Radio emission as a tracer for star formation

Radio continuum emission as a star-formation tracer

Tracing when and where stars are formed in the Universe is one of the key questions in galaxy evolution. Observations at multiple wavelengths tracing stellar populations facilitate our understanding of star formation in galaxies. Unlike the UV SFR tracers, radio continuum emission has the advantage of being unaffected by dust obscuration. Radio emission in typical star-forming galaxies comes from non-thermal synchrotron radiation (~90% at 1.4GHz) and also from thermal free-free emission from ionised HII regions (10% at 1.4GHz), both directly linked to massive star formation. However, the relationship between radio continuum emission and the SFR in galaxies is not well understood because the steps between SF and synchrotron emission (supernova explosion, acceleration of relativistic electrons in the SNR, propagation of cosmic rays throughout the galaxy, energy loss, and escape) remain poorly understood, inhibiting quantitative interpretation of the observed synchrotron spectra and brightness. Infrared and radio luminosities are observed to be related in galaxies. And so current radio-SFR calibrations often rely on the combination of the empirical infrared-radio correlation (IRRC) and the infrared - SFR calibration.

We have compiled the largest sample of IR and radio luminosities for galaxies in the local Universe to date and are studying the relationship between IR and radio emission as a function of galaxy properties to better understand the radio-emission in star-forming and AGN-host galaxies. The catalogue was presented in Molnar, Sargent, Leslie et al. 2021. For our subset of ~2000 high-confidence star-forming galaxies, we find a median qTIR of 2.54 (scatter: 0.17 dex). We show that qTIR correlates with L_1.4, implying a non-linear IRRC with slope 1.11 ± 0.01. Our new L_1.4-SFR calibration, which incorporates this non-linearity, reproduces SFRs from panchromatic SED fits substantially better than previous IRRC-based recipes.

Recent results have shown that the radio-SFR relation depends on galaxy stellar mass (e.g. Smith et al. 2021, Delvecchio et al. 2021) . Follow-up work involving the The LOFAR Two-metre Sky Survey Second data release is currently in prep - watch this space!