Astronomers made use of a fast radio burst, a 40-microsecond flash at radio wavelengths, to evaluate the state of the nearly undetectable gas around an intervening galaxy.

There’s half of our galaxy that we barely know is there — a hot halo of gas that’s only just detectable. Now astronomers have found another way to study the nearly invisible.

Detecting the (Nearly) Undetectable

Milky Way Galaxy
A giant halo surrounds our Milky Way Galaxy, seen here from a hypothetical outsider's viewpoint.
Steven Simpson / S&T

The mass of the hot, gaseous halo that surrounds most massive galaxies, including the Milky Way, is on par with the mass of all the stars in the galaxy itself. In fact, the halo might hold the very gas that will form future stellar generations.

Yet it largely evades detection. A galaxy’s halo gas is extremely hot — “cooler” clouds of some 10,000 kelvin (17,500°F) float within a hotter, million-degree atmosphere. This atmosphere is also incredibly thin, with only a couple hundred atoms within the space of a child’s balloon. So typically, the only way to see all this gas is indirectly, by the way it absorbs the light of background sources such as quasars.

But quasars aren’t the only sources of radiation that can penetrate the cosmos. Like quasars, mysterious and ephemeral fast radio bursts can also travel billions of light-years, carrying the imprints of the cosmic expanse they traverse when they at last arrive at Earth.

Fast radio burst as probe of a galaxy halo
Short bursts of radio waves from FRB 181112, each less than 40 microseconds long, passed through an intervening galaxy's halo, probing its structure.
ESO / M. Kornmesser

Astronomers reported in the September 27th Science what happened when a 40-microsecond radio flash passed through the halo of an intervening galaxy. The results are surprising: The radio waves seem almost entirely undisturbed, indicating a calmer halo than astronomers would have thought.

One Mystery Probes Another

Fast radio bursts offer a new (albeit, still indirect) way to probe halos. Radio waves travel through the vacuum of space at the speed of light. But when they pass through gas, such as the hot gas in a galaxy’s halo, they slow down, and longer wavelengths slow down more than shorter wavelengths. The shorter wavelengths arrive at Earth first; the delay in the arrival of the longer wavelengths is known as the dispersion measure. Astronomers can use the dispersion measure to gauge how much gas the radio waves have passed through.

Xavier Prochaska (University of California) and colleagues probed the halo of a galaxy using the fast radio burst FRB 181112, detected using the Australian Square Kilometer Array Pathfinder (ASKAP). ASKAP immediately pinpointed the source to a specific location on the sky, and astronomers determined the galaxy most likely to host the source.

What the source is remains unclear. In one scenario, highly magnetized neutron stars generate radio-wave flashes, but the jury is still out on whether this scenario holds for fast radio bursts in general.

ASKAP in Western Australia
The ASKAP radio telescope in Western Australia has the ability to pinpoint the sources of ephemeral fast radio bursts.

Regardless what causes these bursts, the end result is that a powerful packet of radio waves has traversed billions of light-years on its way toward Earth. Along the way, according to the precise location mapped out by ASKAP, this packet passed within 95,000 light-years of a foreground galaxy. The intervening galaxy has old stars and a central, supermassive black hole that’s still munching on a midnight snack.

The FRB passed close enough to the galaxy to have passed through its halo, but whatever medium it passed through barely made a dent on the radio signal. If there is a halo around this galaxy, it’s calm and thin. Even the cooler clouds floating in the hotter plasma appear to be few and far between. In other words, the halo is surprisingly boring compared to the turbulent, magnetized plasma astronomers were expecting.

Keep Calm and Carry On

Perseus Cluster
The black hole at the center of an elliptical galaxy in the nearby Perseus Cluster is pushing out cavities that appear like distorted bubbles in the surrounding X-ray-emitting gas. The black hole at the center of the intervening galaxy might have cleared similar cavities around its host.
NASA / CXC / SAO / E. Bulbul & others / XMM / ESA

It’s possible that the once-actively guzzling black hole at the center of this galaxy pushed out jets of material that evacuated the inner halo. It’s also possible that the gas that galaxies swim in is simply more serene than expected.

“Our research appears to reveal something entirely new about galactic halos,” Pocharsky says. “Unless, of course, this galaxy happens to be just some weird exception—and with only one object you can’t be sure about that.”

The team plans to follow up on other fast radio bursts to test this scenario in other galaxies. "I take this as a neat demonstration of what we’ll be able to do with a much larger sample," says Vikram Ravi (Caltech), who was not involved in the study.

Ultimately, though, astronomers may have to turn to other wavelengths, such as X-rays, to directly detect and even map out galaxy halos.


Image of Anthony Barreiro

Anthony Barreiro

October 5, 2019 at 5:00 pm

I've got three questions:

How do you measure the temperature of gas that is barely there? You can't stick a thermometer in it. Do you measure its black body radiation? If it's emitting x-rays or gamma rays it must be really hot?

Why do shorter wavelength radio waves travel through the gas faster than longer wavelengths? Does this have anything to do with refraction?

How can anything be surprisingly boring? 😉

By the way, even though I don't understand what the video is demonstrating, if there are awards for astronomical visualization (the Astvies?), it deserves one. Very exciting!

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Monica Young

January 16, 2020 at 4:47 pm

Hi Anthony, it's true we don't have any thermometers in space to measure this gas ;). If there is blackbody radiation, then yes, you can measure the temperature that way. However, I think in the case of the halo gas, we mostly know it's there because of specific wavelengths of light emitted by some highly ionized atoms, such as highly ionized oxygen. The properties of these spectral lines can be used to gauge the temperature. It's an indirect measurement for sure, though.

The explanation of how ionized gas (i.e., plasma) affects radio waves is a little complex. The short answer is that no, it's not due to refraction, it's because the radio waves are interacting with the electrons in the ionized gas. You can find a more in-depth description here:

I agree, the video is fantastic!

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October 7, 2019 at 9:52 am

Hi Monica,
A question about Black Hole rotation. Is rotational energy lost by a Black Hole having to spin up it accretion disk & having to do so for billions of years. I know some energy will be recovered by the angular momentum of mass passing through the event horizon & "impacting" on the Black Hole, but that energy conserved has to be less than the accretion spin up & consequent heating energy expended?

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Monica Young

January 16, 2020 at 4:38 pm

Really good you say, when matter is accreted onto/into the black hole, it spins up the black hole. But in order for matter to go into the black hole, angular momentum has to be preserved. So what ends up happening is that the accretion disk helps power an outflow, either a slower "wind" or a fast (relativeistically fast) jet. Whichever way material flows outward, it ends up carrying some angular momentum with it, enough to conserve the total angular momentum of the system. Does that help answer your question?

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January 6, 2020 at 12:05 pm

I liked this post and all but my question is: Are scientists coming up with new ways to get information about these hot halos of gas? If so would you make an article on the new way?

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Monica Young

January 16, 2020 at 4:47 pm

They're certainly trying! It's a fascinating field of research that we'll continue to report on.

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