Feb 24, 2021 9:00 AM

Twinkling Black Holes Reveal an Invisible Cloud in Our Galaxy

Cosmic radio backlights are helping scientists size up “missing” forms of matter and might offer clues about what makes up the universe.

At first, Yuanming Wang was not excited. More relieved, maybe. The first -year astrophysics PhD student at the University of Sydney sat in front of her computer, looking at images in which she’d found the signs of radio waves from distant galaxies twinkling, just as she had hoped. But because Wang’s discovery relied more on scouring ones and zeros than peering through a telescope—and the discovery itself was just plain weird—it took awhile for the moment to hit.

Radio wave “twinkling,” known as scintillation, happens when the radio signals from sources like stars and black holes get interrupted as they stream toward Earth. Detecting scintillation from distant galaxies is very rare. Only a tiny fraction of the night sky will yield these signals, and most telescopes are unable to capture variations on such fast timescales. But Wang found hallmarks of scintillation from six galaxies at once. And five appeared to be separate galaxies in a tight, straight line. "I realized, OK, so this is the first detection of such an unusual shape in the sky," Wang says. “That is definitely the first time, and it will imply we found some invisible thing. So at that moment I started to understand this exciting result.”

What excited Wang’s team wasn’t actually the radio waves, each beaming in from billions of light-years away. It’s what was blocking them, making the signals momentarily flicker. They believe it’s an invisible cloud of cold gas, right in our galactic backyard, and that clouds like it may answer a long-standing puzzle about why more than half of the Milky Way’s matter is AWOL.

Cosmological simulations predict that about 95 percent of all matter is dark energy and dark matter, leaving only 5 percent for the baryonic matter made of protons and neutrons. But half of the total baryonic weight has been unaccounted for. Astronomers have gathered compelling evidence that the space between galaxies holds missing baryons in diffuse strands of gas. The problem is that it’s hard to find sparse or cold clouds in a dark sky.

“In astronomy, we're essentially limited to observing things that emit light,” says Tara Murphy, a radio astronomer at the University of Sydney who led the study. “So stars, galaxies, warm dust—we see because they emit thermal radiation and then we observe that. In doing that, we can add up how much we think the galaxy weighs.”

Our universe is so skilled at hiding matter that researchers must concoct new ways of detecting otherwise invisible cosmic features. Wang and Murphy’s team chanced upon a surprising tool: black holes. In a study set to publish in the April issue of Monthly Notices of the Royal Astronomical Society, they report how they used supermassive black holes as backlights to discover a long, slender gas cloud in the Milky Way.

Instead of reading visible light, the team collected radio waves, which reach telescopes unobstructed by Earth’s atmosphere. The five radio-bright spots Wang found stacked beside each other were active galactic nuclei, distant galaxies containing supermassive black holes that gush out radio waves. Radio energy coming from each black hole traveled billions of light-years to Earth, only to be tripped up in the last 10 light-years inside the Milky Way by what Murphy's team concludes is a mysterious cloud of cold gas. “We can only see them because we're using these distant galaxies as backlights that are shining through the cloud,” says Murphy. And since the flickering signals appear in a tight, coherent line, that suggests that they were being obstructed by a single cloud in our galaxy. “There's no other way of seeing these clouds of gas,” says Murphy. “They are completely invisible.”

Illustration: Yuanming Wang/University of Sydney

“It’s a neat trick to play,” says Jillian Scudder, an astrophysicist from Oberlin College not involved in the new study, of the team’s methodology. The data Wang used come from the Australian Square Kilometre Array Pathfinder (ASKAP), a radio telescope that scans skies fast enough to detect brief changes in distant radio emissions. “This kind of time-variability astronomy is going to become more prevalent," Scudder says, because of new radio telescopes like ASKAP.

