Abstract
Aims. Dust is observed in the polar regions of nearby active galactic nuclei (AGN) and it is known to contribute substantially to their mid-IR emission and to the obscuration of their UV to optical emission. We aim to carry out a statistical test to check whether this component is a common feature based on an analysis of the integrated spectral energy distributions of these composite sources. Methods. We selected a sample of 1275 broad-line AGN 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 (if present) and described using the gray or steep extinction curve. Results. 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 AV/NH 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.
Highlights
Modeling the characteristics of dust obscuration is crucial in determining the intrinsic spectral energy distribution (SED) of any astronomical source from the ultraviolet (UV) to the nearinfrared
We investigate the presence of dust along the line of sight of BLAGN by using the impact of adding polar dust reddening to their UV to IR continuum
We found a similar fraction of sources that are better fitted with the introduction of a polar dust component
Summary
Modeling the characteristics of dust obscuration is crucial in determining the intrinsic spectral energy distribution (SED) of any astronomical source from the ultraviolet (UV) to the nearinfrared (near-IR). The introduction of polar dust in the modeling of active galaxies of both types is found to impact the measure of parameters linked to dust, such as the covering factor of obscuring material (Asmus 2019; Toba et al 2021) or the fraction of dust emission coming from the AGN (Mountrichas et al 2021b) The identification of these multiple components and their relative contribution is difficult when only integrated multiwavelength data are available, leading to degeneracies (e.g., Netzer 2015; Hao et al 2013). 2, we describe our composite models (AGN and host galaxy) created with X-CIGALE to fit the SEDs of our sources and using optical-IR colors, we highlight the impact on the optical-IR emission of dust reddening with either a steep (SMC, Pei 1992) or a gray (Gaskell et al 2004, hereafter, G04) extinction curve. In this paper we assume a ΛCDM cosmology with parameters coming from the seven-year data from WMAP (Komatsu et al 2011)
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