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland
e-mail: ashwanitapan@gmail.com
² Laboratório Nacional de Astrofísica, MCTI Rua dos Estados Unidos 154, Bairro das Nações, CEP 37504-364 Itajubá, MG, Brazil
³ Astronomy Department, Universidad de Concepción Casilla 160-C, Concepción 4030000, Chile
⁴ Department of Theoretical Physics and Astrophysics, Faculty of Science, Masaryk University, Kotlářska 2, 611 37 Brno, Czech Republic
⁵ School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel

Received: 28 August 2023
Accepted: 19 October 2023

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 Hβ and Mg II lines and the observed values for NGC 5548 for the line distances implied by the time-delay measurement (for Hβ) 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 Hβ emitting region moves closer to the dustless part of the accretion disc surface.