The radio-to-submillimeter flux density ratio of galaxies as a measure of redshift

We re-examine the technique of determining the redshifts of galaxies from t he ratio of the submillimeter-to-radio continuum flux densities based on a recently published catalog of 850 mum sources. We derived the expected vari ation of this ratio as a function of redshift incorporating the expected av erage luminosity and the spectral energy distribution (SED) of dust emissio n and the radio continuum. We find that the existing data for most of the h igh redshift (z greater than or similar to 1) sources correspond to our new calculation. Amongst the well-identified sources, there is none with an in dex significantly higher than predicted. Sources which have an index lower than predicted are either within the error zone or much lower, the latter p resumably having AGN-dominated radio-continuum emission. We find the median redshift to be similar to 2, which is consistent with that deduced by prev ious work (similar to 3) within the error. We also discuss the various syst ematic effects that can affect the accuracy of the redshift estimate. We ex amine other methods of redshift estimation, like photometric ratio in the s ubmillimeter and locating the peak of the SED in the rest system of the obj ects. We conclude that while the various methods are helpful in identifying high-redshift objects and making a crude estimate of the redshift, they ar e not, at present, accurate enough for a detailed study of redshift distrib ution of the submillimeter galaxies.