2.3. X-ray Absorption as a Probe of the 100 pc Scale Torus

X-ray studies have also provided evidence for obscuring medium on larger scales (~ 100 pc). Indeed, samples of AGNs selected in the soft X-rays ( 2 keV) are characterized by a shortage of edge-on galaxies (Lawrence & Elvis 1982, McLeod & Rieke 1995, Simcoe et al. 1997). This effect is ascribed to circumnuclear material, associated to the galactic gaseous disk, which obscures the AGN when the host galaxy is oriented edge-on. Only a moderate absorbing column (N_H 10²² cm^-2) is required to suppress significantly the soft X-ray emission.

A 100 pc scale component of the obscuring torus is also required to fit the broad IR spectral energy distribution (Granato et al. 1997) and it is also directly observed in the HST images of some objects (eg. Ford et al. 1997, Malkan et al. 1998).