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Stars form from galactic merger

Galaxies merge to form star factory

Do big star-rich galaxies form slowly over over long periods of time, with slow absorption of smaller galaxies and stately formation of stars? Or do they form swiftly when smaller galaxies crash together in a furious burst of star formation? Astrophysics researchers from the United Kingdom, America, France and South Africa – including the University of the Western Cape's (UWC) Astrophysics Group in the Faculty of Natural Sciences – recently published a paper that may have the answer an online issue of Nature.

When HXMM01 was first identified by the Herschel Space Observatory as part of the Multi-tiered Extragalactic Survey, it seemed to be just one massive, sub-millimetre bright galaxy - so called because it emits light with wavelengths of less than a millimetre but larger than those of visible light.

But observation by several telescopes on the ground and in space showed that it was actually composed of two smaller (though still massive) colliding spiral galaxies, connected by a bridge of material. And further analysis using instruments examining the galaxies in the infra-red range (with wavelengths of under a millimetre but larger than visible light) showed that the material in the middle is formed of gas that is rapidly coalescing into stars, at a rate of about 2000 stars formed per year – about a thousand times the rate of star formation in our own Milky Way Galaxy.

By measuring the rate of star formation in the merging galaxies, as well as the amount of gas that still remained in them (about 200 billion solar masses each, or enough to form 200 billion sun-sized stars), the researchers determined how long it would take before the gas resources were exhausted and the galaxies settled down into one big (super-giant, in fact) elliptical galaxy: around 200 million years.

And while that may sound like a very long time, it's really very short when compared to the age of the universe as a whole (around 14 billion years) or even of our own sun (around 5 billion years). And since HXMM01 is 11 billion light years away, this merger happened during a time when our galaxy was only about 3 billion years old

“Largely thanks to the launch of long-wavelength satellites, especially NASA's Spitzer and ESA's Herschel, our quest over the last ten years has been to try to map these processes as we look back in time,” said Dr Mattia Vaccari, study co-author and SKA Postdoctoral Fellow at UWC's Astrophysics Group. “Because light takes time to reach us, observing galaxies far away in space allows us to look into the past. So we can use sensitive instruments to probe distant galaxies and see the various stages of their formation.”

These findings may help explain a persistent astronomy mystery – why examining the distant universe seems to show that when the universe was young, it was already fairly heavily populated with large reddish elliptical-shaped galaxies. For years, scientists have debated how these elliptical galaxies formed from smaller spiral galaxies (like the Milky Way). One hypothesis has it that spiral galaxies slowly grew into ellipticals by slowly absorbing many other galaxies with low star formation rates. Another holds that powerful collisions between galaxies led to increased star formation rates.

This work provides powerful support for the merger/collision idea: it seems that when it comes to star formation, the big galaxies of the early days of the universe grew up very quickly. And the biggest galaxies may be the result of smash-ups rather than slow accumulation