Mysterious dark matter may be killing galaxies

Halos of dark matter may be stripping away gas from galaxies and sending them to an early death by depriving them of the material to make new stars, say scientists who claim to have solved the galactic murder mystery. Researchers based at the International Centre for Radio Astronomy Research (ICRAR) in Australia found that a phenomenon called ram-pressure stripping is more prevalent than previously thought.

The study of 11,000 galaxies shows their gas - the lifeblood for star formation - is being violently stripped away on a widespread scale throughout the local Universe. "The image we paint as astronomers is that galaxies are embedded in clouds of dark matter that we call dark matter halos," said Toby Brown, PhD candidate at ICRAR and Swinburne University of Technology in Australia.

Dark matter is the mysterious material, that despite being invisible accounts for roughly 27 per cent of our Universe, while ordinary matter makes up just five per cent. The remaining 68 per cent is dark energy. "During their lifetimes, galaxies can inhabit halos of different sizes, ranging from masses typical of our own Milky Way to halos thousands of times more massive," Brown said.

"As galaxies fall through these larger halos, the superheated intergalactic plasma between them removes their gas in a fast-acting process called ram-pressure stripping," he said. "You can think of it like a giant cosmic broom that comes through and physically sweeps the gas from the galaxies," he added.

Brown said removing the gas from galaxies leaves them unable to form new stars. "It dictates the life of the galaxy because the existing stars will cool off and grow old," he said. "If you remove the fuel for star formation then you effectively kill the galaxy and turn it into a dead object," he added.

ICRAR researcher Barbara Catinella, said that astronomers already knew ram-pressure stripping affected galaxies in clusters, which are the most massive halos found in the Universe. "This paper demonstrates that the same process is operating in much smaller groups of just a few galaxies together with much less dark matter," Brown said.

"Most galaxies in the Universe live in these groups of between two and a hundred galaxies," he said. "We've found this removal of gas by stripping is potentially the dominant way galaxies are quenched by their surrounds, meaning their gas is removed and star formation shuts down," he added. The study was published in the journal Monthly Notices of the Royal Astronomical Society.