The location of the dust causing internal reddening of active galactic nuclei

TL;DR

This study uses Balmer decrements in AGNs to determine that the dust causing reddening of the broad-line region is primarily located closer to the central black hole than the narrow-line region, indicating an interior dust distribution.

Contribution

It provides observational evidence that the dust responsible for reddening the BLR in AGNs is mainly situated inside the NLR, refining the understanding of dust distribution in active galactic nuclei.

Findings

BLR reddening often exceeds NLR reddening.

NLR reddening sets a lower limit to BLR reddening.

BLR reddening correlates with [O III] line properties.

Abstract

We use the Balmer decrements of the broad-line regions (BLRs) and narrow-line regions (NLRs) of active galactic nuclei (AGNs) as reddening indicators to investigate the location of the dust for four samples of AGNs with reliable estimates of the NLR contribution to the Balmer lines. Intercomparison of the NLR and BLR Balmer decrements indicates that the reddening of the NLR sets a lower limit to the reddening of the BLR. Almost no objects have high NLR reddening but low BLR reddening. The reddening of the BLR is often substantially greater than the reddening of the NLR. The BLR reddening is correlated with the equivalent widths of [O III] lines and the intensity of the [O III] lines relative to broad H\beta. We find these relationships to be consistent with the predictions of a simple model where the additional dust reddening the BLR is interior to the NLR. We thus conclude that the dust causing the additional reddening of the accretion disc and BLR is mostly located at a smaller radius than the NLR.