The dust covering factor in active galactic nuclei

By M. Stalevski, C. Ricci, Y. Ueda, P. Lira, J. Fritz, Maarten Baes. 2016

248 citations recorded by the source corpus.

The primary source of emission of active galactic nuclei (AGNs), the accretion disc, is surrounded by an optically and geometrically thick dusty structure ('the so-called dusty torus'). The infrared radiation emitted by the dust is nothing but a reprocessed fraction of the accretion disc emission, so the ratio of the torus to the AGN luminosity (L-torus/L-AGN) should corresponds to the fraction of the sky obscured by dust, i.e. the covering factor. We undertook a critical investigation of the L-torus/L-AGN as the dust covering factor proxy. Using state-of-the-art 3D Monte Carlo radiative transfer code, we calculated a grid of spectral energy distributions (SEDs) emitted by the clumpy two-phase dusty structure. With this grid of SEDs, we studied the relation between L-torus/L-AGN and the dust covering factor for different parameters of the torus. We found that in the case of type 1 AGNs the torus anisotropy makes L-torus/L-AGN underestimate low covering factors and overestimate high covering factors. In type 2 AGNs L-torus/L-AGN always underestimates covering factors. Our results provide a novel easy-to-use method to account for anisotropy and obtain correct covering factors. Using two samples from the literature, we demonstrated the importance of our result for inferring the obscured AGN fraction. We found that after the anisotropy is properly accounted for, the dust covering factors show very weak dependence on L-AGN, with values in the range of approximate to 0.6-0.7. Our results also suggest a higher fraction of obscured AGNs at high luminosities than those found by X-ray surveys, in part owing to the presence of a Compton-thick AGN population predicted by population synthesis models.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/93cb906c64bb2664a9e38618b039ac8230b592b7

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In Astronomy and Astrophysics

1. D. M. Alexander
2. W. N. Brandt
3. R. Hickox
4. P. Hopkins
5. L. Hernquist
6. R. Maiolino
7. Mara Salvato
8. H. Netzer

Citations in the corpus

• New Spectral Model for Constraining Torus Covering Factors from Broadband X-Ray Spectra of Active Galactic Nuclei (2018, methodology): “Recent calculations by Stalevski et al. (2016) suggest that this simple ratio may be effectively used as an indicator of the torus covering factor (e.g., in large surveys; Maiolino et al. 2007, Treister et al. 2008), provided that anisotropy of the disk and torus radiation is properly accounted for.”
• x-cigale: fitting AGN/galaxy SEDs from X-ray to infrared (2020, methodology): “The typical value is ∆ ≈ 40◦ from observations (e.g. Stalevski et al. 2016).”
• The Rise of Faint, Red Active Galactic Nuclei at z > 4: A Sample of Little Red Dots in the JWST Extragalactic Legacy Fields (2025, unknown): “For the AGN component, we adopt the skirtor2016 module based on a clumpy torus model from M. Stalevski et al. ( 2012, 2016 ) .”
• Universal bolometric corrections for active galactic nuclei over seven luminosity decades (2020, methodology): “Aware of this assumption, to overcome the limitation of having only four AGN templates, and in the meantime to better reproduce the spectral emission of either very luminous type 1 or very low luminous type 2 AGN, we enriched the library with the models by Stalevski et al. (2016) used in D17.”
• Active galactic nuclei: what’s in a name? (2017, background): “…al. 2008; Assef et al. 2013; Mateos et al. 2016), an effect typically referred to as the receding torus (Lawrence 1991), although others have found no evidence of such an effect (Wang and Jiang 2006; Lawrence and Elvis 2010; Hönig et al. 2011; Lacy et al. 2013; Stalevski et al. 2016, see also Sect.”