# Strong evidence of Anomalous Microwave Emission from the flux density spectrum of M31

## TL;DR

This study provides strong evidence of anomalous microwave emission from M31, revealing its spectral characteristics and emission mechanisms across a broad frequency range, with implications for understanding galactic dust and microwave foregrounds.

## Contribution

First detection and detailed spectral analysis of AME in M31, decomposing emission mechanisms from radio to infrared frequencies.

## Key findings

- Strong AME detected at ~25GHz with 1.45 Jy flux density.
- Synchrotron dominates below 10GHz with spectral index -1.10.
- AME overtakes synchrotron and free-free between 20-50GHz.

## Abstract

We have observed the Andromeda galaxy, Messier 31 (M31), at 6.7GHz with the Sardinia Radio Telescope. We mapped the radio emission in the C-band, re-analyzed WMAP and Planck maps, as well as other ancillary data, and we have derived an overall integrated flux density spectrum from the radio to the infrared. This allowed us to estimate the emission budget from M31. Integrating over the whole galaxy, we found strong and highly significant evidence for anomalous microwave emission (AME), at the level of (1.45+0.17-0.19)Jy at the peaking frequency of ~25GHz. Decomposing the spectrum into known emission mechanisms such as free-free, synchrotron, thermal dust, and AME arising from electric dipole emission from rapidly rotating dust grains, we found that the overall emission from M31 is dominated, at frequencies below 10GHz, by synchrotron emission with a spectral index of -1.10+0.10-0.08, with subdominant free-free emission. At frequencies >10GHz, AME has a similar intensity to that of synchrotron and free-free emission, overtaking them between 20GHz and 50GHz, whereas thermal dust emission dominates the emission budget at frequencies above 60GHz, as expected.