Alien invaders pack the Milky Way

Feb 23, 2010
A Hubble Space Telescope image of the typical globular cluster Messier 80, an object made up of hundreds of thousands of stars and located in the direction of the constellation of Scorpius. The Milky Way galaxy has an estimated 160 globular clusters of which one quarter are thought to be ‘alien’. Image: NASA / The Hubble Heritage Team / STScI / AURA

(PhysOrg.com) -- Around a quarter of the globular star clusters in our Milky Way are invaders from other galaxies, new research from Swinburne University of Technology (Australia) shows.

In a paper accepted for publication in the , Swinburne astronomer Professor Duncan Forbes has shown that many of our galaxy’s globular star clusters are actually foreigners - having been born elsewhere and then migrating to our .

“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.

Using 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 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.

“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 are broken-up and their stars assimilated into the Milky Way, the globular star clusters of the 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.”

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

Explore further: The changing laws that determine how dust affects the light that reaches us from the stars

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

Provided by Swinburne University of Technology

4.3 /5 (15 votes)

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srikkanth_kn
1.3 / 5 (3) Feb 24, 2010
Many more may still be waiting. Have they identified them ? Why no mention of that ?
yyz
not rated yet Feb 24, 2010
I think the notion of multiple star forming periods in some globular clusters is well established, no matter the length of time involved. Detailed star formation epochs are being delineated in a number of nearby globular clusters, as a literature search easily shows. The challenge to astrophysicists is to explain these star forming events and find a way to plausibly model their (occasional) occurrence.
yyz
not rated yet Feb 24, 2010
Really ancient submillimeter invaders have been spotted by Hubble: http://128.84.158...18v1.pdf
yyz
not rated yet Feb 26, 2010
Fro"We find that the [latter young] track is dominated by globular clusters associated with the Sagittarius and Canis Major dwarf galaxies. Despite being overly simplistic, its age-metallicity relation can be well represented by a simple closed box model with continuous star formation. The inferred chemical enrichment history is similar to that of the Large Magellanic Cloud, but is more enriched, at a given age, compared to the Small Magellanic Cloud. After excluding Sagittarius and Canis Major GCs, several young track GCs remain. Their horizontal branch morphologies are often red and hence classified as Young Halo objects, however they do not tend to reveal extended horizontal branches (a possible signature of an accreted remnant nucleus). Retrograde orbit GCs (a key signature of accretion) are commonly found in the young track. We also examine GCs that lie close to the Fornax-Leo-Sculptor great circle defined by several satellite galaxies. We find that several GCs are consistent with

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