Radio–FIR correlation: A probe into cosmic ray propagation in the nearby galaxy IC 342

¹ School of Astronomy, Institute for Research in Fundamental Sciences (IPM), PO Box 19395-5531 Tehran, Iran
² Max-Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Max-Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁴ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁵ Canadian Institute for Theoretical Astrophysics (CITA), University of Toronto, 60 St George St, Toronto M5S 3H8, Canada
⁶ INAF – Istituto di Radioastronomia, Via P. Gobetti 101, 40129 Bologna, Italy
⁷ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), 44780 Bochum, Germany
⁸ Astronomical Observatory of the Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland

^⋆ Corresponding author; ftaba@ipm.ir

Received: 13 June 2024
Accepted: 15 September 2024

Abstract

Resolved studies of the correlation between the radio and far-infrared (FIR) emission from galaxies at different frequencies can unveil the interplay between star formation and the relativistic interstellar medium (ISM). Thanks to the LOFAR LoTSS observations combined with VLA, Herschel, and WISE data, we study the role of cosmic rays and magnetic fields in the radio–FIR correlation on scales of ≳200 pc in the nearby galaxy IC 342. The thermal emission traced by the 22 μm emission, constitutes about 6%, 13%, and 30% of the observed radio emission at 0.14, 1.4, and 4.8 GHz, respectively, in star-forming regions and less in other parts. The nonthermal spectral index becomes flatter at frequencies lower than 1.4 GHz (α_n = −0.51 ± 0.09, S_ν ∝ ν^α_n) than between 1.4 and 4.8 GHz (α_n = −1.06 ± 0.19) on average, and this flattening occurs not only in star-forming regions but also in the diffuse ISM. The radio–FIR correlation holds at all radio frequencies; however, it is tighter at higher radio frequencies. A multi-scale analysis shows that this correlation cannot be maintained on small scales due to diffusion of cosmic ray electrons (CREs). The correlation breaks at a larger scale (≃320 pc) at 0.14 GHz than at 1.4 GHz (≃200 pc), indicating that the CREs traced at lower frequencies have diffused a longer path in the ISM. We find that the energy index of CREs becomes flatter in star-forming regions, in agreement with previous studies. Cooling of CREs due to the magnetic field is evident globally only after compensating for the effect of star formation activity that both accelerates CREs and amplifies magnetic fields. Compared with other nearby galaxies, we show that the smallest scale of the radio–FIR correlation is proportional to the propagation length of the CREs on which the ordered magnetic field has an important effect.