University of Leicester
MNRAS-2015-Della Ceca-3227-42.pdf (2.2 MB)

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

Download (2.2 MB)
journal contribution
posted on 2016-01-27, 11:33 authored by R. Della Ceca, F. J. Carrera, A. Caccianiga, P. Severgnini, L. Ballo, V. Braito, A. Corral, G. A. Del Moro, Fernando S. Mateos, A. Ruiz, Michael Geoffrey Watson
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[Subscript: x]/f[Subscript: o]) 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[Subscript: x]/f[Subscript: o] > 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[Subscript: (2−10 keV)] ≥ 1.5 × 10[Superscript: −13] erg cm[Superscript: −2] s[Superscript: −1]) are associated with obscured AGN (NH > 1022 cm[Superscript: −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 10[Superscript: 44] erg s[Superscript: −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, f[Subscript: 24 μm]/f[Subscript: R] ≥ 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.



Monthly Notices of the Royal Astronomical Society, 2015, 447 (4), pp. 3227-3242 (16)

Author affiliation

/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy


  • VoR (Version of Record)

Published in

Monthly Notices of the Royal Astronomical Society


Oxford University Press (OUP)





Acceptance date


Copyright date


Available date


Publisher version