Skip to main content

Research Repository

Advanced Search

A wide search for obscured active galactic nuclei using XMM-Newton and WISE

Rovilos, E.; Georgantopoulos, I.; Akylas, A.; Aird, J.; Alexander, D.M.; Comastri, A.; Del Moro, A.; Gandhi, P.; Georgakakis, A.; Harrison, C.M.; Mullaney, J.R.

A wide search for obscured active galactic nuclei using XMM-Newton and WISE Thumbnail


E. Rovilos

I. Georgantopoulos

A. Akylas

J. Aird

A. Comastri

A. Del Moro

P. Gandhi

A. Georgakakis

C.M. Harrison

J.R. Mullaney


Heavily obscured and Compton-thick active galactic nuclei (AGNs) are missing even in the deepest X-ray surveys, and indirect methods are required to detect them. Here we use a combination of the XMM–Newton serendipitous X-ray survey with the optical Sloan Digital Sky Survey (SDSS), and the infrared WISE all-sky survey in order to check the efficiency of the low X-ray-to-infrared luminosity selection method in finding heavily obscured AGNs. We select the sources which are detected in the hard X-ray band (2–8 keV), and also have a redshift determination (photometric or spectroscopic) in the SDSS catalogue. We match this sample with the WISE catalogue, and fit the spectral energy distributions of the 2844 sources which have three, or more, photometric data points in the infrared. We then select the heavily obscured AGN candidates by comparing their 12 μm luminosity to the observed 2–10 keV X-ray luminosity and the intrinsic relation between the X-ray and the mid-infrared luminosities. With this approach, we find 20 candidate heavily obscured AGNs and we then examine their X-ray and optical spectra. Of the 20 initial candidates, we find nine (64 per cent; out of the 14, for which X-ray spectra could be fitted) based on the X-ray spectra, and seven (78 per cent; out of the nine detected spectroscopically in the SDSS) based on the [O III] line fluxes. Combining all criteria, we determine the final number of heavily obscured AGNs to be 12–19, and the number of Compton-thick AGNs to be 2–5, showing that the method is reliable in finding obscured AGNs, but not Compton thick. However, those numbers are smaller than what would be expected from X-ray background population synthesis models, which demonstrates how the optical–infrared selection and the scatter of the Lx-LMIR relation limit the efficiency of the method. Finally, we test popular obscured AGN selection methods based on mid-infrared colours, and find that the probability of an AGN to be selected by its mid-infrared colours increases with the X-ray luminosity. The (observed) X-ray luminosities of heavily obscured AGNs are relatively low (L2−10keV<1044ergs−1), even though most of them are located in the ‘quasi stellar object (QSO) locus’. However, a selection scheme based on a relatively low X-ray luminosity and mid-infrared colours characteristic of QSOs would not select ∼25 per cent of the heavily obscured AGNs of our sample.

Journal Article Type Article
Publication Date Feb 1, 2014
Deposit Date Jun 25, 2014
Publicly Available Date Jul 29, 2014
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 438
Issue 1
Pages 494-512
Keywords Galaxies: active, Infrared: galaxies, X-rays: galaxies.
Public URL


Published Journal Article (4.2 Mb)

Copyright Statement
This article has been accepted for publication in Monthly notices of the Royal Astronomical Society © 2014 The Authors Published by Oxford University Press on behalf of Royal Astronomical Society. All rights reserved.

You might also like

Downloadable Citations