X-ray observations of sub-mm LABOCA galaxies in the eCDFS

¹ INAF-Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
e-mail: ioannis.georgantopoulos@oabo.inaf.it
² Institute of Astronomy & Astrophysics, National Observatory of Athens, Palaia Penteli, 15236 Athens, Greece
³ Max Planck Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany

Received: 17 June 2010
Accepted: 15 October 2010

Abstract

We explore the X-ray properties of the 126 sub-mm galaxies (SMGs) of the LABOCA survey in the CDFS and the eCDFS regions. SMGs are believed to experience massive episodes of star formation. Our goal is to examine whether star formation coexists with AGN activity, determine the fraction of highly obscured AGN, and finally to obtain an idea of the dominant power mechanism in these sources. Using Spitzer and radio arcsecond positions for the SMGs, we find 14 sources with significant X-ray detections. For most of these there are only photometric redshifts available, with their median redshift at  ~2.3. Taking only the CDFS area into account that has the deepest X-ray observations, we estimate an X-ray AGN fraction of  <26 ± 9% among SMGs. The X-ray spectral properties of the majority of the X-ray AGN that are associated with SMGs are consistent with high obscuration,  >10²³ cm^-2, but there is no unambiguous evidence of Compton-thick sources. Detailed spectral energy distribution fittings show that the bulk of total IR luminosity originates in star forming processes, although a torus component is usually present. Finally, stacking analysis of the X-ray undetected SMGs reveals a signal in the soft (0.5–2 keV) and marginally in the hard (2–5 keV) X-ray bands. The hardness ratio of the stacked signal is relatively soft (−0.40 ± 0.10) corresponding to Γ ~ 1.6. This argues against a high fraction of Compton-thick sources among the X-ray undetected SMGs.