Nearby galaxies in the LOFAR Two-metre Sky Survey

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 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.

Nearby galaxies in the LOFAR Two-metre Sky Survey

“…This is because the CRE lifetime increases at low frequencies and so the cosmic rays travel further from their sites of acceleration. When one studies the spatially resolved radio-SFR relation, the relation becomes sub-linear unlike the integrated radio-SFR relation, which is approximately linear when one accounts for the influence of cosmic-ray transport (Smith et al 2021;Heesen et al 2022). This influence of the cosmic-ray transport on spatially resolved observations (Heesen et al 2014(Heesen et al , 2019 is supported further by Mulcahy et al (2016) who analysed the radial variation of the radio spectral index in NGC 5194.…”

“…In this paper we use data of the second LoTSS data release (LoTSS-DR2; Shimwell et al 2022). The data consist of maps of 39 nearby galaxies at a frequency of 144 MHz, which were presented by Heesen et al (2022), and we use the data to calculate spatially resolved magnetic field strength maps.…”

“…We used the LoTSS-DR2 144 MHz maps at 6 arcsec angular resolution as presented by Heesen et al (2022). We also use their radio spectral index maps, or if not available, the integrated radio spectral index.…”

“…Our galaxy sample consists of a mix of galaxies from the SIRTF Nearby Galaxies Survey (SINGS; Kennicutt et al 2003), the Continuum Halos in Nearby Galaxies: An EVLA Survey (CHANG-ES; Irwin et al 2012), and Key Insights on Nearby Galaxies: A Far-Infrared Survey with Herschel (KINGFISH; Kennicutt et al 2011). We calculate magnetic field strength maps from LOFAR 144 MHz intensity maps and spectral index maps from Heesen et al (2022). The galaxies were chosen to have rich ancillary data, and we include observations from the H i Nearby Galaxy Survey (THINGS; Walter et al 2008) and the HERA CO-Line Extragalactic Survey (HERACLES; Leroy et al 2009).…”

Walk on the Low Side: LOFAR Explores the Low-frequency Radio Emission of GASP Jellyfish Galaxies

“…In principle, this truncation may be the result of the stripping of the ISM magnetic fields and the CRe, together with the ISM, or low-frequency flux losses occurring in the high-density, starforming regions of the disk due to the ionization captures. The former scenario should be characterized by a lack of both radio and Hα emission at the disk edges as result of the ISM stripping, whereas in the latter we should observe Hα emission without a radio counterpart and a local flattening of the radio spectral index at low frequencies due to the loss of the lowenergy CRe (e.g., Lacki 2013;Basu et al 2015;Heesen et al 2022). In Figures 1 and 2 we observe galaxies with signatures of stripping of both the thermal and nonthermal ISM (e.g., the northern edge of JW39), whereas evidence of ionization losses have been observed at the disk edges of JW100 in the form of flat-spectrum emission between 144 MHz and 1.4 GHz (Ignesti et al 2022).…”