Enigmatic deep space radio signals could unlock the secret to the Milky Way’s weirdest habit

Our galaxy is tossing bits of itself out into space, and a burst of radio waves from another galaxy 163 million light years away recently helped astronomers prove it.

The enigmatic radio waves are called fast radio bursts (FRBs) and they’re exactly what they sound like: short, intense blast of radio waves from a distant object. Astronomers have observed hundreds over the last few years, but traced fewer than two dozen back to their origins. But recently, astronomers have pinpointed the sources of another 30 FRBs, which may help us understand what causes them — and how we can use them to explore things much closer to home.

Caltech astronomer Vikram Ravi and his colleagues presented their results at the 241^st meeting of the American Astronomical Society, and in a paper submitted to The Astrophysical Journal.

What’s New — Around 163 million light years from Earth, something dramatic happened in a barred spiral galaxy called IRAS 02044+7048. Whatever it was sent FRBs rippling out into space. On their way to radio antennae in California, those radio waves passed through a halo of gas that surrounds the Milky Way. All that gas dispersed the radio waves, like headlights shining through fog. By measuring exactly how much the radio waves got dispersed, Ravi and his colleagues figured out how much gas is floating unseen around the Milky Way.

And it turns out that some of that mass was missing.

In most of the universe, matter as we know it makes up about 16 percent of the total matter mass — the rest is invisible, mysterious stuff called dark matter, which physicists can detect only by how its gravity affects the visible matter around it. Unlike normal matter, dark matter doesn’t interact with light or radio waves. So when the burst of radio waves from IRAS 02044+7048 scattered as they passed through the Milky Way’s gassy halo, that was entirely thanks to what physicists call baryons: the ordinary kind of matter that makes up the gases, liquids, and solids we can actually perceive and interact with.

That was helpful because much of the material around our galaxy is made of ordinary baryons, but it’s usually invisible because it doesn’t glow, and it doesn’t reflect light from another source. The scattering radio waves helped shed some light on that otherwise invisible gas, and helped Ravi and his colleagues figure out how much of the mass around our galaxy is gas and how much is dark matter.

Surprisingly, baryons made up only about 10 percent of the mass in the Milky Way’s halo. Somehow, it seems that our galaxy is getting rid of material, probably by flinging it completely out of the galactic gravity well.

Why It Matters — The missing matter is actually a key piece of evidence for a theory that astrophysicists have been working on for years: that galaxies regularly toss matter out into space. We think of galaxies as pretty stable structures, and we probably picture a galaxy’s formation as a process of gathering material together, not tossing it away. But it’s apparently more complicated than that.

“This is fundamental to galaxy formation,” says Ravi in a recent press conference. “Matter is funneled in and blown out of galaxies in cycles.”

In other words, vomiting mass out into the void is a normal part of being a galaxy; matter gets tossed out of galaxies thanks to powerful winds from newborn stars, the explosive deaths of dying giant stars (supernovae), and the messy eating habits of the supermassive black holes still growing at the centers of some galaxies. And thanks to radio waves scattering in otherwise invisible gas, we can now measure the results of that process.

Here’s The Background — Ravi and his colleagues wouldn’t have been able to measure how much the radio waves were dispersed on their way through our galaxy’s halo of matter without knowing how far those waves had traveled, and how much material they had to travel through. And figuring that out required about 66 radio antennae, some very advanced software, and a lot of incredibly precise math.

FRBs were first discovered in 2007 as short, intense bursts of radio waves, and have detected hundreds more since. Figuring out where FRBs come from has been a challenge; astronomers have only managed to trace 21 out of those hundreds back to known galaxies. Until recently, that is.

Using a radio telescope called the Deep Synoptic Array — dozens of 4.5-meter radio antennae lined up in the California desert — Ravi and his colleagues have managed to trace 30 more Fast Radio Bursts back to their host galaxies since February 2022. This batch includes the one that helped them measure the Milky Way’s missing matter.

Tracking FRBs back to their starting points may eventually help physicists understand exactly what causes these short, loud cosmic phenomena.

So far, the primary suspect is a type of recently deceased star called a magnetar. Astronomers at several observatories caught one of these highly-magnetized dead stars red-handed emitting an FRB back in 2020, so we know beyond a reasonable doubt that magnetars are definitely one cause of FRB. But Ravi and his colleagues say their 30 newly-identified sources raise a question about whether magnetars are the only cause.

“Magnetars like those in the Milky Way are formed during episodes of intense star formation,” says Ravi. But several of the galaxies that produced DSA’s 30 FRBs are aging galaxies, long past their star-forming years. “To find fast radio bursts from galaxies that have mostly stopped forming stars was surprsing,” says Ravi.

What’s Next — At the moment, 63 radio antennae are scanning the skies at DSA, but by the end of the year the total will be up to 110. The telescope scans the whole sky in a continuous loop — one small chunk at a time. But each of those small chunks contains about 10 million galaxies, so pinpointing which of the 10 million produced a given Fast Radio Burst is no small feat.

“To pinpoint the origins, you need to resolve it by 1 part in 10 million,” says Ravi. “You have to line up the signals from all the different antennas to roughly 10 picoseconds.” In other words, Ravi and his colleagues have to measure extremely tiny differences, about 1/1000th of a degree, in the angle at which the radio waves hit each antenna in the array.

And that takes a lot of processing power: “The data rate is equivalent to watching 28,000 Netflix movies all at once.”

The DSA’s eventual goal is to observe 300 FRBs over the next three years.