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

TL;DR

The paper probes what governs the RC–SFR relation in local star-forming galaxies by applying a non-parametric Random Forest to a large LoTSS-DR2–based sample, revealing that SFR is the primary predictor of radio luminosity with a significant, but smaller, contribution from stellar mass. By fitting a mass-dependent RC–SFR model and performing bias corrections through mock simulations, the authors derive debiased relations of the form $L_{150 m{MHz}} = L_c \psi^{\beta} (M_*/10^{10} M_{\odot})^{\gamma}$, finding $\beta \approx 1.06$–$1.11$ and $\gamma \approx 0.24$–$0.26$ depending on the SFR/M* source, with results consistent across photometric and spectroscopic datasets. The study discusses potential metallicity effects and residual AGN contamination, and compares its mass-dependent calibration with prior works, attributing differences to sample selection and AGN treatment. The findings improve RC–SFR calibrations for future radio surveys and demonstrate the utility of RF methods for handling censored radio samples in galaxy evolution studies. The work has practical implications for using radio luminosity as a dust-unbiased SFR tracer in large surveys and for interpreting how galaxy properties shape RC–SFR across cosmic time.

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 ($z\,\leq$\,0.3) star-forming galaxies (SFGs) using the second data release of the 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 SDSS-DR8. Along with the spectroscopically obtained SFRs and stellar mass values, we also use SFRs and stellar masses derived using photometric SED-fitting from the \textit{GALEX}--SDSS--\textit{WISE} Legacy Catalogue (GSWLC) 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 $>5σ$ 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 timescales. 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.

Figures (11)

• Figure 1: Sky coverage of MPA--JHU DR8 and LoTSS-DR2. The light and dark grey regions indicate sources from the MPA--JHU and LoTSS-DR2 catalogues respectively. The black points represent sources that belong to the parent sample as defined in Section \ref{['selection']}, i.e. sources that have optical as well as radio observations.
• Figure 2: Distribution of gas phase metallicity, velocity dispersion, radio luminosity and redshift for our samples. The 18,828 galaxies which contain LoTSS flux densities given in the 2023MartinH_optcounterparts catalogue are shown in dark grey. For the sample of 35,099 galaxies (18,828 sources with catalogued radio flux densities and aperture flux densities measured for remaining 16,271 sources), the distributions are shown in light grey.
• Figure 3: Comparison of SFR and stellar mass values derived using the photometric (GSWLC-X2) and spectroscopic (MPA--JHU) datasets for sources with 150 MHz flux densities in the LoTSS catalogue (in dark grey) and sources with radio luminosities measured with aperture photometry (in light grey). The inset plot shows the difference between SFR and stellar mass, and all values have been converted to our adopted Chabrier IMF (where required).
• Figure 4: The relationship between redshift and 150 MHz luminosity for our samples. All sources in the LoTSS-DR2 catalogue with redshifts $z \leq 0.3$ are shown in light grey, while the subset of sources with LoTSS-DR2 catalogue matches in our spectroscopic sample are shown in black. These have been complemented by the sources for which we have measured aperture 150 MHz flux densities, which are shown in dark grey (as indicated in the legend).
• Figure 5: Upper panel: relative feature importance of SFR, stellar mass, gas-phase metallicity and velocity dispersion in predicting radio luminosities for our sample of SFGs, when included in the model. The importance values of parameters in each model sum to 1. Lower panel: permuted relative importance for the same features. The numbers below each bar correspond to the legend, and to the enumerated explanations in section \ref{['ml']}.