The infrared-radio correlation of star-forming galaxies is stronglyM⋆-dependent but nearly redshift-invariant sincez∼ 4

Abstract

Over the past decade, several works have used the ratio between total (rest 8−1000μm) infrared and radio (rest 1.4 GHz) luminosity in star-forming galaxies (qIR), 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 ofqIRwith redshift, finding a mild but significant decline that is yet to be understood. Here, for the first time, we calibrateqIRas a function ofbothstellar mass (M⋆) and redshift, starting from anM⋆-selected sample of > 400 000 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 withM⋆, with more massive galaxies displaying a systematically lowerqIR. A secondary, weaker dependence on redshift is also observed. The best-fit analytical expression is the following:qIR(M⋆, z) = (2.646 ± 0.024) × (1 + z)( − 0.023 ± 0.008)–(0.148 ± 0.013) × (log M⋆/M⊙ − 10). Adding the UV dust-uncorrected contribution to the IR as a proxy for the total SFR would further steepen theqIRdependence onM⋆. 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 novelM⋆-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.