The mystery of fast radio bursts (FRBs) just got a little bit weirder.
A team of astronomers has just traced a burst called FRB 20190208A back to the distant galaxy that spat it out – and they found a tiny, faint dwarf galaxy that seems to be more than halfway across the observable Universe.
That's a really unusual place to find one of these mysterious signals, and it confirms they are way more complex than we currently understand.
"The majority of fast radio burst host galaxies seem to be massive, star-forming galaxies – perhaps implying that most FRBs are produced by magnetars formed from core collapse supernovae," astronomer Danté Hewitt of the University of Amsterdam told ScienceAlert.
"However, the faintness of the FRB 20190208A host galaxy implies that it's one of the least massive FRB host galaxies we've ever seen – so that was definitely surprising!"
Fast radio bursts are an intriguing cosmic conundrum. They are huge spikes of radio waves that appear in radio telescope observations, lasting just milliseconds, but discharging in that time as much energy as 500 million Suns.
Most of them flare just once, randomly, making them impossible to predict, and very difficult to trace back to a source. Some, however, are repeat offenders, continuing to spit out signals, sometimes randomly, sometimes in a timed pattern. These are usually a bit easier to trace, because astronomers can watch for them and study them more closely.
We don't currently have a good grasp on what makes them. There's a growing body of evidence that erupting magnetars are the culprit for at least some of them; but the different ways FRBs can present indicates that we don't have the whole story. Looking at where they come from is one way to fill in some of the gaps.
This brings us to FRB 20190208A, a repeating burst first detected in February 2019. Hewitt and his colleagues used radio telescopes to observe the location of the burst for a total of 65.6 hours. In that time, between February 2021 and August 2023, they caught the source bursting twice more.
This information allowed them to pinpoint its location in the sky. They then used optical telescopes to take deep sky observations to see what sort of galaxy might be lurking there.
"Our initial attempts at identifying a host galaxy revealed no source at the FRB position. We were a little baffled," Hewitt said.
"There are a few possible explanations in such a case, but seemingly 'hostless' FRBs are quite rare (since most FRB sources appear to be in massive galaxies). But then, when we saw the images from the Gran Telescopio Canarias, there was a very exciting 'Oh wow! Look! There's actually a faint smudge right where the bursts are coming from' moment."
Dwarf galaxies are difficult to see, which is only exacerbated by distance. Because this one is so faint, the researchers were unable to derive a confident distance measurement; but, by looking at the way the radio light of the FRB dispersed as it traveled through space, it could be a light travel time of some 7 billion years.
That would make FRB 20190208A one of the most distant FRBs ever detected, which is very nifty. But it's the identity of the tiny galaxy that really sparks the imagination.
"This host galaxy is most likely 10-100 times fainter than the vast majority of other FRB host galaxies, perhaps on par with the Magellanic Clouds," Hewitt said.
"Naturally, dwarf galaxies such as these do not house a lot of the stars in the Universe. So finding an FRB in such a galaxy may indicate that there are environmental conditions (e.g. the metallicity: whether the gas is pure hydrogen or not) that are conducive to the production of (some) FRB sources."
So far, only a small number of FRBs have been localized. What's interesting is that more repeating FRBs have been traced to dwarf galaxies than non-repeating FRBs. This could be an observation bias; but it could alternatively mean that the conditions in dwarf galaxies are somehow more conducive to the production of repeating FRBs.
Dwarf galaxies, Hewitt explained, are known to host some of the most massive stars in the Universe due to their low metallicity. When these stars die, they don't go quietly, blowing up the sky in core-collapse supernovae. Those collapsed cores then go on to become highly magnetized neutron stars, or magnetars.
"Finding repeating FRB sources in dwarf galaxies thus potentially links these repeating FRB sources with massive star progenitors," Hewitt said. "It's a little poetic. When the most massive stars die, they unleash some of the most energetic explosions in the Universe; and then maybe, the remnants of those explosions continue to scream into the void, repeatedly producing FRBs."
We're not quite yet at the point where we can solve this fascinating puzzle. But we're getting closer. It's discoveries like these that take us, step by painstaking step, towards a full understanding of what is behind these wild, huge explosions in the sky.
"It's also a little bit of a cautionary tale for the future," Hewitt told ScienceAlert.
"The story of FRB 20190208A tells us that in order to robustly associate an FRB with a host galaxy, we will sometimes need both a very precise position from radio arrays, as well as very deep imaging using the largest optical telescopes we currently have at our disposal. That is simply not something that can be done yet for thousands of sources."
The team's research has been published in The Astrophysical Journal Letters.