Dense stellar cluster may show how massive galaxies get their start
NASA, ESA and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration
Even the mightiest galaxies start life as a small seed. Now researchers think they’ve identified a sprouting seed of a giant elliptical galaxy, churning out new stars just 3 billion years after the Big Bang. The discovery could help astronomers understand how the most massive galaxies in the universe are built.
Giant elliptical galaxies are big and boring. These nearly featureless behemoths can hold trillions of ancient stars. Because these silent titans have been largely dormant for billions of years astronomers had thought that elliptical galaxies were relics from an earlier time.
But researchers don’t see this type of galaxy when they peer farther out into space — and hence further back in time. Instead, they see dead galaxies that are massive but much more compact. Astronomers suspect that giant ellipticals descended from these distant compact cousins, growing by cannibalizing small galaxies that passed too close. Confirming this idea is difficult because, outside of computer simulations, astronomers can’t watch a galaxy grow. All they have are snapshots from discrete moments in cosmic history.
Now Erica Nelson, an astrophysicist at Yale University, and colleagues have identified what they think is the seed of an elliptical galaxy in the constellation Ursa Major — a massive compact galaxy that is still growing, churning out about 90 suns a year. The Milky Way, by comparison, makes only a couple of new stars annually. The researchers calculated the rate at which the galaxy is generating stars based on its color, as measured by three space telescopes.
In less than 3 billion years, the tiny galaxy, named GOODS-N-774 , had already accumulated about 150 billion suns’ worth of mass — roughly as much as the Milky Way but squished into a ball just 7 percent as wide. Most likely, the researchers argue, the galaxy built itself up in well under a billion years and is already slowing down.
The mass and size of GOODS-N-774 is similar to the dead compact galaxies astronomers see at later times, which in turn look a lot like the dense hearts of giant ellipticals that exist today. Nelson and colleagues concludein a paper posted online June 15 at arXiv.org and to appear in Nature that GOODS-N-774 is probably an ancestor of the dead cores and, eventually, today’s giants.
“I think it’s a very impressive result,” says Guillermo Barro, an astrophysicist at the University of California, Santa Cruz. “We were expecting something like this, but it’s great to have confirmation.” What’s unique about this discovery, he says, is that the researchers were able to directly weigh the compact galaxy. Until now, astronomers have had to infer the mass of similar galaxies by adding up a galaxy’s light and then doing some calculations. Nelson’s team instead measured the speed at which gas whipped around inside GOODS-N-774. The speed of the gas reveals the mass of the galaxy.
The next question, Barro says, is how such galaxy cores form. To build so many suns so quickly, these cores need to squish a lot of gas and stars into a relatively tiny volume. One possible way, he says, is to smash together smaller galaxies, though GOODS-N-774 doesn’t show any hints of carnage from a collision. Alternatively, a younger, less-dense core might suck down more gas than its star-building factories can handle, which could make the galaxy unstable and cause it to crumple.
E. Nelson et al. A massive galaxy in its core formation phase three billion years after the Big Bang. arXiv:1406.3350. Posted June 15, 2014. Nature, in press, 2014.
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