What is Killing Off the Universes Galaxies?

Across the Universe, as we speak (well, millions of years ago, but whatever) galaxies are being killed.

So this raises the question a team of global researchers are searching for the answer to – and they just might have it. What is killing off the universe’s galaxies? Short answer: gas-stripping.

Based out of the International Centre for Radio Astronomy Research (ICRAR) in Western Australia, the researchers have revealed phenomenon called ram-pressure stripping is more prevalent than previously thought. Ram-pressure drives gas from galaxies, sending them to an early death by depriving them of the material to make new stars.

The study of 11,000 galaxies shows their gas is being violently stripped away on a widespread scale throughout the local Universe.

Toby Brown, leader of the study and PhD candidate at ICRAR and Swinburne University of Technology, said the image we paint as astronomers is that galaxies are embedded in clouds of dark matter that we call dark matter halos. 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 5 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,” Mr 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”.

Mr Brown said describes it as like a giant cosmic broom that comes through and physically sweeps the gas from the galaxies. This dictates the life of the galaxy because 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,” Mr Brown said.

ICRAR researcher Dr Barbara Catinella, co-author of the study, said 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.

“Most galaxies in the Universe live in these groups of between two and a hundred galaxies,” said Dr Catinella, “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.”

Mr Brown said the other main process by which galaxies run out of gas and die is 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,” he 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.”

The study combined 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. [Royal Astronomical Society]


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