Nearby galaxies in LoTSS-DR2: insights into the non-linearity of the radio-SFR relation

TL;DR

This study investigates the radio-SFR relation in nearby galaxies using LOFAR data, revealing a super-linear relation influenced by galaxy size and mass, and highlights the non-linearity and spatial variation of radio spectral indices.

Contribution

It provides the first detailed analysis of the radio-SFR relation at 144 MHz, demonstrating its super-linear nature and linking spectral index variations to galaxy properties and star formation activity.

Findings

Radio-SFR relation at 144 MHz is super-linear with $L_{144} \\propto SFR^{1.4-1.5}$

Mean spectral index between 144 and 1400 MHz is -0.56, consistent with cosmic ray electron injection

Galaxies with higher SFRs have steeper radio spectra and larger, more massive galaxies are better electron calorimeters.

Abstract

Context. Cosmic rays and magnetic fields are key ingredients in galaxy evolution, regulating both stellar feedback and star formation. Their properties can be studied with low-frequency radio continuum observations, free from thermal contamination. Aims. We define a sample of 76 nearby (< 30 Mpc) galaxies, with rich ancillary data in the radio continuum and infrared from the CHANG-ES and KINGFISH surveys, which will be observed with the LOFAR Two-metre Sky Survey (LoTSS) at 144 MHz. Methods. We present maps for 45 of them as part of the LoTSS data release 2 (LoTSS-DR2), where we measure integrated flux densities and study integrated and spatially resolved radio spectral indices. We investigate the radio-SFR relation, using star-formation rates (SFR) from total infrared and H $\alpha$ + 24-$\mu$m emission. Results. The radio-SFR relation at 144 MHz is clearly super-linear with $L_{144} \propto SFR^{1.4-1.5}$. The mean integrated radio spectral index between 144 and $\approx$1400 MHz is $\langle \alpha\rangle = -0.56 \pm 0.14$, in agreement with the injection spectral index for cosmic ray electrons (CRE). However, the radio spectral index maps show a variation of spectral indices with flatter spectra associated with star-forming regions and steeper spectra in galaxy outskirts and, in particular, in extra-planar regions. We found that galaxies with high star-formation rates (SFR) have steeper radio spectra; we find similar correlations with galaxy size, mass, and rotation speed. Conclusions. Galaxies that are larger and more massive are better electron calorimeters, meaning that the CRE lose a higher fraction of their energy within the galaxies. This explains the super-linear radio-SFR relation, with more massive, star-forming galaxies being radio bright. We propose a semi-calorimetric radio-SFR relation, which employs the galaxy mass as a proxy for the calorimetric efficiency.