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Exploring the active galactic nuclei population with extreme X-ray-to-optical flux ratios (fx/fo > 50)

AutorDella Ceca, R.; Carrera, Francisco J. ; Caccianiga, A.; Severgnini, P.; Ballo, Lucía ; Braito, V.; Corral, Amalia ; Moro, A. del; Mateos, Silvia ; Ruiz, Ángel; Watson, M. G.
Palabras claveX-rays: galaxies
Quasars: general
Galaxies: active
BL Lacertae objects: general
Galaxies: nuclei
Fecha de publicación2015
EditorOxford University Press
CitaciónMonthly Notices of the Royal Astronomical Society 447(4): 3227-3242 (2015)
ResumenThe 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 (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 (∼70 per cent of the sample) in the bright flux regime explored by our survey (f(2−10 keV) ≥ 1.5 × 10−13 erg cm−2 s−1) are associated with obscured AGN (NH > 1022 cm−2), 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 1044 erg s−1). 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, f24 μm/fR ≥ 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.
Versión del editorhtpps://doi.org/10.1093/mnras/stu2665
Identificadoresdoi: 10.1093/mnras/stu2665
e-issn: 1365-2966
issn: 0035-8711
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