The diversity of SCUBA-selected galaxies

We present extensive observations of a sample of distant, submillimetre (submm) galaxies detected in the field of the massive cluster lens, Abell1835, using the Submm Common-User Bolometer Array (SCUBA). Taken in conjunction with earlier observations of other submm-selected sources (Ivison et al. 1998; Smail et al. 1999; Soucail et al. 1999) we now have detailed, multi-wavelength observations of seven examples of the submm population, having exploited the combination of achromatic amplification by cluster lenses and lavish archival datasets. These sources, all clearly at z > 1, illustrate the wide range in the radio and optical properties of distant submm-selected galaxies. We include detailed observations of the first candidate ‘pure’ starburst submm galaxy at high redshift, a z = 2.56 interacting galaxy which shows no obvious sign of hosting an AGN. The remaining sources have varying degrees of inferred AGN activity (three from seven of the most luminous show some evidence of the presence of an AGN) although even when an AGN is obviously present it is still not apparent if reprocessed radiation from this source dominates the submm emission. In contrast with the variation in the spectral properties, we see relatively homogeneous morphologies for the population, with a large fraction of merging or interacting systems. Our study shows that virtually identical spectral energy distributions are seen for galaxies which exhibit strikingly different optical/UV spectral-line characteristics. We conclude that standard optical/UV spectral classifications are misleading when applied to distant, highly obscured galaxies and that we must seek other means of determining the various contributions to the overall energy budget of submm galaxies and hence to the far-infrared extragalactic background.