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Half of the atoms making up everything around you are intergalactic interlopers. Large galaxies like our Milky Way amassed half their matter from neighbouring star clusters up to a million light years away, according to a new simulation.

“We did not realise how much of the mass in today’s Milky Way-like galaxies was actually ‘stolen’ from the winds of other galaxies,” says corresponding author at Northwestern University in Illinois

The theft occurs after a death. When some stars reach the end of their life cycle, they become massive supernovae, spewing high-speed gas out into the universe. The matter in these ejections is picked up by galactic winds, streams of charged particles powered by the exploding supernovae.

It was previously thought that galactic winds couldn’t be the source of much intergalactic matter transfer because they weren’t powerful enough to cross the vast distances that separate neighbouring galaxies. Turns out, they’re stronger than we thought.

“We assumed that the winds were confined to the galaxies they came from – that they could recycle by falling back onto the galaxy that ejected them, but not transfer much mass from one galaxy to another,” says Faucher-Giguère.

Over a galaxy’s lifetime, it will swap matter continuously with its neighbours and the journey between one galaxy and another could take anywhere from several hundred million to 2 billion years, he says.

Across the universe

Using 3D models of galaxy evolution, Faucher-Giguère and his co-author Daniel Anglés-Alcázar simulated the path matter inside galaxies would have taken through the universe from the big bang through to today. More accurate simulations of supernovae revealed that the galactic winds were moving matter faster than previously thought.

They found that in galaxies with 100 billion stars or more, the galactic winds actually ferried in about 50 per cent of the matter present today.

“Galactic winds as a mode of transfer has been underappreciated,” says at the University of Washington in Seattle. “Daniel Anglés-Alcázar uses one of the best simulations to do a detailed particle tracking analysis and really laid it all out for us.”

Faucher-Giguère and Anglés-Alcázar found that for larger galaxies like our own, this intergalactic Gulf Stream is the primary contributor to their growth, allowing them to snatch away matter from their smaller counterparts.

The intergalactic transfer of matter is less crucial for the growth of smaller galaxies, which rely more on local galactic winds to keep any matter that might be ejected from supernovae within their system. Faucher-Giguère thinks the Milky Way receives its matter from the nearby small and large Magellanic clouds, two dwarf galaxies between 160,000 and 200,000 light years away.

Werk says tracking the flow of matter from the origin of the universe to present day, and understanding where the atoms that make up the air we breathe and water we drink is one of the fundamental problems in astrophysics. “It’s one of the holy grails of extra galactic cosmology,” she says. “Now, we’ve found that half these atoms come from outside our galaxy.”

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