Cosmic axion background propagation in galaxies

TL;DR

This paper models axion-like particle (ALP) to photon conversion in various galactic environments, concluding that the Milky Way's conversion signal is too weak to explain the cluster X-ray excess, but starburst galaxies could have higher conversion probabilities.

Contribution

It provides detailed simulations of ALP-photon conversion across different galaxy types, incorporating magnetic field and medium effects, and assesses their potential to explain cosmic X-ray phenomena.

Findings

Conversion probability in the Milky Way is too small to explain the cluster X-ray excess.

Galaxy clusters have a much higher ALP-photon conversion probability than the Milky Way.

Starburst galaxies may have comparable conversion probabilities to clusters due to their strong magnetic fields.

Abstract

Many extensions of the Standard Model include axions or axion-like particles (ALPs). Here we study ALP to photon conversion in the magnetic field of the Milky Way and starburst galaxies. By modelling the effects of the coherent and random magnetic fields, the warm ionized medium and the warm neutral medium on the conversion process, we simulate maps of the conversion probability across the sky for a range of ALP energies. In particular, we consider a diffuse cosmic ALP background (CAB) analogous to the CMB, whose existence is suggested by string models of inflation. ALP-photon conversion of a CAB in the magnetic fields of galaxy clusters has been proposed as an explanation of the cluster soft X-ray excess. We therefore study the phenomenology and expected photon signal of CAB propagation in the Milky Way. We find that, for the CAB parameters required to explain the cluster soft X-ray excess, the photon flux from ALP-photon conversion in the Milky Way would be unobservably small. The ALP-photon conversion probability in galaxy clusters is 3 orders of magnitude higher than that in the Milky Way. Furthermore, the morphology of the unresolved cosmic X-ray background is incompatible with a significant component from ALP-photon conversion. We also consider ALP-photon conversion in starburst galaxies, which host much higher magnetic fields. By considering the clumpy structure of the galactic plasma, we find that conversion probabilities comparable to those in clusters may be possible in starburst galaxies.