Mysterious radio waves the size of several galaxies could be galactic winds from stars that exploded billions of years ago, according to new research.
The winds, which can travel at up to 2,000km per second, were only discovered in 2019 and have so far remained a mystery.
But a team of American astrophysicists now believe the waves are shells formed by galactic winds – possibly from massive exploding stars known as supernovae.
Computer simulations demonstrating galactic winds blowing over 750 million years offer an explanation for the radio circles which have left scientists perplexed up to this point.
Back in 2019, the newly-completed Australian Square Kilometer Array Pathfinder (ASKAP) telescope picked up something no one had ever seen before: radio wave circles so large they contained entire galaxies at their centres.
When they observed these radio rings, a team of researchers from the University of California San Diego studying ‘starburst’ galaxies believed they could provide the answers as to their origin.
In their study, published in the journal Nature, researchers delved into massive starburst galaxies that can drive these ultra-fast outflowing winds.
Starburst galaxies have an exceptionally high rate of star formation.
When stars die and explode, they expel gas from the star and its surroundings back into interstellar space.
If enough stars explode near each other at around the same time, the force of these explosions can push the gas out of the galaxy itself into outflowing winds – which can travel at up to 2,000km per second.
Astrophysicist and lead study author Dr Alison Coil described the galaxies as very interesting.
New technology allowed ASKAP to scan large portions of the sky at very faint limits which made odd radio circles detectable for the first time in 2019.
These circles were enormous: hundreds of kiloparsecs across, where a kiloparsec is equal to 3,260 light-years.
For comparison, our galaxy, the Milky Way, is around 30 kiloparsecs across.
Since the radio circles were detected, a multitude of theories has swirled to explain their origin, from planetary nebulae – an emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives – to black hole mergers.
But radio data alone could not determine if any of these theories was correct.
Dr Coil and her colleagues thought it possible the radio rings were a development from the later stages of the starburst galaxies they’d been studying, and began to look into radio circle 4 – the first one discovered, which is can be seen from the Northern Hemisphere.
Until 2019, odd radio circles had previously only been observed through their radio emissions, without any optical data.
Dr Coil’s team, however, used an integral field spectrograph at the W.M. Keck Observatory in Maunakea, Hawaii, to look at radio circle 4 which revealed a tremendous amount of highly luminous, heated, compressed gas – far more than is seen in the average galaxy.
Using optical and infrared imaging data, the researchers determined that the stars inside circle 4 galaxy were around six billion years old.
‘There was a burst of star formation in this galaxy, but it ended roughly a billion years ago,’ Dr Coil said.
Dr Cassandra Lochhaas, a postdoctoral fellow at the Harvard & Smithsonian Center for Astrophysics and a co-author on the study, ran a series of computer simulations to replicate the size and properties of the large-scale radio ring, including the large amount of shocked, cool gas in the central galaxy.
Dr Lochhas’ simulations demonstrated outflowing galactic winds blowing for 200 million years before they shut off.
When the galactic wind stopped, a forward-moving shock continued to propel high-temperature gas out of the galaxy and created a radio ring, whilst a reverse shock sent cooler gas falling back onto the galaxy.
The simulation played out over 750 million years — within the ballpark of the estimated one-billion-year stellar age of ORC 4.
‘To make this work you need a high-mass outflow rate, meaning it’s ejecting a lot of material very quickly,’ Dr Coil said.
‘And the surrounding gas just outside the galaxy has to be low density, otherwise the shock stalls. These are the two key factors.
‘It turns out the galaxies we’ve been studying have these high-mass outflow rates. They’re rare, but they do exist.
‘I really do think this points to odd radio circles originating from some kind of outflowing galactic winds.’
Dr Coil added that these outflowing winds could not only help astronomers understand odd radio circles, but could also help astronomers understand the outflowing winds themselves.
‘Odd radio circles provide a way for us to ‘see’ the winds through radio data and spectroscopy,” she said.
‘This can help us determine how common these extreme outflowing galactic winds are and what the wind life cycle is.
‘They can also help us learn more about galactic evolution: do all massive galaxies go through an odd radio circle phase?
‘Do spiral galaxies turn elliptical when they are no longer forming stars? I think there is a lot we can learn about and from these odd radio circles.’
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