XMM-Newton surveys of the Canada-France Redshift Survey fields -: I.: The submillimetre/X-ray relation

First results from XMM-Newton observations of the Canada-France Redshift Survey (CFRS) 3-h, 10-h and 14-h fields are presented. Limited regions of two of the XMM surveys (3 and 14 h) are compared with the Canada-UK Deep Submillimetre Surveys (CUDSS) undertaken with SCUBA. None of the 27 SCUBA sources in the 3-h field is detected by XMM , while one of the 23 SCUBA sources in the 14-h field is found to coincide with an X-ray source. The SCUBA population as a whole is not significantly detected in either the 0.5-2 keV or the 2-10 keV X-ray bands, even after coadding the X-ray flux at the SCUBA positions, in both fields. The 18 X-ray sources within the CUDSS 3-h map yield a mean submillimetre (submm) flux of 0.48 +/- 0.27 mJy after coadding the submm flux at the X-ray positions. Using this result we place an upper limit on the contribution of AGN to the submm background at 850 mum of similar to7 per cent. Conversely we estimate the contribution of submm sources to the 0.5-2 keV X-ray background to be <16.5 per cent. These results strongly support the conclusion that the two backgrounds are caused by different processes, in the one case nucleosynthesis in stars, in the other accretion on to black holes. We conclude that it is possible for SCUBA sources in general to contain AGN, as long as they are Compton-thick and are at z > 2.3. The ratio of the X-ray to submm flux for the X-ray sources, however, implies that even when a galaxy does contain an AGN, most of the energy heating the dust is from young stars and not from the active nucleus.