Cosmic-ray electron propagation in the peculiar barred spiral galaxy NGC 2442
Abstract
Face-on spiral galaxies offer a favorable geometry for studying magnetic-field structures and their impact on cosmic-ray propagation, as projection effects and structural overlap are significantly reduced. We aim to investigate the transport of cosmic-ray electrons (CREs) in the nearby face-on spiral galaxy NGC 2442 and assess how the galactic environment and magnetic-field structure influence their propagation. We combined radio continuum (RC) observations from ASKAP at 943 MHz, MeerKAT at 1.28 and 1.7 GHz, and ATCA at 5 GHz with optical Hα and infrared data to characterize the galactic environment. We then compared these observations with 2D CRE transport simulations to evaluate the role of the magnetic field and the surrounding environment in shaping CRE propagation. NGC 2442 exhibits an integrated RC spectrum that is steeper than those of most nearby galaxies, with spectral indices of α=-0.96 =-1.21 for the total emission and α_ ̊m nt for the synchrotron emission over 408 MHz--5 GHz. The spectrum shows a break near 1 GHz, indicating substantial radiative aging of the CRE population. Under the equipartition assumption, we derive a mean magnetic-field strength of 10.8, , . We identify a steep-spectrum synchrotron ``island'' in the southeastern region, with an average synchrotron spectral index of α ∼ -1.09 and no clear Hα, infrared, far-ultraviolet or near-ultraviolet counterpart, indicating that the CREs are unlikely to be injected in situ. Our 2D simulations using CRPropa show that anisotropic diffusion along ordered magnetic fields allows CREs to reach the island region much more efficiently than isotropic diffusion. G. The RC--star formation rate smoothing analysis indicates effective CRE propagation lengths of ∼ 0.65--0.89 kpc at 943--1700 MHz and ∼ 0.44 kpc at 5 GHz, corresponding to effective diffusion coefficients on the order of 10^ 28 -1 NGC 2442 provides evidence that the combined effects of environmental disturbances and ordered magnetic fields can strongly regulate CRE propagation in disturbed spiral galaxies.