ALMA sees galaxy merger in distant universe
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People grow. You are taller now than when you were five years old. Galaxies, like our own Milky Way, also grow. Right now, the Milky Way galaxy is larger than it was many billions of years ago. But galaxies do not grow on their own, like people. They only can get bigger and fatter by gobbling up other galaxies.
Long ago, when the Universe was just one or two billion years old, there were only small galaxies. But thanks to their mutual gravity, these small galaxies sometimes attracted one another. The result: they collided and merged into a larger galaxy. That’s how the growth of big galaxies like the Milky Way started.
The merging of two galaxies is a violent process, just like the collision of two cars on the highway. The gas in the galaxies is stirred up. Shockwaves arise, gas clouds are compressed, and new stars are born. So if you find a galaxy with a baby boom of new stars, it’s probably the result of a recent merger.
Using ALMA, astronomers have now found one of the best examples of a galaxy merger in the distant universe. Because the galaxy is so far away, its light took more than 12.5 billion years to reach us. Thant means that we are seeing the galaxy as it was 12.5 billion years ago, when the universe was in its infancy.
Seen through an optical telescope, the galaxy is hardly visible. The normal light of the newborn stars is absorbed by thick clouds of dust. But the dust is heated by the stars, and starts to glow at other wavelengths. ALMA can detect this glow, and astronomers can then deduce the star formation rate in the galaxy. It turns out to be a thousand times higher than the star formation rate in our own Milky Way!
The new ALMA observations support the idea that small galaxies in the early universe collided and merged to form larger ones. ALMA also found a few other galaxies close to the merging one. Together, they may start to form a small galaxy cluster.
The galaxy that ALMA has studied is called AzTEC-3. It was discovered years ago with the James Clerk Maxwell Telescope at Mauna Kea, Hawaii, using a camera called AzTEC – hence the name. It is over 12.5 billion light years away, in the small constellation Sextant (the constellation is named after an old astronomical instrument). ALMA provided astronomers with the first detailed views of AzTEC-3.
The discovery of the high star formation rate of AzTEC-3 was made by Dominik Riechers, an astronomer at Cornell University in Ithaca, New York. Dominik worked together with ten other astronomers, from the United States, Germany and Croatia. They published the result of their work in a professional magazine called The Astrophysical Journal.