The dust covering factor in active galactic nuclei

Abstract: The primary source of emission of active galactic nuclei (AGN), the accretion disk, 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 disk emission, so the ratio of the torus to the AGN luminosity ($L_{\text{torus}}/L_{\text{AGN}}$) should correspond to the fraction of the sky obscured by dust, i.e. the covering factor. We undertook a critical investigation of the $L_{\text{torus}}/L_{\text{AGN}}$ 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 $L_{\text{torus}}/L_{\text{AGN}}$ 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 $L_{\text{torus}}/L_{\text{AGN}}$ underestimate low covering factors and overestimate high covering factors. In type 2 AGNs $L_{\text{torus}}/L_{\text{AGN}}$ 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 $L_{\text{AGN}}$, with values in the range of $\approx0.6-0.7$. 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.

The Dust Covering Factor in Active Galactic Nuclei

The paper titled "The Dust Covering Factor in Active Galactic Nuclei" presents an in-depth analysis of the link between the luminosity ratio of the dusty torus and the active galactic nucleus (AGN) and the dust covering factor. Authored by M. Stalevski et al., this study employs a comprehensive 3D Monte Carlo radiative transfer code to simulate the spectral energy distributions (SEDs) emitted by the clumpy structure of the dusty torus surrounding AGNs. The authors elucidate how the observed ratio of the torus luminosity to the AGN luminosity, ( L_{\text{torus}}/L_{\text{AGN}} ), serves as a proxy for assessing the fraction of the sky obscured by dust, commonly known as the covering factor.

Methodological Insights

The research hinges on generating a grid of SEDs which are critically examined to understand their dependence on various torus parameters such as the optical depth, geometry, and the emission properties of the central accretion disk. The study distinguishes itself by addressing the significant role of anisotropy both in the emission from the AGN accretion disk and the absorptive and re-emissive properties of the surrounding dusty torus.

Key parameters studied include:

1. Optical Depth ((\tau_{9.7})): Models range from optically thin to moderately thick scenarios, showcasing how thicker dust can lead to significant anisotropy in IR emissions.
2. Geometrical Properties: The study examines the implications of the torus's radius and its density gradient on anisotropy and emission patterns.
3. Radiation Transfer Dynamics: The simulation accounts for the intricate scattering, absorption, and emission processes, leading to a nuanced understanding of how the AGN's inherent luminosity is redistributed by the dust.

Results and Interpretation

The findings illustrate that for type 1 AGNs, the physically motivated models reveal ( L_{\text{torus}}/L_{\text{AGN}} ) tends to underestimate covering factors especially at low geometrical values, whereas, for type 2 AGNs, the ratio consistently underestimates the covering factor. Such distinctions arise from the intrinsic anisotropy of the accretion disk's radiation and dust optical thickness. The paper delivers a correction method for ( L_{\text{torus}}/L_{\text{AGN}} ) to better approximate the true covering factor, significantly contributing to a more precise characterization of obscured AGN populations.

Implications for AGN Studies

The results of this study hold important ramifications for cosmological models, particularly in understanding the evolution and frequency of obscured versus unobscured AGNs over cosmic time. Correcting ( L_{\text{torus}}/L_{\text{AGN}} ) allows researchers to refine obscured AGN fractions, addressing discrepancies observed between infrared-based studies and X-ray surveys—essential for constructing accurate population synthesis models of the cosmic X-ray background.

In broader contexts, the proposed methodology to accurately estimate the covering factor underscores the intricate physical processes that dominate AGN environments. Ensuring that observational biases are minimized is crucial for advancing the comprehension of AGN physics, and this work serves as a foundation for future explorations into how anisotropic emissions can affect the interpretation of active galaxies.

Overall, the paper by Stalevski et al. signifies a step towards resolving long-standing debates regarding the obscuration of AGNs and further underlines the necessity for sophisticated models that account for the multifaceted nature of dust emissions and AGN luminosity.