Macquarie astronomers find a well of serenity in deep space.

A massive galaxy four billion light-years from Earth is surrounded by a halo of tranquil gas.

The finding, which reveals a galactic halo much less dense and less magnetised than expected, was made by a team of astronomers that included two researchers from Macquarie University.

Stuart Ryder and Lachlan Marnoch from the Department of Physics and Astronomy worked with colleagues from the International Centre for Radio Astronomy Research in WA, the Kavli Institute for the Physics and Mathematics of the Universe, in Japan, the University of California, CSIRO and others. The resulting study has been published in the journal Science.

The research marks a significant increase in detecting the nature of the cool and enriched halo gas that is known to extend for many light-years around galaxies, but which has until now proved challenging to study.

Stuart, Lachlan and the rest of the international team were able to gain insight into the conditions in the halo around the distant galaxy by tracing the path of a fast radio burst, dubbed FRB 181112.

“Our role was to obtain the European Southern Observatory Very Large Telescope images and spectroscopy, without which we wouldn’t even have been aware that the burst had passed through the halo of a foreground galaxy,” explains Stuart.

“That was the big surprise here, in that we had always assumed passage through halo gas would have a more significant impact on the burst profile than it did.”

The research represented a particularly nice outcome for Lachlan Marnoch – a Masters student working under Dr Ryder.

“Lachlan was responsible for processing the ESO data, and thus is also a co-author,” explains Stuart. “Not many Masters students get their name on a paper in Science before they even submit their thesis!”

The research was led by University of California Professor of Astronomy and Astrophysics J. Xavier Prochaska. He says he expected the fast radio burst to be distorted as it passed through the halo, but instead it was unaffected – indicating that the gas was more diffuse than thought, and with a much weaker magnetic field.

“So, our observations can inform theories about how matter is ejected and how magnetic fields are transported from the galaxy,” he explains.

“Our research appears to reveal something entirely new about galactic halos. Unless of course, this galaxy happens to be just some weird exception — and with only one object you can’t be sure about that.”

The research used a fast radio burst that was detected in November by the Australian Square Kilometre Array Pathfinder (ASKAP), in outback Western Australia.

The telescope is a precursor to the Square Kilometre Array (SKA), which will be the world’s largest radio telescope when it’s built in the next decade.

The paper, titled The low density and magnetization of a massive galaxy halo exposed by a fast radio burst, is available here:  https://science.sciencemag.org/lookup/doi/10.1126/science.aay0073