Radio telescopes have been a boon for astronomers looking to tease out missing baryons hiding as diffuse masses in space. (Murphy says astronomers have an old joke about finding “odd socks”—a sock lost deep in space wouldn’t emit light or be heavy enough to affect anything around it. “So in a sense, that's what we're doing. We're trying to find those odd socks to explain those missing baryons," she says.) Unlike their optical cousins, radio telescopes don’t limit a scientist to seeing only hot things that produce visual light. “When we look out with a radio telescope, we're seeing a sort of secret universe—a different view,” says Murphy. “What we're seeing is those places where electrons are accelerated to relativistic speeds in really extreme places, like where black holes are forming.”

Just last year, researchers from the US and Australia reported thefirst measurements of missing matter between galaxies using ASKAP. This warm-hot intergalactic medium, or WHIM, measurement is the strongest direct evidence that warm-hot gas accounts for a huge chunk of missing cosmic baryons. The researchers “weighed” the universe by analyzing enormous blasts of radio waves from super distant galaxies called fast radio bursts. As the fast radio bursts sailed billions of light-years through conventionally undetectable matter, the waves themselves changed in ways they physically wouldn’t if traveling through a void. That effect on the radio bursts implied a quantifiable “weight” of matter encountered on the long journey, which jibed with cosmological predictions—suggesting intergalactic space contains plenty of sparse, invisible gas clouds.

The WHIM result found cosmic matter between galaxies, but that’s just one part of the missing baryon puzzle. “Amusingly, there’s a second one, which is on galaxy scales,” says J. Xavier Prochaska, a UC Santa Cruz scientist who measured the WHIM with fast radio bursts for last year’s study. (Prochaska was not affiliated with the new work.) Within the Milky Way’s disk and surrounding halo, scientists expect hundreds of billions of suns’ worth of baryonic mass—and only about 20 percent of that shows up. “That's the Missing Baryon Problem 2: Where are all the baryons of this galactic system?” says Prochaska. “We continue to debate whether that material is there and we haven't seen it, or if in the making of a galaxy most of it’s been ejected.” In other words, is it still there but too dim to find, or did it blow away after the galaxy formed?

Video: Yuanming Wang/University of Sydney

The work that led to Wang spotting an invisible cloud launched serendipitously with hand-me-down data from one of Murphy’s previous experiments peering through one of Earth’s most discerning eyes. Located deep in the Western Australia desert, ASKAP’s 36 dishes measure 12 meters across and can take quick, detailed snapshots of the sky. (Researchers recently detected 3 million galaxies—1 million of which were new—in 300 hours.) Since detection happens so fast, astronomers can chop a 10-hour night’s radio bounty into high-res 15-minute increments. This lets astronomers observe events that unfold very rapidly. Analyzing how those snapshots evolve, Murphy thought, could uncover galactic radio emissions varying on extraordinarily short timescales.

“The reason that rapid variability is interesting is because it's usually a sign of something extreme going on,” says Murphy. Detecting extreme events can mean spotting hidden supernovae or catching nearby stars releasing flares so large that they wipe out any potential for life on planets in their orbit. This fast variability is hard to observe, however, since it requires a radio source to be far away (small in our field of view) and for whatever is obstructing it to be large and close to home.

In 2019, Murphy worked on an unrelated investigation of the radio wave aftermath of the merging of two neutron stars. The team used ASKAP to scan a tract of the cosmos nine and 33 days after the merger. But after that analysis ended, the data remained a treasure trove for further analysis of variations in the night sky. “We got like 30,000 galaxies—30,000 radio sources—in that field. So I had to deal with lots of data,” Wang says.

Wang wanted to find the most capricious radio signals in the sky. She wrote a script to weed out the data from stagnant radio blips they didn't care about, but was still left with thousands of radio sources that appeared to be varying. The vast majority had uninteresting explanations, or were artifacts of the detection process. Still, Wang scrutinized each one. “So I just click, click, click, click for several days,” says Wang, “and eventually,I found it.”

Of the 30,000 distant galaxies, only six were actually scintillating rapidly. “Of those six, five were in a dead straight line,” says Murphy. “When you discover something like that, you think there's something strange going on here.”

