Nearby galaxies in the LOFAR Two-metre Sky Survey. I. Insights into the non-linearity of the radio-SFR relation

Heesen, V.V.HeesenStaffehl, M.M.StaffehlBasu, A.A.BasuBeck, R.R.BeckStein, M.M.SteinTabatabaei, F. S.F. S.TabatabaeiHardcastle, M. J.M. J.HardcastleChyży, K. T.K. T.ChyżyShimwell, T. W.T. W.ShimwellAdebahr, B.B.AdebahrBeswick, R.R.BeswickBomans, D. J.D. J.BomansBOTTEON, AndreaAndreaBOTTEONBrinks, E.E.BrinksBrüggen, M.M.BrüggenDettmar, R. -J.R. -J.DettmarDrabent, A.A.DrabentDE GASPERIN, FrancescoFrancescoDE GASPERINGürkan, G.G.GürkanHeald, G. H.G. H.HealdHorellou, C.C.HorellouNikiel-Wroczynski, B.B.Nikiel-WroczynskiPALADINO, RositaRositaPALADINOPiotrowska, J.J.PiotrowskaRöttgering, H. J. A.H. J. A.RöttgeringSmith, D. J. B.D. J. B.SmithTasse, C.C.Tasse2025-03-202025-03-2020220004-6361http://hdl.handle.net/20.500.12386/36900Context. 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 that are 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-star formation rate (SFR) relation using SFRs derived from total infrared and Hα + 24-μm emission. Results: The radio-SFR relation at 144 MHz is clearly super-linear with L144 MHz ∝ SFR1.4−1.5. The mean integrated radio spectral index between 144 and ≈1400 MHz is ⟨α⟩= − 0.56 ± 0.14, in agreement with the injection spectral index for cosmic ray electrons (CREs). However, the radio spectral index maps show 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 SFRs 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 that employs the galaxy mass as a proxy for the calorimetric efficiency. Tables and fits maps are only available at the CDS via anonymous ftp to <A href="http://cdsarc.u-strasbg.fr/">cdsarc.u-strasbg.fr</A> (ftp://130.79.128.5) or via <A href="http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/664/A83">http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/664/A83</A>...STAMPAenNearby galaxies in the LOFAR Two-metre Sky Survey. I. Insights into the non-linearity of the radio-SFR relationArticle10.1051/0004-6361/2021428782-s2.0-85137026749https://www.aanda.org/articles/aa/full_html/2022/08/aa42878-21/aa42878-21.htmlhttps://api.elsevier.com/content/abstract/scopus_id/85137026749FIS/05 - ASTRONOMIA E ASTROFISICA