Magellanic clouds 'just visiting'
The Large Magellanic Cloud may be just visiting the Milky Way on its way to intergalactic space. But if this was true, this would force us to rethink how galaxies evolve (Image: AURA/NOAO/NSF)
The Milky Way's closest galactic neighbours, the Magellanic clouds, are simply paying us a visit rather than being cannibalised, according to a controversial paper.
New measurements show these satellite galaxies are moving too quickly to have been captured and disrupted by the more massive Milky Way.
Instead, the paper says the clouds are in a hyperbolic orbit and, like spacecraft getting a gravitational 'boost' from planets, they will be flung out into intergalactic space.
Dr Gurtina Besla from the Harvard-Smithsonian Center for Astrophysics presented the paper at a recent workshop on the Magellanic system held at the Australia Telescope National Facility.
Details are also available on the arXiv website.
According to workshop organiser, Dr Erik Muller, the paper provoked both intense interest and scepticism.
"Most astronomers were excited but cautious because the measurements have been taken over a very short time and the error bars seem to be quite large," he says.
But some astronomers rejected the findings.
"The real problem is that it challenges the way we understand the evolution of the Magellanic system and how galaxies grow," says Muller.
"There are a lot of things about the Magellanic system that are not well understood," he says. "But we do know that it is interacting with our galaxy."
One of the proofs of that interaction is the long filament of neutral hydrogen gas known as the Magellanic stream which spans the 180,000 light-years between dwarf galaxies and the Milky Way.
Astronomers think that the stream probably evolved through a combination of gravitational effects, the lighter gas being pulled from the galaxies more readily than their dense stars, an effect known as ram pressure stripping.
But if the new measurements are correct, and the Magellanic system is making only one pass of our galaxy, the Magellanic stream would not have had time to form.
Galaxy evolution
And the implications for accepted galactic evolutionary theory are just as profound.
The growth of large spiral galaxies like the Milky Way is currently attributed to accretion as they gobble up small galaxies like the Magellanic clouds.
This new evidence would make such scenarios less likely, so new evolutionary models would need to be developed.
Although some people challenged Besla's paper, Muller says it is important not to be complacent especially with something as complex as the Magellanic system.
"I like the fact that just when we think we have settled things and we're getting somewhere we're required to question it again."
The Magellanic system includes the Large and Small Magellanic Clouds orbiting each other, the bridge of dust and gas between them, and various tidal stream components.
