Alien Invaders of the Milky Way

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The Milky Way has an estimated 160 orbiting globular clusters -hundreds of thousands, and sometimes millions of stars all bound together in a tight ball due to their gravity- of which one quarter are thought to be ‘alien’ invaders from other galaxies, according to research from Swinburne University of Technology (Australia). 


Swinburne astronomer Professor Duncan Forbes has shown that many of our galaxy’s globular star clusters are actually aliens – having been born elsewhere and then migrating to our Milky Way.

“It turns out that many of the stars and star clusters we see when we look into the night sky are not natives, but aliens from other galaxies,” said Forbes. “They have made their way into our galaxy over the last few billion years.”

Previously astronomers had suspected that some star clusters, which contain around a million stars each, were foreign to our galaxy, but it was difficult to positively identify which ones. Globular clusters, which are found in the halo of a galaxy, contain considerably more stars and are among the oldest stars in the universe. Almost every large galaxy has been found to possess a system of globular clusters.

Using Hubble Space Telescope data, Forbes, along with his Canadian colleague Professor Terry Bridges, examined old star clusters within the Milky Way galaxy.

They then compiled the largest ever high-quality database to record the age and chemical properties of each of these clusters.

“Using this database we were able to identify key signatures in many of the star clusters that gave us tell-tale clues as to their external origin,” Forbes said.

“We determined that these foreign-born globular star clusters actually make up about one quarter of our Milky Way globular star cluster system. That implies tens of millions of accreted stars – those that have joined and grown our galaxy – from globular star clusters alone.”

The researchers’ work also suggests that the Milky Way may have swallowed-up more dwarf galaxies than was previously thought.

6a00d8341bf7f753ef01310f34fc49970c.jpg “We found that many of the foreign clusters originally existed within dwarf galaxies – that is ‘mini’ galaxies of up to 100 million stars that sit within our larger Milky Way. Our work shows that there are more of these accreted dwarf galaxies in our Milky Way than was thought. Astronomers had been able to confirm the existence of two accreted dwarf galaxies in our Milky Way – but our research suggests that there might be as many as six yet to be discovered."

"Although the dwarf galaxies are broken-up and their stars assimilated into the Milky Way, the globular star clusters of the dwarf galaxy remain intact and survive the accretion process. This will have to be explored further, but it is a very exciting prospect that will help us to better understand the history of our own galaxy.”

In addition to migration of Dwarf galaxy remnants, another team of Australian researchers discovered that the Milky Way galaxy has been cannibalizing on the Sagittarius dwarf galaxy and others.

The Sagittarius dwarf is a fascinating object, located at only 24 kiloparsecs from the Sun and 16 kiloparsecs from the Galactic Center (i.e. 75 000 and 50 000 light-years respectively), is the nearest known satellite of the Milky Way. The dwarf was discovered only recently in 1994, hidden to us by foreground Galactic stars.

"The Sagittarius dwarf is a cosmic lightweight weighing 10,000 times less than our Milky Way," Dr Stefan Keller said. "It has ventured too close to our galaxy and is now getting stretched out and slowly torn apart, a bit like spaghetti being wound round a fork" and being swallowed by our own Galaxy after complete disruption caused by Galactic tides,

In the latest issue of the Astrophysical Journal, Dr Keller and researchers from the Australian National University reveal that a large band of stars at the edge of the Milky Way were chomped off of the smaller Sagittarius galaxy.

But it’s not just Sagittarius who has reason to call the Milky Way a bad neighbor. According to Dr Keller, this is not the first time the Milky Way has nibbled on its neighbors.

“Early in the life of the Milky Way galaxy mergers such as this occurred on a much more frequent basis, contributing substantially to the mass of the Milky Way,” Dr Keller said. "The devouring of the Sagittarius dwarf is like the after-dinner mint on top of what has been an extensive banquet for the Milky Way."

Forbes’ research was carried out in Canada as part of an Australian Research Council International Fellowship.

More information: Accreted versus in situ Milky Way globular clusters, Monthly Notices of the Royal Astronomical Society, Available at http://arxiv.org/abs/1001.4289

Casey Kazan via Swinburne University of Technology

Image top of page: 47 Tucanae is what astronomers refer to as a "globular cluster." Globular clusters are compact regions with anywhere from ten thousand to several million stars. 47 Tucanae is an ancient cluster of several million stars located about 15,000 light years from Earth. The stars in 47 Tucanae are an estimated 10-12 billion years old, placing them among the oldest in our galaxy. Because the stars in clusters like 47 Tucanae are about the same age, they make perfect laboratories for the study of stellar life cycles. Credit: Southern African Large Telescope (SALT) project

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