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Star factory found in
faraway galaxy

KEITH COOPER
ASTRONOMY NOW
Posted: November 11, 2009


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A young galaxy that existed just over a billion years after the big bang has been found to be making stars at the furious rate of about fifty per year, showing that star formation and galaxy growth was a much quicker process in the distant past than it is today.

The galaxy, with its large star-forming regions, was made visible to astronomers at Durham University thanks to a gravitational lens seen by the Hubble and Spitzer space telescopes and the Gemini North Telescope on Mauna Kea, Hawaii. A gravitational lens is created when the gravity of a massive object – in this case a large cluster of galaxies called MS 1358+62 – bends and magnifies the light of even more distant galaxies behind it. This allowed the smeared and warped image of a galaxy that existed 12.5 billion years ago to be seen. Bright knots of light can be seen in the lensed image, which are individual regions of star formation 650 light years across.

The lensed galaxy is seen as the red arc just below and to the left of the large elliptical galaxy in the centre that is part of the lensing cluster MS 1358+62. The lensing has created two images of the galaxy, with one mirror-imaged, so that you can see the pattern of bright knots (the star forming regions) are reversed. Image: Dr Johan Richard, Durham University.

“It has all the hallmarks of being a normal galaxy from that early time,” says lead scientist Dr Mark Swinbank. “Looking at the individual star-forming regions, we see that stars are forming much more rapidly than they do in the present day.”

His team measured an average star-formation rate of 42 stars per year in this young galaxy, which is only 6,000 light years across, and this is an order of magnitude greater than the average star formation rate in our own Galaxy, the Milky Way.

These new results, the most detailed ever made of a galaxy this far back in time, suggest that star formation was much more efficient in the distant past. “The Universe was much denser in these early times,” Swinbank tells Astronomy Now. “There was more gas around to form stars, and the rate at which gas cooled and formed stars was much higher.”

The standard models of galaxy evolution show that they grow either by gas falling onto a galaxy and forming stars, or by mergers with other galaxies. Swinbank admits it is not possible to tell from the observations which of the two is driving the star formation, but the results add to evidence that massive galaxies formed much quicker than had been previously believed. This galaxy has a mass of 700 million Suns, and had it been forming stars constantly at the observed rate it would have reached this size within just 17 million years. However, galaxies tend to form stars in bursts with quieter lulls in between, says Swinbank, so the actual age of the galaxy is probably older, perhaps hundreds of millions of years. It also shows how our own Galaxy may have been born. “It is the size that we would expect the Milky Way to have been when it was ten percent its current age,” says Swinbank.

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