Discovery of large-scale magnetic fields in the halo of hot gas that surrounds galaxies

28 February 2023

Photo: J. English/CHANG-ES collaboration

A recent Quantum Universe paper shows that nearby galaxies are surrounded by a magnetised halo of gas. This halo is dubbed the circumgalactic medium and even our own galaxy the Milky Way has one.

Magnetic fields themselves are invisible and that makes measuring them in intergalactic space very hard. But light is changed by magnetic fields. Polarisation is perhaps known from certain sunglasses that filter out light of certain polarisation. This study relies on background sources of polarised light that lie behind nearby galaxies. By using an effect called Faraday rotation, the magnetic field and ionised gas around galaxies can be measured very accurately. Faraday rotation denotes the rotation of the plane in which light waves (in our case radio waves) oscillate. This rotation is proportional to the magnetic field strength and we observe more rotation when the sightline to the background source passes through the halo of the galaxy that is being studied, showing that the halo is magnetised. Faraday rotation is best studied when radio waves are used, and this can be done with the LOFAR telescope where Hamburg is an important partner.

This discovery means that some of the web of cosmic magnetic fields that are present in the universe come from galaxies as opposed to being merely remnants of the early universe. The first author of the paper, Dr. Volker Heesen, staff scientist at Hamburg Observatory, elaborates that "Our results need to be confirmed with further observations that can inform us about the physics of galactic outflows and the way galaxies evolve." This measurement of magnetic fields in the galactic halo will help the understanding of when and how magnetic fields are created in the Universe.