Polar dust obscuration in broad-line active galaxies from the XMM-XXL field

TL;DR

This study investigates the presence and characteristics of polar dust obscuration in broad-line active galaxies using XMM-XXL data, revealing that about half may contain polar dust with varying extinction properties, affecting spectral energy distribution modeling.

Contribution

It introduces a method to identify and characterize polar dust obscuration in AGN using a new modeling approach with X-CIGALE, highlighting the prevalence and properties of polar dust in a large sample.

Findings

Approximately 50% of AGN may contain polar dust.

60% of dusty AGN exhibit a steep extinction curve.

Ignoring polar dust leads to overestimating stellar contributions.

Abstract

Dust is observed in the polar regions of nearby AGN and it is known to contribute substantially to their mid-IR emission and to the obscuration of their UV to optical emission. We selected a sample of 1275 BLAGN in the XMM-XXL field, with optical to infrared photometric data. These AGN are seen along their polar direction and we expect a maximal impact of dust located around the poles when it is present. We used X-CIGALE, which introduces a dust component to account for obscuration along the polar directions, modeled as a foreground screen, and an extinction curve that is chosen as it steepens significantly at short wavelengths or is much grayer. By comparing the results of different fits, we are able to define subsamples of sources with positive statistical evidence in favor of or against polar obscuration and described using the gray or steep extinction curve. We find a similar fraction of sources with positive evidence for and against polar dust. Applying statistical corrections, we estimate that half of our sample could contain polar dust and among them, 60% exhibit a steep extinction curve and 40% a flat extinction curve; although these latter percentages are found to depend on the adopted extinction curves. The obscuration in the V-band is not found to correlate with the X-ray column density, while A_V/N_H ratios span a large range of values and higher dust temperatures are found with the flat, rather than with the steep extinction curve. Ignoring this polar dust component in the fit of the spectral energy distribution of these composite systems leads to an overestimation of the stellar contribution. A single fit with a polar dust component described with an SMC extinction curve efficiently overcomes this issue but it fails at identifying all the AGN with polar dust obscuration.