Most galaxies age slowly. It typically takes a couple billion years for them to run out of gas and the other raw materials they need to keep growing and making stars. Now, astronomers examining a few rebel galaxies that live fast and die young have discovered that their star-making gas was somehow suddenly ejected out to space—causing them to redden and age prematurely.
Using the Westerbork Synthesis Radio Telescope in the Netherlands, an international team led by Ivy Wong from the University of Western Australia studied four young galaxies that appear to be making the transition from blue and active to red and dead in less than a billion years. These blue galaxies lie between 275 million and 375 million light-years away from us, and all four of them are right on the verge of switching off their star formation.
These rapidly reddening galaxies, they found, had expelled most of their reservoir of atomic hydrogen gas. However, the cause of this sudden gas depletion is still unclear.
"One possibility is that it could be blown out by the galaxy's supermassive black hole," Wong says in an International Centre for Radio Astronomy Research (ICRAR) release. "Another possibility is that the gas could be ripped out by a neighboring galaxy." But that seems less likely, she adds: "The galaxies in the pilot project are all isolated and don't appear to have others nearby."
In the image of galaxy J0836 above, the expelled gas cloud lies along a path of material radiating strongly in radio wavelengths, Science reports, which suggests that the gas reservoir was expelled at a time when the black hole at the center of the parent galaxy was much more active than it is at the moment.
Until now, many astronomers didn’t think dying galaxies would have any gas left to see. "We selected four galaxies right at the time where this gas ejection should be occurring," says study co-author Kevin Schawinski from the Institute for Astronomy in Switzerland. "It was amazing to see that this is exactly what happens!" And that means there will be larger surveys in the future to better understand this sort of sudden stellar shutdown.
Their findings were published in the Monthly Notices of the Royal Astronomical Society this week. (Also available at arXiv.)