A team of researchers at Northwestern University and elsewhere has discovered that — contrary to previously standard lore — up to 50% of the matter in our Galaxy may come from distant galaxies.
According to Angles-Alcazar et al, up to 50% of matter in the Milky Way Galaxy may have an extragalactic origin. Image credit: NASA / JPL.
After the Big Bang about 13.7 billion years ago, the Universe was filled with a uniform gas — no stars, no galaxies.
But there were tiny perturbations in the gas, and these started to grow by force of gravity, eventually forming stars and galaxies. After galaxies formed, each had its own identity.
Using supercomputer simulations, Northwestern University astrophysicist Dr. Daniel Anglés-Alcázar and co-authors found a major and unexpected new mode for how galaxies, including our own Milky Way, acquired their matter — intergalactic transfer.
The simulations show that supernova explosions eject copious amounts of gas from galaxies, which causes atoms to be transported from one galaxy to another via powerful galactic winds.
“Given how much of the matter out of which we formed may have come from other galaxies, we could consider ourselves space travelers or extragalactic immigrants,” Dr. Anglés-Alcázar said.
“It is likely that much of the Milky Way’s matter was in other galaxies before it was kicked out by a powerful wind, traveled across intergalactic space and eventually found its new home in the Milky Way.”
Galaxies are far apart from each other, so even though galactic winds propagate at several hundred km per second, this process occurred over several billion years.
“This study transforms our understanding of how galaxies formed from the Big Bang,” said co-author Dr. Claude-André Faucher-Giguère, also of Northwestern University.
“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.”
By tracking in detail the complex flows of matter in the simulations, the scientists found that gas flows from smaller galaxies to larger galaxies, such as the Milky Way, where the gas forms stars.
This transfer of mass through galactic winds can account for up to 50% of matter in the larger galaxies.
“In our simulations, we were able to trace the origins of stars in Milky Way-like galaxies and determine if the star formed from matter endemic to the galaxy itself or if it formed instead from gas previously contained in another galaxy,” Dr. Anglés-Alcázar said.
The findings, published in the Monthly Notices of the Royal Astronomical Society (arXiv.org preprint), open a new line of research in understanding galaxy formation.
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Daniel Anglés-Alcázar et al. 2017. The cosmic baryon cycle and galaxy mass assembly in the FIRE simulations. Mon Not R Astron Soc 470 (4): 4698-4719; doi: 10.1093/mnras/stx1517
