A three-dimensional diagnostic diagram for Seyfert 2 galaxies: Probing X-ray absorption and Compton thickness

We present and discuss a three-dimensional diagnostic diagram for Seyfert 2 galaxies obtained using X-ray and CO mi data on a large sample of objects (reported in the Appendix). The diagram shows the Ka 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 CO mi 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 greater than or equal to 10(24) cm(-2).