2015A&A...573A..45M

We study the evolution of the radio spectral index and far-infrared/radio correlation (FRC) across the star-formation rate - stellar masse (i.e. SFR-M_*) plane up to z∼2. We start from a stellar-mass-selected sample of galaxies with reliable SFR and redshift estimates. We then grid the SFR-M_* plane in several redshift ranges and measure the infrared luminosity, radio luminosity, radio spectral index, and ultimately the FRC index (i.e. q_FIR) of each SFR-M_*-z bin. The infrared luminosities of our SFR-M_*-z bins are estimated using their stacked far-infrared flux densities inferred from observations obtained with the Herschel Space Observatory. Their radio luminosities and radio spectral indices (i.e. α, where S_ν∝ ν^–α) are estimated using their stacked 1.4GHz and 610MHz flux densities from the Very Large Array and Giant Metre-wave Radio Telescope, respectively. Our far-infrared and radio observations include the most widely studied blank extragalactic fields - GOODS-N, GOODS-S, ECDFS, and COSMOS - covering a total sky area of ∼2.0deg². Using this methodology, we constrain the radio spectral index and FRC index of star-forming galaxies with M_*>10¹⁰M_☉ and 0<z<2.3. We find that α^1.4GHz_610MHz does not evolve significantly with redshift or with the distance of a galaxy with respect to the main sequence (MS) of the SFR-M_* plane (i.e. Δlog(SSFR)_ MS_{=log[SSFR(galaxy)/SSFR}MS_(M*_,z)]). Instead, star-forming galaxies have a radio spectral index consistent with a canonical value of 0.8, which suggests that their radio spectra are dominated by non-thermal optically thin synchrotron emission. We find that the FRC index, q_FIR, displays a moderate but statistically significant redshift evolution as q_ FIR_(z)=(2.35±0.08)x(1+z)^{–0.12±0.04}, consistent with some previous literature. Finally, we find no significant correlation between q_FIR and Δlog(SSFR)_MS, though a weak positive trend, as observed in one of our redshift bins (i.e. Δ[q_FIR]/Δ[Δlog(SSFR)_MS]=0.22±0.07 at 0.5<z<0.8), cannot be firmly ruled out using our dataset.