What factors shape the radio luminosity of star-forming galaxies? A new calibration from LoTSS-DR2

ABSTRACT Radio observations offer a dust-unobscured view of galaxy star formation via the radio continuum–star formation rate (RC–SFR) relation. Emerging evidence of a stellar mass dependence in the RC–SFR relation raises the broader question of how other galaxy properties may influence this relation. In this work, we study the dependence of the global RC–SFR relation on galaxy properties in local ($z\, \le$ 0.3) star-forming galaxies (SFGs) using the second data release of the LOw Frequency ARray (LOFAR) Two-Metre Sky Survey (LoTSS-DR2). Employing a non-parametric decision-tree regression algorithm, we identify the most important galaxy properties for estimating the radio luminosity using a sample of 18‌‌‌ 828 emission-line-classified SFGs based on spectroscopic data from the Sloan Digital Sky Survey (SDSS)-DR8. Along with the spectroscopically obtained SFRs and stellar mass values, we also use SFRs and stellar masses derived using photometric spectral energy distribution-fitting from the GALEX–SDSS–WISE Legacy Catalogue for the same sample. We find that a galaxy’s SFR is most important for predicting the radio luminosity, followed by the stellar mass, at ${\gt} 5\sigma$ significance. Complementing the LoTSS catalogue 150 MHz flux densities with aperture photometry for the rest of the emission-line classified sample (35‌ 099 galaxies in total), we obtain a new calibration of the RC–SFR relation, which does not change significantly whether we use spectroscopic or photometrically derived SFRs and stellar masses, despite the fact that the methods probe star formation on different characteristic time-scales. Our results highlight the utility of decision-tree algorithms for handling censored radio-selected galaxy samples, which will be useful for future spectroscopic surveys of radio sources.