Australasian Science: Australia's authority on science since 1938

Out of the Galactic Plain

By Stephen Luntz

Vast jets of charged particles have been mapped escaping the galactic plane, confirming observations from space telescopes.

In 2010 the Fermi Gamma-ray Space Telescope found evidence of what were called “Fermi Bubbles” – areas of high-energy particles apparently travelling away from the galactic plane. While it was thought that these bubbles might be originating from the centre of the galaxy, their distance and size could not be established.

Now the Centre of Excellence for All-Sky Astrophysics (CAASTRO) has used the Parkes radiotelescope to measure these distances by detecting changes to the polarisation of radio waves emitted by these particles as they pass through clouds of gas between us and the emission site.

“We can see there is a lot of gas between us and the bubbles, and much more as we go to the plane of the galaxy,” says CAASTRO’s Deputy Director, Prof Lister Staverley-Smith. “This is what we would expect if we are a long way from the source. We can’t place it exactly at the centre of the galaxy, but it is at least halfway there.”

The outflows extend for 50,000 light years. “The source of the energy has been somewhat of a mystery, but we know there is a lot there – about a million times as much energy as a supernova explosion,” Staverley-Smith adds. Proposed sources are the gas jets surrounding the supermassive black hole at the centre of the galaxy and the star formation processes nearby. “We think the latter is more likely,” says Staverley-Smith.

The centre of the galaxy has many giant stars, with solar winds hundreds of times stronger than the sun’s and short lifespans ending in supernova explosions. It is thought these collectively produce the particles, and the galaxy’s pressure gradient forces them above and below the plane in a manner Staverley-Smith compares to the mushroom cloud of a nuclear bomb. Curvature is attributed to interactions with star formation regions in the galactic halo.

Lying in the galactic plane, the particles pose no threat to Earth but Staverley-Smith thinks the Earth’s magnetic field would probably provide sufficient protection if we were exposed.

Further study has promise. “We found that the outflows’ radiation is not homogenous but that it actually reveals a high degree of structure – which we suspect is key to how the galaxy’s overall magnetic field is generated and maintained,” says CAASTRO Director, Prof Bryan Gaensler.