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 f raction of the SCUBA sources which make most of the background at 850 mu m, thus providing a link between the two spectral windows. We have tackled th is issue by comparing data at 2-10 keV and at 850 mu m for a sample of 34 s ources at fluxes (or limiting fluxes) which resolve most of the background in the two bands. We present here new SCUBA observations, and new correlati ons between separate data sets retrieved from the literature. Similar corre lations presented by others are added for completeness. None of the 11 hard X-ray (2-10 keV) sources has a counterpart at 850 mu m, with the exception of a Chandra source in the SSA13 field, which is a candidate type 2, heavi ly absorbed QSO at high redshift. The ratios F-850 mu m/F-5keV (mostly uppe r limits) of the X-ray sources are significantly lower than the value obser ved for the cosmic background. In particular, we obtain that 2-10 keV sourc es brighter than 10(-15) erg s(-1) cm(-2), which make at least 75% of the b ackground in this band, contribute for less than 7% to the submillimeter ba ckground. Out of the 24 SCUBA sources, 23 are undetected by Chandra. The ra tios F-850 mu m/F-5keV (mostly lower limits) of these SCUBA sources indicat e that most of them must be powered either by starburst activity, or by an AGN which is obscured by a column N-H > 10(25) cm(-2), With a reflection ef ficiency in the hard X rays significantly lower than 1% in most cases. Howe ver, AGNs of this type could not contribute significantly to the 2-10 keV b ackground.