AzTEC Millimetre Survey of the COSMOS field - II. Source count overdensity and correlations with large-scale structure

We report an overdensity of bright submillimetre galaxies (SMGs) in the 0.15 deg(2) AzTEC/COSMOS survey and a spatial correlation between the SMGs and the optical-IR galaxy density at z less than or similar to 1.1. This portion of the COSMOS field shows a similar to 3 sigma overdensity of robust SMG detections when compared to a background, or 'blank-field', population model that is consistent with SMG surveys of fields with no extragalactic bias. The SMG overdensity is most significant in the number of very bright detections (14 sources with measured fluxes S(1.1mm) > 6 mJy), which is entirely incompatible with sample variance within our adopted blank-field number densities and infers an overdensity significance of >> 4 sigma. We find that the overdensity and spatial correlation to optical-IR galaxy density are most consistent with lensing of a background SMG population by foreground mass structures along the line of sight, rather than physical association of the SMGs with the z less than or similar to 1.1 galaxies/clusters. The SMG positions are only weakly correlated with weak-lensing maps, suggesting that the dominant sources of correlation are individual galaxies and the more tenuous structures in the survey region, and not the massive and compact clusters. These results highlight the important roles cosmic variance and large-scale structure can play in the study of SMGs.