Cosmic traffic accident in the early universe
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Every now and then, galaxies in the universe crash into each other. Some of those cosmic traffic accidents have been studied in much detail by telescopes on the ground and in space. Most of them are tens or hundreds of millions of light-years away.
Now, ALMA has seen a similar galactic smash-up at the edge of the observable universe, at 12.7 billion light-years away. Despite this enormous distance, ALMA was able to see that there are indeed two galaxies involved. They are some 30,000 light-years apart from each other – about a quarter of the diameter of our Milky Way galaxy – close enough to call it a cosmic crash.
At an earlier stage, the two galaxies must have passed each other at an even closer range. Thanks to their mutual gravity, shock waves were generated in the gas that fills both galaxies. From these shock waves, many new stars started to form. In fact, ALMA found that the birth rate of new stars in the two galaxies is about a thousand times higher than in our own Milky Way.
Apart from gas, the two galaxies also contain large amounts of dust. The dust absorbs the bright radiation from the newborn stars. As a result, the dust becomes warm and starts to glow at infrared wavelengths. This infrared radiation arrives at Earth as millimeter and submillimeter radiation, because the waves are stretched by the expansion of the universe.
The discovery is special, because the galaxies are so far away. Their light has taken 12.7 billion years to reach Earth. That means that ALMA sees the galaxies as they were 12.7 billion years ago, when the universe was just over one billion years old. No one had expected to find such massive star-forming galaxies so early in the history of the universe.
Within a few hundred million years, the two galaxies will have merged into one large elliptical galaxy. At an even later stage, it may well end up as the core of a large cluster of galaxies.
What?
A number of years ago, the European Herschel Space Observatory discovered a faint speck of infrared light in the sky. However, Herschel couldn’t tell how far away it was. The APEX telescope – a small forerunner of ALMA – was able to measure its distance: 12.7 billion light-years. Now, ALMA has studied the remote object in detail. Thanks to its ultra-sharp vision, ALMA could see that it’s actually two galaxies close to each other, on the verge of a crashing into each other. Moreover, it turned out that both galaxies are producing new stars at an extremely high rate. The galaxy pair is known as ADFS-27.
Who?
The ALMA observations of ADFS-27 were carried out by a large international team of astronomers., led by Dominik Riechers. Dominik is an astronomer at Cornell University in Ithaca, New York. He and his colleagues have published the new result in a professional astronomy magazine, called The Astronomical Journal.
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