Daniel MOLNAR
(University of Sussex)
The calibration of radio continuum emission as a high-resolution, dust unbiased star formation tracer relies to a large extent on the locally well observed inrared-radio correlation (IRRC). With the advent of new, high sensitivity radio instruments (such as the SKA) it is crucial to quantify its redshift dependence, before we can safely use it to measure star formation rates in the early Universe. Recent studies have revealed a trend for mildly decreasing infrared/radio luminosity ratios in star-forming galaxies at increasingly high redshifts. This contradicts theoretical predictions, highlighting our poor understanding of the physical processes that link radio emission to star formation. Combining new, high sensitivity 3 GHz radio data, Herschel observations and morphological classification based on HST images in the COSMOS field out to z ~ 1.5, we have found, for the first time, significantly different redshift trends for infrared/radio ratios of spheroid- and disk-dominated star-forming galaxies (SFGs). Disk-dominated SFGs, the simplest star-forming systems in our sample, have a constant IRRC, while spheroid-dominated galaxies show a decreasing trend consistent with other studies. The different behaviour of median infrared/radio luminosity ratios is due to to a relative radio excess in spheroid-dominated SFGs at high redshift, hinting at a possible low-level AGN activity, which does not express itself clearly in standard AGN diagnostics, which were initially used to identify and remove AGNs from our sample. These results demonstrate the need for a more in-depth understanding of the IRRC before we can accurately use radio as a star formation tracer at high redshifts.
I will also present preliminary results of our local reference sample IRRC study.