2021A&A...647A.123D

Over the past decade, several works have used the ratio between total (rest 8-1000 µm) infrared and radio (rest 1.4GHz) luminosity in star-forming galaxies (q_IR), often referred to as the infrared-radio correlation (IRRC), to calibrate the radio emission as a star formation rate (SFR) indicator. Previous studies constrained the evolution of q_IR with redshift, finding a mild but significant decline that is yet to be understood. Here, for the first time, we calibrate q_IR as a function of both stellar mass (M_*) and redshift, starting from an M_*-selected sample of >400000 star-forming galaxies in the COSMOS field, identified via (NUV-r)/(r-J) colours, at redshifts of 0.1<z<4.5. Within each (M_*,z) bin, we stacked the deepest available infrared/sub-mm and radio images. We fit the stacked IR spectral energy distributions with typical star-forming galaxy and IR-AGN templates. We then carefully removed the radio AGN candidates via a recursive approach. We find that the IRRC evolves primarily with M_*, with more massive galaxies displaying a systematically lower q_IR. A secondary, weaker dependence on redshift is also observed. The best-fit analytical expression is the following: q_IR(M_*,z)=(2.646±0.024)x(1+z)^{(–0.023±0.008)}-(0.148±0.013)x(logM_*/M_☉-10). Adding the UV dust-uncorrected contribution to the IR as a proxy for the total SFR would further steepen the q_IR dependence on M_*. We interpret the apparent redshift decline reported in previous works as due to low-M_* galaxies being progressively under-represented at high redshift, as a consequence of binning only in redshift and using either infrared or radio-detected samples. The lower IR/radio ratios seen in more massive galaxies are well described by their higher observed SFR surface densities. Our findings highlight the fact that using radio-synchrotron emission as a proxy for SFR requires novel M_*-dependent recipes that will enable us to convert detections from future ultra-deep radio surveys into accurate SFR measurements down to low-M_* galaxies with low SFR.