A wide search for obscured Active Galactic Nuclei using XMM-Newton and WISE

摘要

We use a combination of the XMM-Newton serendipitous X-ray survey with theoptical SDSS, and the infrared WISE all-sky survey in order to check theefficiency of the low X-ray to infrared luminosity selection method in findingheavily obscured AGN. We select sources in the 2-8 keV X-ray band which have aredshift determination in the SDSS catalogue. We match this sample with theWISE catalogue, and fit the SEDs of the 2844 sources which have three, or more,photometric data-points in the infrared. We then select the heavily obscuredAGN candidates by comparing their 12 micron AGN luminosity to the observed 2-10keV X-ray luminosity and their expected intrinsic relation. With this approachwe find 20 candidates, and we examine their X-ray and optical spectra. Of the20 initial candidates, we find nine (64%; out of the 14, for which X-rayspectra could be fit) based on the X-ray spectra, and seven (78%; out of thenine detected spectroscopically in the SDSS) based on the [OIII] line fluxes.Combining all criteria, we determine the final number of heavily obscured AGNto be 12-19, and the number of Compton-thick AGN to be 2-5, showing that themethod is reliable in finding obscured AGN, but not Compton-thick. Howeverthose numbers are smaller than what would be expected from X-ray backgroundpopulation synthesis models, which demonstrates how the optical-infraredselection and the scatter of the L_x-L_MIR relation introduced by observationalconstraints limit the efficiency of the method. Finally, we test popularobscured AGN selection methods based on mid-infrared colours, and find that theprobability of an AGN to be selected by its mid-infrared colours increases withthe X-ray luminosity. However, a selection scheme based on a relatively lowX-ray luminosity and mid-infrared colours characteristic of QSOs would notselect ~25% of the heavily obscured AGN of our sample. (abridged)