Correlations between bright submillimetre sources and low-redshift galaxies

We present evidence for a positive angular correlation between bright submillimetre (sub-mm) sources and low-redshift galaxies. The study was conducted using 39 sources selected from three contiguous, flux-limited SCUBA surveys, cross-correlated with optical field galaxies with magnitudes R < 23 (with a median redshift of z≃ 0.5). We find that the angular distribution of sub-mm sources is skewed towards overdensities in the galaxy population, consistent with 25 ± 12 per cent being associated with dense, low-redshift structure. The signal appears to be dominated by the brightest sources with a flux density S850 μm > 10 mJy. We conduct Monte Carlo simulations of clustered sub-mm populations, and find that the probability of obtaining these correlations by chance is less than 0.4 per cent. The results may suggest that a larger than expected fraction of sub-mm sources lies at z≃ 0.5. Alternatively, we argue that this signal is most likely caused by gravitational lensing bias, which may be entirely expected given the steep sub-mm source counts. Implications for future sub-mm surveys are discussed.