An excess of sub – millimetre sources towards z ∼ 1 clusters

Deep sub-millimetre observations using SCUBA are presented of the central regions of four high redshift clusters which have been extensively studied optically: CL0023+0423 (z = 0.84), J0848+4453 (z = 1.27), CL1604+4304 (z = 0.90) and CL1604+4321 (z = 0.92). 10 sub–millimetre sources are securely detected towards these four clusters at 850μm, with two further tenuous detections; the raw 850μm source counts exceed those determined from blank–field surveys by a factor ∼ 3 − 4. In particular, towards CL1604+4304, 6 sources are detected with S850μJy > 4 mJy making this the richest sub-mm field discovered to date. Corrections for gravitational lensing by these high redshift clusters reduce these excess sources counts, but are unlikely to account for more than about half of the excess, with the remainder presumably directly associated with cluster galaxies. The 450 to 850μm flux density ratios of the detected sources are systematically higher (at a significance level > 98%) than those determined for blank–field selected sources, consistent with them being at the cluster redshifts. If subsequent identifications confirm cluster membership, these results will demonstrate that the optical Butcher–Oemler effect is also observed at sub-mm wavelengths.