Galactic ‘superwinds’ fling heavy metals into deep space

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Spiral galaxies such as the Milky Way are shown in the centre, surrounded by the circumgalactic medium (black). But the circumgalactic medium contains very hot gas, shown in red, orange and white that outweighs the central galaxies.

Winds howling from galaxies propel heavy elements produced by star formation up to a million light-years away, a new study shows.

Researchers in the US and Europe, led by the University of Colorado, Boulder’s Benjamin Oppenheimer, showed there was more oxygen, carbon and iron in the haloes surrounding galaxies than within the galaxies themselves, carried by “galactic superwinds” that travel at more than 1,500 kilometres per second.

This elemental “wastefulness” divested galaxies of ingredients to build more stars and planets, Oppenheimer says: “As it turns out, galaxies aren’t very good at recycling.”

Galaxies don’t exist in a vacuum. They’re usually surrounded by a very thin gassy cloud called the circumgalactic medium, which is thought to shuttle elements in and out of the galaxy.

The circumgalactic medium has a massive reach, too. While a galaxy such as our Milky Way is around 100,000 light-years across, the circumgalactic medium can extend a million light-years.

To see exactly what’s in the circumgalactic medium, Oppenheimer and colleagues used an instrument on the Hubble Space Telescope called the cosmic origin spectrograph, and performed simulations.

It didn’t matter if they modelled a spiral galaxy, such the Milky Way, or an elliptical galaxy – the circumgalactic medium surrounding them was rife with heavy elements, some containing more than half the heavy elements in the galaxies themselves.

And sweeping those huge amounts of heavy elements a million light-years into space required a strong breeze – at least 1,500 kilometres per second, they calculated – generated by a galaxy’s central supermassive black hole and exploding stars.

“This is a violent and long-lasting process that can take over 10 billion years, which means that in a galaxy like the Milky Way, this […] oxygen we’re observing has been there since before the Sun was born,” Oppenheimer says.

The work was published in the Monthly Notices of the Royal Astronomical Society.

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