Nearly half our bodies may have been made in outer space

Nearly half of the atoms that make up our bodies may have formed of 'extra-galactic matter' from beyond the Milky Way and travelled to the solar system on intergalactic winds driven by giant exploding stars, astronomers claim.

The dramatic conclusion emerges from computer simulations that change the earlier understandings of how galaxies are formed. The simulations reveal how galaxies grow over aeons by absorbing huge amounts of material that is blasted out of neighbouring galaxies when stars called supernovas explode at the end of their lives.

Powerful supernova explosions can fling trillions of tonnes of atoms into space with such ferocity that they escape their home galaxy's gravitational pull and fall towards larger neighbours in enormous clouds that travel at hundreds of kilometres per second.

Astronomers have long known that elements forged in stars can travel from one galaxy to another, but the latest research is the first to reveal that up to half of the material in the Milky Way and similar-sized galaxies can arrive from smaller galactic neighbours.

Much of the hydrogen and helium that falls into galaxies forms new stars, while heavier elements, themselves created in stars and dispersed in the violent detonations, become the raw material for building comets and asteroids, planets and life.

Daniel Anglés-Alcázar, an astrophysics postdoctoral fellow from Northwestern University who led the study, said, "Given how much of the matter out of which we formed may have come from other galaxies, we could consider ourselves space travellers or extra-galactic immigrants.

"It is likely that much of the Milky Way's matter was in other galaxies before it was kicked out by a powerful wind, travelled across intergalactic space and eventually found its new home in the Milky Way.''

He added, "What this new mode implies is that up to one-half of the atoms around us – including in the solar system, on Earth and in each one of us – comes not from our own galaxy but from other galaxies, up to one million light years away.''

The findings open a new line of research in understanding galaxy formation, the researchers say, and the prediction of intergalactic transfer can now be tested.

''Our origins are much less local than we previously thought'', said Faucher-Giguère. ''This study gives us a sense of how things around us are connected to distant objects in the sky.''

The researchers ran supercomputer simulations to watch what happened as galaxies evolved over billions of years. They noticed that as stars exploded in smaller galaxies, the blasts ejected clouds of elements that fell into neighbouring, larger galaxies. The Milky Way absorbs about one sun's-worth of extragalactic material every year.

''The surprising thing is that galactic winds contribute significantly more material than we thought,'' said Anglés-Alcázar. ''In terms of research in galaxy evolution, we're very excited about these results. It's a new mode of galaxy growth we've not considered before.''

The simulations showed that elements carried on intergalactic winds could travel a million light years before settling in a new galaxy, according to a report in the Monthly Notices of the Royal Astronomical Society.

Claude-André Faucher-Giguère, another astronomer on the team, said that before their simulations, galaxies were thought to grow primarily by absorbing material left over from the Big Bang. ''What we did not anticipate, and what's the big surprise, is that about half of the atoms that end up in Milky Way-like galaxies come from other galaxies,'' he said. ''It gives us a sense of how we can come from very far corners of the universe.''