Hydrogen gas is the lifeblood of galaxies, which use it to make new stars and stay young. While many processes stop star formation, an international team of astronomers has discovered that one of them might have been underestimated.
In a new study, published in the Monthly Notices of the Royal Astronomical Society, researchers have discovered that 11,000 galaxies are experiencing ram-pressure stripping that's strangling them. As these objects fall towards the center of the galaxy clusters they inhabit, the hot intergalactic medium acts as a wind, removing the galaxy's cool gas reservoir.
“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,” PhD candidate Toby Brown, from the International Centre for Radio Astronomy Research and Swinburne University, said in a statement. “It 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.”
Galaxies are surrounded by large “halos” of dark matter, which are about five times the mass of the visible stuff in those galaxies. These halos stretch and extend across galaxy clusters and their huge gravitational forces change how galaxies move and interact.
“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,” said Mr Brown. “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. You can think of it like a giant cosmic broom that comes through and physically sweeps the gas from the galaxies.”
This hot “wind” of plasma was supposed to be effective in killing off star formation only in clusters with very large halos, but the new research shows that more modest sizes have a sizable effect too.
“This paper demonstrates that the same process is operating in much smaller groups of just a few galaxies together with much less dark matter,” said Mr Brown. “Most galaxies in the Universe live in these groups of between two and a hundred galaxies.”
Ram-pressure stripping is an incredibly fast process on the cosmic scale, which can bring “death” to galaxies in just a few tens of millions of years. This could help explain some of the mysteries still surrounding galaxy evolution.