Axion-like particles and circular polarisation of active galactic nuclei

TL;DR

This paper proposes that axion-like particles interacting with photons in extragalactic magnetic fields can explain observed large-scale polarisation alignments of quasars, including effects on circular polarisation.

Contribution

It introduces a wave packet approach to photon-axion mixing and explores the implications for circular polarisation in quasar light.

Findings

Photon-axion mixing can produce large-scale polarisation alignments.

The model predicts specific signatures in circular polarisation.

Extragalactic magnetic fields influence photon propagation through axion-like particles.

Abstract

The measurements of the linear polarisation of visible light from quasars give strong evidence for large-scale coherent orientations of their polarisation vectors in some regions of the sky. We show that these observations can be explained by the mixing of the photons with very light pseudoscalar (axion-like) particles in extragalactic magnetic fields during their propagation. We present a new treatment in terms of wave packets and discuss the circular polarisation.