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Description
We present and discuss a three-dimensional diagnostic diagram for Seyfert 2 galaxies obtained using X-ray and [O III] data on a large sample of objects (reported in the Appendix). The diagram shows the K{alpha} iron line equivalent width as a function of both the column density derived from the photoelectric cutoff and the 2-10 keV flux normalized to the [O III] optical-line flux (the latter corrected for extinction and assumed to be a true indicator of the source intrinsic luminosity). We find that the hard X-ray properties of type 2 objects depend on a single parameter, the absorbing column density along the line of sight, in accordance with the unified model. The diagram can be used to identify Compton-thick sources and to isolate and study peculiar objects. From this analysis we have obtained a column density distribution of Seyfert 2 galaxies that is thought to be a good approximation of the real distribution. A large population of heavily absorbed objects is discovered, including many Compton-thick candidates. Our results indicate that the mean log N_H_cm^-2^ in type 2 Seyfert galaxies is 23.5 and that as many as 23%-30% of sources have N_H_=>10^24^cm^-2^.
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