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http://hdl.handle.net/20.500.12386/34663
Title: | A new technique to efficiently select Compton-thick AGN | Authors: | SEVERGNINI, Paola CACCIANIGA, Alessandro DELLA CECA, Roberto |
Issue Date: | 2012 | Journal: | ASTRONOMY & ASTROPHYSICS | Number: | 542 | First Page: | A46 | Abstract: | Aims: We present a new efficient diagnostic method, based on mid-infrared and X-ray data, to select local (z < 0.1) Compton-thick AGN with the aim of estimating their surface and space density. Methods: We define a region in the X-ray-to-mid-IR [F(2-12 keV)/F25ν25] vs. X-ray color (HR) plane associated to Compton-thick AGN, i.e. [F(2-12 keV)/F25ν25] < 0.02 and HR > - 0.2. On the basis of this selection method we build up a sample of 43 Compton-thick AGN candidates using data from IRAS Point Source and 2XMM-Newton catalogues. In order to test the efficiency of the proposed method in selecting Compton-thick AGN we use the results of the X-ray spectral analysis performed on all the sources of our sample (presented in a parallel work). After taking into account the different selection effects, we have estimated the number of Compton-thick in the local Universe and their density down to the IRAS flux limit of F25 = 0.5 Jy. Results: 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 Compton-thick region are actually Compton-thick AGN. Twenty percent are newly-discovered Compton-thick AGN. We then estimate the surface density of Compton-thick AGN down to the IRAS PSC catalogue flux limit (F25 = 0.5 Jy) that turns out to be ρCT ~ 3 × 10-3 src deg-2. After estimating an equivalent IR-to-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 μm emission for the sources in our sample. By considering only the 25 μm AGN emission, we estimate a surface density of Compton-thick AGN which is consistent with the results found by hard X-ray surveys. Finally, we estimated the co-moving space density of Compton-thick AGN with intrinsic LX > 1043 erg s-1 (0.004 < z < 0.06): ΦC-thick ~ (3.5-0.5+4.5)×10-6 Mpc-3. The prediction for Compton-thick AGN based on the synthesis model of X-ray background in Gilli et al. (2007) is consistent with this value. | URI: | http://hdl.handle.net/20.500.12386/34663 | URL: | http://arxiv.org/abs/1204.4359v1 https://www.aanda.org/articles/aa/full_html/2012/06/aa18417-11/aa18417-11.html |
ISSN: | 0004-6361 | DOI: | 10.1051/0004-6361/201118417 | Bibcode ADS: | 2012A&A...542A..46S | Fulltext: | open |
Appears in Collections: | 1.01 Articoli in rivista |
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AA_2012_542_A46.pdf | PDF editoriale | 286.51 kB | Adobe PDF | View/Open |
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