EDP Sciences
Free access
Volume 484, Number 3, June IV 2008
Page(s) 671 - 678
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20078790
Published online 16 April 2008

A&A 484, 671-678 (2008)
DOI: 10.1051/0004-6361:20078790

Searching for mid-IR obscured AGN in the Chandra deep field North

I. Georgantopoulos1, A. Georgakakis2, M. Rowan-Robinson2, and E. Rovilos1, 3

1  Institute of Astronomy & Astrophysics, National Observatory of Athens, Palaia Penteli, 15236, Athens, Greece
    e-mail: ig@astro.noa.gr
2  Astrophysics Group, Blackett Laboratory, Imperial College, Prince Consort Road, SW7 2BZ, UK
3  Present address: Max Planck Institut für Extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany

Received 4 October 2007 / Accepted 28 March 2008

The efficiency of mid-infrared selection methods for finding obscured AGN is investigated using data in the Chandra Deep Field North. It is shown that samples of AGN candidates compiled on the basis of mid-infrared colours only suffer substantial contamination from normal galaxies. X-ray stacking analysis reveals a soft mean X-ray spectrum for these sources, consistent with $\Gamma\approx$ 2.1. This suggests that star-forming galaxies and not obscured AGN dominate the stacked signal. In contrast AGN selection methods that combine mid-infrared with optical criteria are more successful in finding heavily obscured AGN candidates. A method similar to the one proposed by Fiore et al. (2008) is adopted to select extremely red objects (R - [3.6] > 3.7 mag) with high 24 $\mu$m to optical flux ratio $(f_{\rm 24~\mu m}/f_{R}>1000)$. About 80% of these sources are not detected at X-ray wavelengths. Stacking the X-ray photons at the positions of these sources shows a flat mean X-ray spectrum ($\Gamma$$\approx$ 0.8), which suggests Compton-thick sources, low-luminosity and moderately obscured ($N_{\rm H}$ ~ 8 $\times$ 1022 cm-2) AGN, or a combination of the two. The mid-infrared colours and luminosities of these sources are consistent with ULIRGs at z $\approx$ 2, while HST/ACS images, available for the optically brighter of these sources, show disturbed optical morphologies in many of them. The evidence above suggests that this population includes systems in the process of formation.

Key words: X-rays: general -- X-rays: diffuse background -- X-rays: galaxies -- infrared: galaxies

© ESO 2008