Abstract
The cosmic history of the growth of supermassive black holes in galactic centers 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 (Fx/Fo) 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 Fx/Fo >50, EXO50 sources hereafter), using a well defined sample of seven X-ray sources extracted from the 2XMM catalogue. Five EXO50 sources (about 70 percent of the sample) in the bright flux regime explored by our survey (f(2-10 keV) > 1.5E-13 cgs) are associated with obscured AGN (Nh > 1.0E22 cm-2), spanning a redshift range between 0.75 and 1 and characterised by 2-10 keV intrinsic luminosities in the QSO regime (e.g. well in excess to 1.0E44 cgs). We did not find compelling evidence of Compton Thick 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), 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.
Highlights
IntroductionThe study of high-z-obscured quasars (type 2 QSOs: the high luminosity counterpart of Seyfert 2 galaxies) is one of the hot topics of current extragalactic astronomy
The study of high-z-obscured quasars is one of the hot topics of current extragalactic astronomy
There are no Compton thick active galacic nuclei (AGN) (NH in excess to ∼1024 cm−2) amongst the three type 2 QSO, neither is suspected their presence in the two still spectroscopically unidentified sources, since their X-ray spectra are at odds with that usually observed in Compton thick AGN
Summary
The study of high-z-obscured quasars (type 2 QSOs: the high luminosity counterpart of Seyfert 2 galaxies) is one of the hot topics of current extragalactic astronomy. In spite of the outstanding progress, obtained by using medium and deep X-ray surveys (see e.g. Brusa et al 2010 and references therein), the weakness, both in the X-ray and in the optical band, of the selected sources with high values of the X-ray-to-optical flux ratio usually prevents from a detailed analysis of the individual objects (see e.g. Koekemoer et al 2004; Civano, Comastri & Brusa 2005)
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