Largest merger of galaxies discovered
点击量： 时间：2018-01-10 02:02:05
By Maggie McKee (Image: NASA/JPL-Caltech/K Rines/CfA) (Image: NASA/ESA/Hubble Heritage Team/STScI/AURA/Rines et al.) Four massive galaxies are colliding in the largest galactic merger ever seen, new observations reveal. The smash-up is shedding light on how the biggest galaxies in the universe form – and why many of them stopped giving birth to stars billions of years ago. Astronomers classify mergers according to the relative sizes of the galaxies involved. Minor mergers unite galaxies of vastly different size – marrying a ‘dwarf’ galaxy with one the size of the Milky Way, for example – while major mergers join those of roughly equal size. Now, researchers led by Kenneth Rines of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US, have found the largest major merger ever seen. It involves a quartet of galaxies at the centre of a galactic cluster known as CL0958+4702, which lies about 5 billion light years from Earth. Three of the merging galaxies are the size of the Milky Way, while the other is about three times as massive. “This is the largest major merger in terms of total stellar mass,” Rines told New Scientist, adding that the next runner-up is the merger of two Milky Way-sized bodies known as the Antenna Galaxies (see image below right). Using infrared observations by the Spitzer Space Telescope and optical images from the WIYN Observatory in Arizona, US, the team also discovered a colossal, fan-shaped ‘plume’ of old, red stars trailing about 360,000 light years from the merger, apparently tossed out of the galaxies as they spiralled towards each other. “That’s the other fairly amazing thing – the number of stars in the plume is about three Milky Ways’ worth,” says Rines. Eventually, about half of those stars will likely fall into the merged galaxies – which are expected to coalesce into a single mammoth galaxy in about 100 million years – while the other half will float freely outside it, he says. That suggests other free-floating stars previously found within galaxy clusters were also ejected from their birth galaxies during major mergers, says Rines: “There’s been speculation about where this ‘intracluster light’ comes from – did the stars form in between the galaxies? That seems pretty unlikely.” But the find sheds light on an even more fundamental mystery, he says: “This answers the question of how you form the most massive galaxies in the universe.” That is because the most massive galaxies yet observed – weighing about 10 times the Milky Way – are found at the centres of nearby galaxy clusters, which are seen as they are now, 13.7 billion years after the big bang. The four newly discovered merging galaxies lie 5 billion light years away, meaning astronomers see them as they were 5 billion years ago. “If we could see this cluster as it is today, that merger would be long over,” Rines explains, adding that it would resemble the behemoths observed at the heart of nearby clusters. Interestingly, the four merging galaxies are made of old, red stars, suggesting each had lost the gas necessary to form new stars long beforehand, within 5 billion years of the big bang. That agrees with other recent observations showing that galaxies within clusters – which typically contain hundreds of galaxies – contain fewer young stars than those lying outside clusters. “There’s a decades-long debate over why that is,” says Rines. “The two camps are nature vs nurture.” In the nature scenario, galaxies grew at different rates in the early universe depending on the density of dark matter in their birthplace. “The idea would be that galaxy formation happened sooner in dense environments that eventually formed clusters than it did in underdense environments,” he says. That means the galaxies in crowded clusters simply formed, merged and exhausted their gas supplies earlier than those in smaller groups, such as the Milky Way, or voids of relatively empty space. In the nurture scenario, the galaxy’s surrounding environment would shape its star formation rate. Thus, interactions with other galaxies strip or heat the gas, preventing it from forming stars, says Rines. “Nature vs nurture can be described as, ‘Does the gas use itself up in forming stars or does something happen to it from other galaxies?