An improved view of cosmic-ray transport and the galactic outflow in NGC 253
The nearly edge-on starburst galaxy NGC 253 has been observed to exhibit extended halo emission in multiple bands, making it an ideal laboratory for studying the transfer of matter from the disk to the halo. We aim to determine how the cosmic-ray electrons (CREs) flow from the disk to the halo and understand what drives their propagation. By combining data from multiple observations, we generated improved total intensity images at 943 MHz with a resolution of 13 from the Australian SKA Pathfinder (ASKAP), and at 216 MHz with a resolution of 45 from the Murchison Widefield Array (MWA). The 1D advection and diffusion equations were solved, and the solutions were fitted to the observed synchrotron emission intensity and spectral-index profiles to constrain the propagation model and parameters. The ASKAP total intensity map has an rms noise of 16 μJy beam -1 $, reaching the classical confusion limit, and the MWA map has an rms noise of 1 mJy beam^-1. The sensitivities are significantly improved in comparison to previous observations at similar frequencies. In the ASKAP image, we identify a clear loop-like structure in the northwestern radio spur, extending vertically up to sim9,kpc above the disk, while the southeastern spur reaches heights of sim8,kpc. The synchrotron emission intensity profiles perpendicular to the disk can be fitted with exponential components in the central regions and with Gaussian components in the outer regions. This result implies that CREs in these two regions propagate differently. By jointly fitting the vertical synchrotron emission intensity profiles at 943 MHz and 216 MHz, together with the corresponding synchrotron spectral-index profiles, our results provide the clearest evidence to date that CREs are transported from the disk by advection in the central region and by diffusion elsewhere in NGC 253. The advection speed in the central region increases exponentially with height and reaches the escape speed to form a superwind of CREs at about 5.5 kpc. This superwind is associated with regions in the disk with active star formation and X-ray emission, indicating a bulk motion of baryons caused by the advection. The combined thermal, magnetic, cosmic-ray, and ram pressures exceed the gravitational pressure below $|z|łesssim5.5 kpc, and this overpressure condition accelerates the superwind. High-sensitivity low-frequency radio observations provide an important probe of the transport of CREs. With these observations, we have revealed a newly detailed view into the kinematic origin of the superwind from the center of NGC 253.