Our cannibal galaxy
Story by Julian Cribb
To see its starfields hung in radiant splendour across the night sky, you would scarcely suspect the Milky Way of being a cannibal, a gigantic buzz-saw of a galaxy that has chopped its neighbours into bits and ingested their fragments into its own clouds of stars.
But that is what a fascinating new piece of ‘galactic archaeology’ by Swinburne astrophysicist Professor Duncan Forbes and his colleague Dr Terry Bridges, of Queen’s University, Canada, has revealed.*
In a painstaking analysis of the age and metallic composition of almost 100 galactic clusters – groups of one million or so stars – out of the 160 clusters that comprise our galaxy, the researchers conclude that up to a quarter are aliens … born elsewhere and at a different time to the majority of clusters in our own Milky Way.
Astronomers have long suspected that the galaxies we see today are accretions consisting of the remnants of other galaxies, but the true extent of this intergalactic churning and star-exchange has never before been quite so evident. In fact, say Professor Forbes and Dr Bridges, about a quarter of the galactic clusters in the Milky Way today may themselves be the remnants of between six and eight smaller galaxies which it has chewed up and partially absorbed over time.
Notable among the youngsters are clusters with a strong family resemblance to the remnants of the dwarf galaxies Sagittarius and Canis Major, both of which appear to have been torn apart by the vast tidal forces of gravity in the past. It now seems they passed close enough to the Milky Way, the dominant local galaxy, for it to rob them of large clusters of their stars, some of which are only half the general age of the clusters in our galaxy.
“A great circle in the sky connects the Fornax, Leo (I and II) and Sculptor galaxies,” Professor Forbes says. “One possibility is that some clusters were tidally stripped from the Fornax galaxy as it crossed the orbit of the Milky Way. Another possibility is that these clusters came from the remains of a completely disrupted dwarf galaxy that was torn to pieces.”
Another set of star clusters in our Milky Way exhibit a contrary motion to most of the others, making it quite likely they have been drawn in from outside.
The cluster known as Omega Centaurus and another called M54, may indeed be the remnant nuclei of ruined dwarf galaxies which our own has devoured.
Yet another group of star clusters has signatures unusually rich in helium and other elements, pointing to formation processes somewhat different to those of clusters in our galaxy as a whole.
The team found some galactic clusters as young as a mere two billion years – less than half the age of the Earth – suggesting that the process of disruption and accretion is proceeding more or less continuously, as star clusters are born from gas clouds in dwarf galaxies past which the Milky Way hurtles, absorbing some of them on its journey.
Similar star-stripping it appears is now starting to befall the Magellanic Clouds, large and small, which have approached close enough to the Milky Way to be feeling the power of its gravitational hunger, Professor Forbes says. And a vast event, the collision of the Milky Way with the giant spiral galaxy Andromeda, is due to take place in five billion years from now.
“The universe seems in some ways to be a very violent place, with all these interactions, mergers and collisions taking place, as the giant galaxies cannibalise the smaller ones. But on the other hand, so vast are the distances that even when two galaxies collide the stars do not come into contact with one another, although they are subject to each other’s gravitational influence.”
Instead, he says, astronomers speculate these mergers may bring about the change from the classic spiral-shaped galaxy to the larger and more chaotic elliptical form – and in some cases possibly back again.
The team’s galactic archaeology has yielded the largest high-quality database recording the age and chemical properties of each of the Milky Way star clusters, revealing ‘layer by layer’ the deep history of our own star system and its neighbours.
“Using this data from the Hubble Space Telescope we’ve been able to identify key signatures in many of the galactic clusters that differentiate them from the bulk of the population and point to an external origin,” Professor Forbes says. “This led us to conclude that tens of millions of the stars we can see each night in our own galaxy are outsiders, drawn in from other galactic bodies.
“Previously astronomers considered our galaxy might have absorbed stars from a couple of others. These latest data provide evidence that the Milky Way may be far hungrier than we imagined, and has swallowed pieces of as many as six or eight.”
* Professor Forbes and Dr Bridges’ paper, ‘Accreted versus in situ Milky Way globular clusters’, appears in a recent issue of the Monthly Notices of the Royal Astronomical Society. Professor Forbes’ research was carried out in Canada as part of an Australian Research Council International Fellowship.
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This article was originally published in Swinburne Magazine issue 10, July 2010.