# Spectral energy distribution and radio halo of NGC 253 at low radio frequencies

## TL;DR

This study presents low-frequency radio observations of NGC 253, modeling its spectral energy distribution, revealing a complex interplay of starburst and extended emissions, and detecting its large-scale radio halo with characteristic structures.

## Contribution

First detailed broadband radio spectral modeling of NGC 253 at low frequencies, identifying absorption features and halo morphology using MWA data.

## Key findings

- Central starburst region modeled as free-free absorbed synchrotron plasma.
- Extended emission shows spectral flattening at low frequencies.
- Detected large-scale X-shaped radio halo extending ~8 kpc.

## 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).