To Wang and Murphy, something strange also meant that there might be something wrong. Their team had to confirm that their result wasn’t just some weird one-off. They reimaged the sky from a different angle so the interesting feature appeared elsewhere, ruling out unreliable pixels. But in the end, they couldn’t blame it on telescope misbehavior. “So then you're left with the idea that this must be something astronomical,” says Murphy. “It must be real.”

Encouraged, Wang and Murphy collected more snapshots of the scintillating signals over 11 months—seven nights of observation in all. That timespan let them tease out the size and shape of what they believe to be an interfering gas cloud, as the backlights shifted in relation to Earth, the first example of such an approach. Their results show that the filament of gas is thin and about a third of one light-year long—20,000 times longer than Earth’s distance from the Sun.

How did this weird cloud form? Murphy's team can't know for sure, but they think a star’s immense gravity shredded a gas cloud into these proportions. Black holes are known to create these gas streams, but none are nearby. “So rather than a black hole,” Murphy says, “we have some kind of plasma cloud that's been disrupted by a star and stretched it out so that we have this long tidal stream.”

One aspect of the cloud stumped Murphy’s team. She says only warm charged gas, plasma, could cause the twinkling. But based on her team’s models, they think the cloud could form its shape only by moving fast—about 30 kilometers per second—and that means that a larger portion of it would actually be very cold. So cold, in fact, that hydrogen droplets inside could freeze like snowflakes.

Françoise Combes, a Collège de France astrophysicist not involved with the work, is sold on the team’s find. In fact, Combes’ own work two decades ago hypothesized that not only do cold clouds exist but also that they make up a large portion of the Milky Way’s missing baryons. She thinks this cloud is likely just the small tip of a much larger fractal cloud structure throughout the galactic disk. “Scintillations are the signature of the existence of this hierarchy of molecular cloud scales,” she wrote in an email to WIRED. “There is plenty of space to have a large fraction of dark baryons under the form of cold molecular clouds.”

“It's an intriguing data set that I think we've all been waiting for many years,” says Ue-Li Pen, a theoretical astrophysicist who studies scintillation at the University of Toronto and was not involved in the work. But Pen is not convinced by the hydrogen snow cloud interpretation. Though it’s possible that cold hydrogen clouds exist in galaxies, he says, their abundance is a contentious subject. Previous papers, Pen notes, have surmised the clouds may account for some of what we consider dark matter in the universe, too. As a counter to the snow cloud theory, Pen has an alternate idea. He thinks that an edge of magnetized interstellar gas could stretch and scintillate radio waves, like the distortion in light reflecting off a lake.

Murphy acknowledges that the snow cloud hypothesis is unproven. “This is just one hypothesis, and there may be others that fit our data,” she says. “Getting more data is key.”

Prochaska feels that Murphy and Wang’s measurements are convincing evidence that cold clouds can exist and account for missing baryons. He says this Milky Way–centered work is “very complimentary” to his work that instead focuses on the space between galaxies. “I love nontraditional approaches to attacking fundamental type questions,” he says. “It’s a really cool technique.”

The technique reinforces that with new ways of finding matter, we’re sure to find new ways that matter exists in the galaxy. “There are a lot of places where gas can hide in a galaxy,” says Scudder. “I think it's not going to be one answer for the missing baryon problem. I think it's going to be a contribution of a lot of things that are all a little bit more important than we thought they were."

The new method doesn’t allow for “weighing” baryons like Prochaska did last year, but astronomers hope radio telescopes like ASKAP will continue illuminating otherwise invisible cosmic behavior. Other researchers doing similar work think the find is a great example of why radio astronomy is getting more attention. “This is surprising. It's new. And I think it could also broaden my own perspectives when I interpret my data,” says Sthabile Kolwa, an astronomer at the University of Johannesburg, South Africa, who was not involved in the study. Kolwa says the report of a rare filament and cold hydrogen cloud is fascinating. “We expect it to be there, but it just hasn't been seen,” Kolwa says.

As she continues her hunt for more weird cases of scintillation, Wang says her reflexive skepticism will fade faster. “Next time,” she jokes, “I will definitely be very excited.”