Massive galaxies found in early Universe Read time: 3 minutes If you’ve just created your own little school garden, you don’t expect to find a large tree growing there within a week. Likewise, astronomers didn’t expect large galaxies to exist already at a time when the Universe was very young. Still, that’s exactly what they found, using the large sensitivity and the sharp vision of ALMA. The Universe was born some 13.8 billion years ago as an expanding ‘sea’ of very hot gas. Gravity created clumps in the gas, from which the small ‘building blocks’ of galaxies grew. Later, these small clumps collided and merged, slowly growing into ever-larger galaxies. ALMA has now discovered two galaxies that were already pretty large when the Universe was only 780 million years old – twenty times younger than it is now. Moreover, the two galaxies are about to merge into a much larger one. Apparently, massive galaxies grew much faster than everyone had expected. The two galaxies were first discovered by the South Pole Telescope at Antarctica, as a single, extremely faint speck of infrared light. ALMA studied the faint object in much more detail. It turned out that there were actually two objects, very close together. They can hardly be seen because they are extremely distant. So far away, in fact, that their light took about 13 billion years to reach Earth. That’s why they are seen as they appeared 13 billion years ago, when the Universe was still very young. The two galaxies are producing new stars at an incredibly high rate. The largest of the two also contains large amounts of dust, according to the ALMA observations. Since dust can only form when stars are dying, there must have been an even earlier generation of stars. The two early galaxies are less than 25,000 light-years apart. They are bound to merge into an even larger galaxy. The astronomers also discovered a huge halo (or cloud) of mysterious dark matter surrounding the two galaxies. Studying such early objects will help scientists to better understand the very early evolution of the Universe and the formation of the first galaxies. What? The two galaxies that ALMA discovered are so far away that their light took 13 billion years to reach us. The larger of the two contains large amounts of gas (270 billion times the mass of the Sun) and of dust (3 billion solar masses). New stars are produced in this galaxy at a rate of a few thousand per year. In the smaller galaxy, a few hundred new stars are born each year. This galaxy contains about 35 billion solar masses in the form of stars. The two galaxies, known as SPT0311-58, are some 25,000 light-years apart and will eventually merge into a single massive galaxy. Who? The two massive galaxies in the early Universe were discovered by a large international team of astronomers. The team was led by Dan Marrone. Dan is an associate professor of astronomy at the University of Arizona in Tucson. The team published their discovery in Nature, a professional weekly magazine of science. ALMA URL