Title: Spectral energy distribution and radio halo of NGC 253 at low radio frequencies,
Abstract: We present new radio continuum observations of NGC253 from the Murchison
Widefield Array at frequencies between 76 and 227 MHz. We model the broadband
radio spectral energy distribution for the total flux density of NGC253 between
76 MHz and 11 GHz. The spectrum is best described as a sum of central starburst
and extended emission. The central component, corresponding to the inner 500pc
of the starburst region of the galaxy, is best modelled as an internally
free-free absorbed synchrotron plasma, with a turnover frequency around 230
MHz. The extended emission component of the NGC253 spectrum is best described
as a synchrotron emission flattening at low radio frequencies. We find that 34%
of the extended emission (outside the central starburst region) at 1 GHz
becomes partially absorbed at low radio frequencies. Most of this flattening
occurs in the western region of the SE halo, and may be indicative of
synchrotron self-absorption of shock re-accelerated electrons or an intrinsic
low-energy cut off of the electron distribution. Furthermore, we detect the
large-scale synchrotron radio halo of NGC253 in our radio images. At 154 - 231
MHz the halo displays the well known X-shaped/horn-like structure, and extends
out to ~8kpc in z-direction (from major axis).