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 submi llimetre-wave surveys, because their positions are known at best to only se veral arcsec. Centimetre-wave VLA observations are required in order to det ermine positions to subarcsec accuracy, and so to allow reliable optical id entifications to be made. Carilli & Yun pointed out that the ratio of the r adio to submillimetre-wave flux densities provides a redshift indicator for dusty star-forming galaxies, when compared with the tight correlation betw een the far-infrared and radio flux densities observed in low-redshift gala xies. This method does provide a useful, albeit imprecise, indication of th e distance to a submillimetre-selected galaxy. Unfortunately, it does not p rovide an unequivocal redshift estimate, as the degeneracy between the effe cts of increasing the redshift of a galaxy and decreasing its dust temperat ure is not broken.