The C-Band All-Sky Survey (C-BASS): New Constraints on the Integrated Radio Spectrum of M 31

TL;DR

This study uses new radio observations to analyze M31's integrated spectrum, detecting anomalous microwave emission and characterizing its synchrotron spectral index, providing new constraints on the galaxy's radio emission components.

Contribution

The paper presents the first integrated spectrum of M31 at 4.76GHz and combines all available radio data to constrain the synchrotron spectral index and AME distribution.

Findings

AME detected at 3σ significance near 30GHz with flux density 0.27±0.09 Jy

Synchrotron spectral index of M31 is flatter than the Milky Way at -0.66±0.03

AME emissivity in M31 is lower than typical Galactic sources, suggesting non-uniform distribution

Abstract

The Andromeda galaxy (M31) is our closest neighbouring spiral galaxy, making it an ideal target for studying the physics of the interstellar medium in a galaxy very similar to our own. Using new observations of M31 at 4.76GHz by the C-Band All-Sky Survey (C-BASS), and all available radio data at $1^\circ$ resolution, we produce the integrated spectrum and put new constraints on the synchrotron spectral index and anomalous microwave emission (AME) from M31. We use aperture photometry and spectral modelling to fit for the integrated spectrum of M31, and subtract a comprehensive model of nearby background radio sources. The AME in M31 is detected at $3\sigma$ significance with a peak near 30GHz and flux density $0.27\pm0.09$Jy. The synchrotron spectral index of M31 is flatter than our own Galaxy at $\alpha = -0.66 \pm 0.03$ with no strong evidence of spectral curvature. The emissivity of AME, averaged over the total emission from M31 is lower than typical AME sources in our Galaxy, implying that AME is not uniformly distributed throughout M31 and instead is likely confined to sub-regions -- this will need to be confirmed using future higher resolution observations around 20--30GHz.