Testing the connection between the X - ray and submillimetre backgrounds using Chandra

The powerful combination of the Chandra X-ray telescope, the SCUBA submillimetre-wave camera and the gravitational lensing effect of the massive galaxy clusters A 2390 and A 1835 has been used to place stringent X-ray flux limits on six faint submillimetre SCUBA sources and deep submillimetre limits on three Chandra sources which lie in fields common to both instruments. One further source is marginally detected in both the X-ray and submillimetre bands. For the SCUBA sources our results are consistent with starburst-dominated emission. The objects for which the strongest constraints can be placed, including SMMJ 14011+0252 at z = 2.55, can only host powerful active galactic nuclei if they are both Compton-thick and any scattered X-ray flux is weak or itself absorbed. The lensing amplification for the sources are in the range 1.5–7, assuming that they lie at z > ∼ 1. The brightest detected X-ray source has a faint extended optical counterpart (I ≈ 22) with colours consistent with a galaxy at z ≃ 1. The X-ray spectrum of this object is hard, implying strong intrinsic absorption with a column density of about 10 cm and an intrinsic (unabsorbed) 2–10 keV luminosity of 3 × 10 erg s . This object is therefore a Type-II quasar. The weakest detected X-ray sources are not detected in HST images down to I ≃ 26.