Model

The dusty torus as a two-phase medium

We modeled the distribution of the dust in the torus as a two-phase medium, consisting of a large number of high-density clumps embedded in a smooth dusty component of low density. Previously, we have found that an advantageous property of such two-phase medium is that it can produce attenuated silicate features, while at the same time have a pronounced NIR emission, which is challenging for both smooth and clumpy models alone (Stalevski et al. 2012a,Stalevski et al. 2012c). Since then, several lines of evidences were found providing further supporting that the torus is indeed a multiphase structure. Studies using hydrodynamical simulations and taking into account other processes such as self--gravity of the gas, radiative cooling and heating due to supernovae or accretion disk, found that the interstellar medium around the AGN would result in a multiphase filamentary structure (Wada 2009, Wada 2012). Using the two--phase model SEDs grid of Stalevski et al. (2012a), Roseboom et al. (2013) were able to reproduce the observed ratio of NIR to total IR luminosity in a large sample of sources selected from the WISE catalogue, something which models consisting of only clumps are not able to achieve. Analysing a large AGN sample from the WISE catalogue, Assef et al. (2013) found that the observed reddening distribution can be explained by a torus-like structure in which thick dust clouds are embedded in a diffuse inter-cloud dust medium. Another indication of a presence of smooth, low-density inter-cloud medium comes from constant baseline level absorption, between the strong absorption events, in type 2 AGN well-monitored with RXTE (Markowitz et al. 2014). Studying variable reddening in Narrow-line Seyfert 1, Leighly et al. (2015) concluded that it is "plausible that the occulting material is a clump embedded in a larger region of dusty gas responsible for the longer timescale reddening changes, and both are associated with the torus''. Finally, such two-phase structures have been actually observed in the central regions of Milky Way (the so-called Central Molecular Zone and Circumnuclear Disk) and it has been suggested that they represent a remnant of a dusty torus that may have played a role in past AGN phases of our Galaxy (Molinari et al. 2011, Ponti et al. 2013).