No abstract

Unified schemes of active galactic nuclei (AGN) require an obscuring dusty torus around the central source, giving rise to Seyfert 1 line spectrum for pole-on viewing and Seyfert 2 characteristics in edge-on sources. Although the observed IR is in broad agreement with this scheme, the behavior of the 10 micron silicate feature and the width of the far-IR emission peak remained serious problems in all previous modeling efforts. We show that these problems find a natural explanation if the dust is contained in about 5-10 clouds along radial rays through the torus. The spectral energy distributions (SED) of both type 1 and type 2 sources are properly reproduced from different viewpoints of the same object if the visual optical depth of each cloud is larger than about 60 and the clouds' mean free path increases roughly in proportion to radial distance.Comment: 11 pages, submitted to ApJ Letter

According to unified schemes of Active Galactic Nuclei (AGN), the central engine is surrounded by dusty, optically thick clouds in a toroidal structure. We have recently developed a formalism that for the first time takes proper account of the clumpy nature of the AGN torus. We now provide a detailed report of our findings in a two-paper series. Here we present our general formalism for radiative transfer in clumpy media and construct its building blocks for the AGN problem -the source functions of individual dusty clouds heated by the AGN radiation field. We show that a fundamental difference from smooth density distributions is that in a clumpy medium, a large range of dust temperatures coexist at the same distance from the radiation central source. This distinct property explains the low dust temperatures found close to the nucleus of NGC1068 in 10 µm interferometric observations. We find that irrespective of the overall geometry, a clumpy dust distribution shows only moderate variation in its spectral energy distribution, and the 10µm absorption feature is never deep. Furthermore, the X-ray attenuating column density is widely scattered around the column density that characterizes the IR emission. All of these properties are characteristic of AGN observations. The assembly of clouds into AGN tori and comparison with observations is presented in the companion paper.

From extensive radiative transfer calculations we find that clumpy torus models with N 0 ∼ 5-15 dusty clouds along radial equatorial rays successfully explain AGN infrared observations. The dust has standard Galactic composition, with individual cloud optical depth τ V ∼ 30-100 at visual. The models naturally explain the observed behavior of the 10µm silicate feature, in particular the lack of deep absorption features in AGN of any type. The weak 10µm emission feature tentatively detected in type 2 QSO can be reproduced if in these sources N 0 drops to ∼2 or τ V exceeds ∼100. The clouds angular distribution must have a soft-edge, e.g., Gaussian profile, the radial distribution should decrease as 1/r or 1/r 2 . Compact tori can explain all observations, in agreement with the recent interferometric evidence that the ratio of the torus outer to inner radius is perhaps as small as ∼5-10. Clumpy torus models can produce nearly isotropic IR emission together with highly anisotropic obscuration, as required by observations. In contrast with strict variants of unification schemes where the viewing-angle uniquely determines the classification of an AGN into type 1 or 2, clumpiness implies that it is only a probabilistic effect; a source can display type 1 properties even from directions close to the equatorial plane. The fraction of obscured sources depends not only on the torus angular thickness but also on the cloud number N 0 . The observed decrease of this fraction at increasing luminosity can be explained with a decrease of either torus angular thickness or cloud number, but only the latter option explains also the possible emergence of a 10µm emission feature in QSO2. X-ray obscuration, too, has a probabilistic nature. Resulting from both dusty and dust-free clouds, X-ray attenuation might be dominated by the dust-free clouds, giving rise to the observed type 1 QSO that are X-ray obscured. Observations indicate that the obscuring torus and the broad line region form a seamless distribution of clouds, with the transition between the two regimes caused by dust sublimation. Torus clouds may have been detected in the outflow component of H 2 O maser emission from two AGN. Proper motion measurements of the outflow masers, especially in Circinus, are a promising method for probing the morphology and kinematics of torus clouds.

Through matches with the Sloan Digital Sky Survey (SDSS) catalogue we identify the location of various families of astronomical objects in WISE colour space. We identify reliable indicators that separate Galactic/local from extragalactic sources and concentrate here on the objects in our Galaxy and its closest satellites. We develop colour and magnitude criteria that are based only on WISE data to select asymptotic giant branch (AGB) stars with circumstellar dust shells, and separate them into O-rich and C-rich classes. With these criteria we produce an all-sky map for the count ratio of the two populations. The map reveals differences between the Galactic disc, the Magellanic Clouds and the Sgr Dwarf Spheroidal galaxy, as well as a radial gradient in the Large Magellanic Cloud (LMC) disc. We find that the C:O number ratio for dusty AGB stars increases with distance from the LMC centre about twice as fast as measured for near-IR selected samples of early AGB stars. Detailed radiative transfer models show that WISE colours are well explained by the emission of centrally heated dusty shells where the dust has standard properties of interstellar medium (ISM) grains. The segregation of different classes of objects in WISE colour space arises from differences in properties of the dust shells: those around young stellar objects have uniform density distributions while in evolved stars they have steep radial profiles.