Galaxy smashups dominated early universe, study finds
and World Science staff
A new study has found that the biggest galaxies in the universe formed through multiple, grand smashups early in the history of the universe.
The discovery could help explain seemingly unrelated processes such as how massive black holes and stars came to be, according to the researchers, Christopher J. Conselice of The University of Nottingham, U.K. and colleagues.
The team used the deepest images taken by the Hubble Space Telescope to study galaxies when they were two billion years old. At that time, the universe is believed to have been about one-seventh its current age. Astronomers can look back at the universe as it was a long time ago simply by looking further out, because the light from distant places takes so long to reach us.
Peering into that early period, the astronomers found a swath of what they said were spectacular galaxy mergers. These mergers would lead to the creation of new stars from colliding gas clouds, the researchers said. The events would also probably feed and increase the size of black holes lurking in the centre of all galaxies, they added.
The work is helping to confirm what scientists have long suspected, Conselice said: massive galaxies form when smaller galaxies merge. “The most massive galaxies we see in today’s universe, which are passive and old, were once undergoing rapid mergers with each other,” he explained.
While distant galaxies have been studied for over a decade, it has been a mystery how they evolved into the galaxies we see today. Young galaxies have very low masses and astronomers have long been puzzled by how these systems turn into massive galaxies.
Conselice said a typical massive galaxy in today’s universe has undergone four to five mergers.
The mergers are rare today, with only about one per cent of galaxies merging, but 10 billion years ago, nearly all massive galaxies were undergoing mergers, he added. Moreover, almost all of the collisions occurred from the birth of the universe to about six billion years ago. The universe is estimated to be somewhere around 14 billion years old.
Why it stopped is a puzzle, he said. “All this merging activity was somehow curtailed by an unknown process.”
The findings are published in the February 20th edition of the Astrophysical Journal.
The research may hold clues about the formation of our own galaxy, Conselice said. The Milky Way contains spiral arms, which aren’t thought to form through mergers. But at the centre of our galaxy is a huge, dense ball of stars called a bulge, featuring many old stars and a massive black hole. That probably formed through mergers, he said.
The research could also help astronomers to see into the Milky Way’s future, he added. Our galaxy will itself merge with Andromeda, our nearest neighbouring large galaxy, in about a billion years. This would see the destruction of the spiral disk and dramatically reshape our galaxy, significantly changing the positions of stars we see in the night sky, Conselice said.
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