Dust temperature and the submillimetre–radio flux density ratio as a redshift indicator for distant galaxies

It is difficult to identify the distant galaxies selected in existing submillimetre-wave surveys, because their positions are known at best to only several arcseconds. Centimetrewave VLA observations are required in order to determine positions to subarcsecond accuracy, and so to allow reliable optical identifications to be made. Carilli & Yun pointed out that the ratio of the radio to submillimetre-wave flux densities provides a redshift indicator for dusty star-forming galaxies, when compared with the tight correlation between the far-infrared and radio flux densities observed in low-redshift galaxies. This method does provide a useful, albeit imprecise, indication of the distance to a submillimetre-selected galaxy. Unfortunately, it does not provide an unequivocal redshift estimate, as the degeneracy between the effects of increasing the redshift of a galaxy and decreasing its dust temperature is not broken.