Constraining the Radio-Submm Redshift Indicator using data from the SCUBA Local Universe Galaxy Survey

Many of the faint submm sources uncovered by deep SCUBA surveys still remain unidentified at most, if not all other wavelengths. The most successful method so far of obtaining accurate positions and hence allowing more secure identifications has been to use centimetre wavelength imaging with the VLA. This has led to a tempting approach for obtaining redshift estimates for this population (Carilli & Yun 1999,2000), which relies on the tight FIR-radio relationship and takes advantage of the steep spectral slope in the submm. In this paper we use the submm data from the SCUBA Local Universe Galaxy Survey (SLUGS) to estimate the usefulness of, and the uncertainties in, the radio-submm redshift estimator. We investigate the dependencies of the submm-radio spectral index on various galaxy properties and find that there is no significant trend with dust temperature or 850 micron luminosity. The ratio of 850 micron/1.4 GHz flux was found to decrease with increasing radio and FIR luminosity, and we propose that this is due to a component of the dust not associated with recent star formation but which is instead heated to 15-20 K by the general interstellar radiation field.