What's killing our galaxies?

Across the universe, galaxies are being killed, and scientists want to know whodunnit. Now, astronomers based at the International Centre for Radio Astronomy Research (ICRAR) have laid their hands on the culprit — a cosmic phenomenon called ram-pressure stripping.

As explained by Toby Brown, leader of the new study and a PhD candidate at ICRAR and Swinburne University of Technology, galaxies are embedded in clouds of dark matter called dark matter halos. “During their lifetimes,” he said, “galaxies can inhabit halos of different sizes, ranging from masses typical of our own Milky Way to halos thousands of times more massive.

“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,” Brown continued.

“You can think of it like a giant cosmic broom that comes through and physically sweeps the gas from the galaxies.”

Brown said that removing gas from galaxies leaves them unable to form new stars, while the existing stars will cool off and grow old. “If you remove the fuel for star formation then you effectively kill the galaxy and turn it into a dead object,” he explained.

Astronomers already knew ram-pressure stripping affected galaxies in clusters, which are the most massive halos found in the universe. But the new study has revealed that “the same process is operating in much smaller groups of just a few galaxies together with much less dark matter”, according to Brown.

The researchers came to this conclusion through the use of an innovative technique combining the largest optical galaxy survey ever completed — the Sloan Digital Sky Survey — with the largest set of radio observations for atomic gas in galaxies — the Arecibo Legacy Fast ALFA survey. The results of their work have been published in the Monthly Notices of the Royal Astronomical Society.

An artist’s impression of ram-pressure stripping of galaxy NGC 4921, sending it to an early death. Image credits: ICRAR, NASA, ESA, the Hubble Heritage Team (STScI/AURA).

Having now observed the same cause of death in 11,000 galaxies, Brown believes that stripping is “potentially the dominant way galaxies are quenched by their surrounds”. This is in contrast to the other main process by which galaxies die, known as strangulation.

“Strangulation occurs when the gas is consumed to make stars faster than it’s being replenished, so the galaxy starves to death,” Brown said.

“It’s a slow-acting process. On the contrary, what ram-pressure stripping does is bop the galaxy on the head and remove its gas very quickly — of the order of tens of millions of years — and astronomically speaking that’s very fast.”

Top image credit: ©iStockphoto.com/Kurt Paris