Sub-mm and X-ray background: Two unrelated phenomena?

Obscured AGNs are thought to contribute a large fraction of the hard X-ray background (2–10 keV), and have also been proposed as the powerhouse of a fraction of the SCUBA sources which make most of the background at 850μm, thus providing a link between the two spectral windows. We have tackled this issue by comparing data at 2–10 keV and at 850μm for a sample of 34 sources at fluxes (or limiting fluxes) which resolve most of the background in the two bands. We present here new SCUBA observations, and new correlations between separate data sets retrieved from the literature. Similar correlations presented by others are added for completeness. None of the 11 hard X-ray (2–10 keV) sources has a counterpart at 850 μm, with the exception of a Chandra source in the SSA13 field, which is a candidate type 2, heavily absorbed QSO at high redshift. The ratios F850μm/F5keV (mostly upper limits) of the X-ray sources are significantly lower than the value observed for the cosmic background. In particular, we obtain that 2–10 keV sources brighter than 10−15 erg s−1cm−2, which make at least 75% of the background in this band, contribute for less than 7% to the submillimeter background. Out of the 24 SCUBA sources, 23 are undetected by Chandra. The ratios F850μm/F5keV (mostly lower limits) of these SCUBA sources indicate that most of them must be powered either by starburst activity, or by an AGN which is obscured by a column NH > 1025 cm−2, with a reflection efficiency in the hard X rays significantly lower than 1% in most cases. However, AGNs of this type could not contribute significantly to the 2–10 keV background.