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|Title:||Exploring the active galactic nuclei population with extreme X-ray-to-optical flux ratios (fx/fo > 50)||Authors:||DELLA CECA, Roberto
Carrera, F. J.
Del Moro, A.
Watson, M. G.
|Issue Date:||2015||Journal:||MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY||Number:||447||Issue:||4||First Page:||3227||Abstract:||The cosmic history of the growth of supermassive black holes in galactic centres parallels that of star formation in the Universe. However, an important fraction of this growth occurs inconspicuously in obscured objects, where ultraviolet/optical/near-infrared emission is heavily obscured by dust. Since the X-ray flux is less attenuated, a high X-ray-to-optical flux ratio (f<SUB>x</SUB>/f<SUB>o</SUB>) is expected to be an efficient tool to find out these obscured accreting sources. We explore here via optical spectroscopy, X-ray spectroscopy and infrared photometry the most extreme cases of this population (those with f<SUB>x</SUB>/f<SUB>o</SUB> > 50, EXO50 sources hereafter), using a well-defined sample of seven X-ray sources extracted from the 2XMM catalogue. Five EXO50 sources (∼70 per cent of the sample) in the bright flux regime explored by our survey (f<SUB>(2-10 keV)</SUB> ≥ 1.5 × 10<SUP>-13</SUP> erg cm<SUP>-2</SUP> s<SUP>-1</SUP>) are associated with obscured AGN (N<SUB>H</SUB> > 10<SUP>22</SUP> cm<SUP>-2</SUP>), spanning a redshift range between 0.75 and 1 and characterized by 2-10 keV intrinsic luminosities in the QSO regime (e.g. well in excess to 10<SUP>44</SUP> erg s<SUP>-1</SUP>). We did not find compelling evidence of Compton thick active galacic nuclei (AGN). Overall, the EXO50 type 2 QSOs do not seem to be different from standard X-ray-selected type 2 QSOs in terms of nuclear absorption; a very high AGN/host galaxy ratio seems to play a major role in explaining their extreme properties. Interestingly, three out of five EXO50 type 2 QSO objects can be classified as extreme dust-obscured galaxies (EDOGs, f<SUB>24 μm</SUB>/f<SUB>R</SUB> ≥ 2000), suggesting that a very high AGN/host ratios (along with the large amount of dust absorption) could be the natural explanation also for a part of the EDOG population. The remaining two EXO50 sources are classified as BL Lac objects, having rather extreme properties, and which are good candidates for TeV emission.||Acknowledgments:||This research has made use of data obtained from the XMM – Newton satellite and data obtained from the High Energy Astrophysics Science Archive Research Center (HEASARC), provided by NASA's Goddard Space Flight Center. We acknowledge partial financial support from ASI grants (no. I/023/05/0, n. I/088/06/ and I/037/12/0) and from the Italian Ministry of Education, Universities and Research (PRIN2010-2011, grant no. 2010NHBSBE). The research leading to these results has received funding from the European Commission Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 267251 ‘Astronomy Fellowships in Italy’ (AstroFIt). SM and FJC acknowledge financial support by the Spanish Ministry of Economy and Competitiveness through grants AYA2010-21490-C02-01 and AYA2012-31447 and from the ARCHES project, funded by the 7th Framework of the European Union (project no. 313146). AR acknowledges financial support by the Spanish Ministry of Economy and Competitiveness through grant AYA2012-31447. Based on data from the WISE , which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the US Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society and the Higher Education Funding Council for England. The SDSS Web Site is http://www.sdss.org/ . Based on observations made with the Telescopio Nazionale Galileo – operated by the Centro Galileo Galilei- and the Gran Telescopio de Canarias installed in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias, in the island of La Palma (Spain). We thank the anonymous referee for useful comments that have improved the quality of the paper.||URI:||http://hdl.handle.net/20.500.12386/23607||URL:||http://arxiv.org/abs/1501.01444v1
|Appears in Collections:||1.01 Articoli in rivista|
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checked on Oct 27, 2020
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