A new technique to efficiently select Compton-thick AGN
  We present a new efficient diagnostic method, based on mid-infrared and X-ray
data, to select local (z<0.1) Compton-thick (CT) AGN with the aim of estimating
their surface and space density. We define a region in the X-ray/IR vs. HR
plane associated to CT AGN, i.e. F(2-12keV)/F25nu25<0.02 and HR>-0.2. We build
up a sample of 43 CT AGN candidates using data from IRAS-PSC and 2XMM
catalogue. In order to test the efficiency of the proposed method in selecting
CT AGN we use the results of the X-ray spectral analysis performed on all the
sources of our sample. After taking into account the different selection
effects, we have estimated the number of CT in the local Universe and their
density down to the IRAS flux limit of F25=0.5Jy. We find that the diagnostic
plot proposed here is an efficient method to select Compton-thick AGN in the
nearby Universe since ~84% of the sources populating the proposed CT region are
actually CT AGN. Twenty percent are newly-discovered CT AGN. We then estimate
the surface density of CT AGN down to the IRAS PSC catalogue flux limit
(F25=0.5Jy) that turns out to be ~3e-3 src deg-2. After estimating an
equivalent IR-hard X-ray limiting flux, we compare our result with those found
with SWIFT-BAT. We find that the surface density derived here is a factor 4
above the density computed in the hard X-ray surveys. This difference is
ascribed, at least in part, to a significant contribution (~60-90%) of the
star-forming activity to the total 25 mic emission for the sources in our
sample. By considering only the 25 mic AGN emission, we estimate a surface
density of CT AGN which is consistent with the results found by hard X-ray
surveys. Finally, we estimated the co-moving space density of CT AGN with
intrinsic LX>1e43 erg s-1 (0.004<z<0.06): ~3.5e-6 Mpc-3. The prediction for CT
AGN based on the synthesis model of XRB in Gilli et al.(2007) is consistent
with this value.
