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 arcsec. Centimetre-wave VLA observations are required in order to determine positions to subarcsec 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.