大质量星系能吞食附近小星系增加质量

Massive galaxies¹ in the Universe have stopped making their own stars and are instead snacking on nearby galaxies, according to research by Australian scientists. Astronomers² looked at more than 22,000 galaxies and found that while smaller galaxies are very efficient at creating stars from gas, the most massive galaxies are much less efficient at star formation, producing hardly any new stars themselves, and instead grow by eating other galaxies.   Dr Aaron Robotham, who is based at the University of Western Australia node of the International Centre for Radio Astronomy Research (ICRAR), said smaller 'dwarf³' galaxies were being eaten by their larger counterparts.   "All galaxies start off small and grow by collecting gas and quite efficiently⁴ turning it into stars," he said.   "Then every now and then they get completely cannibalised by some much larger galaxy⁵."   The study was released today in the journal Monthly Notices of the Royal Astronomical⁶ Society, which is published by Oxford⁷ University Press.   Dr Robotham, who led the research, said our own Milky⁸ Way is at a tipping point and is expected to now grow mainly by eating smaller galaxies, rather than by collecting gas.   "The Milky Way hasn't merged⁹ with another large galaxy for a long time but you can still see remnants of all the old galaxies we've cannibalised," he said.   "We're also going to eat two nearby dwarf galaxies, the Large and Small Magellanic Clouds, in about four billion years."   But Dr Robotham said the Milky Way is eventually going to get its comeuppance when it merges¹⁰ with the nearby Andromeda Galaxy in about five billion years.   "Technically¹¹, Andromeda will eat us because it's the more massive one," he said.