If the obscuring torus has the same gas-to-dust ratio as in the Galactic ISM, and the dust is characterized a Galactic extinction curve, then the nuclear region of Sy2s should suffer a visual extinction that is related to the gaseous column density by the formula AV = 5 x 10-22 NH (cm-2). In general this is not the case: AV is lower than expected from the NH measured in the X-rays. This was first pointed out by Maccacaro et al. (1982). A visual extinction lower than that expected from the NH measured in the X-rays is also required to fit the IR spectrum of AGNs (Granato et al. 1997). We have collected a sample of Seyferts which both show X-ray (cold) absorption and whose optical or IR broad lines are not completely suppressed. The ratios between the broad lines provide information on the dust reddening towards the nucleus; however, the broad emission lines must be used with much care, since the extreme conditions of the broad line clouds can affect the intrinsic line ratios through radiative transport effects. By assuming the standard extinction curve we can estimate the visual extinction. The resulting distribution for the AV / NH ratio, relative to the Galactic standard value, is shown in Fig. 3. Most of the AGNs in our sample are characterized by a deficit of dust absorption with respect to what expected from the NH measured in the X-rays, in agreement with early claims. At higher, quasar-like luminosities there are even more extreme examples of this effect: objects that, although absorbed in the X-rays, do not show significant dust absorption in the optical and appear as type 1, broad line AGNs have been recently discovered in hard X-ray and radio surveys (Sambruna et al. 1999, Akiyama et al. 2000, Reeves et al. 1997). Puzzling enough, the early Chandra surveys presented to date have found only a few type 1 QSOs absorbed in the hard X-rays; this issue will be shortly discussed in Sect. 7.
Figure 3. Distribution of the AV / NH ratio, relative to the Galactic standard value, for a sample of absorbed AGNs.
The origin of the reduced AV / NH ratio is not clear. An obvious explanation is that the dust-to-gas ratio is much lower than Galactic or that in the inner part of the obscuring torus the dust is sublimated by the strong UV radiation field. However, if the dust content in the absorbing medium is significantly reduced, especially at the inner face, then most of the UV ionizing photons are absorbed by the atomic gas. This should create a huge HII region, which would emit strong (~ narrow) hydrogen lines corresponding to a large covering factor, i.e. much brighter than the emission lines from the NLR (see also Netzer & Laor, 1993). Also, a simple shortage of dust grains with respect to the gas mass would not explain other peculiar properties of the dust in AGNs, such as the absence of the silicate absorption feature in the mid-IR spectra of most Sy2s (Clavel et al. 2000) and the absence of the carbon dip in the UV spectra of some reddened Sy1s.
Another interesting possibility is that the dust extinction curve is much flatter than the standard Galactic. The high density of the gas in the circumnuclear region of AGNs is likely to favor the growth of large grains (probably through coagulation) which, in turn, should flatten the extinction curve and make it featureless. This effect is directly observed in the dense clouds of our Galaxy (Draine 1995). Within the context of the optical versus X-ray absorption, the effect of a flat extinction curve (due to grain coagulation) is twofold: 1) given the same dust mass, the effective visual extinction is lower, and 2) the broad lines ratio gives a deceiving (low) measure of the extinction. A more thorough discussion of the whole issue is given in Maiolino et al. (2000b).