astrophysics

Simulations find ghostly whirls of dark matter trailing galaxy arms

astronomy, astrophysics, Dark Matter, galaxies, Science, simulations / Shannon Garcia / June 7, 2025

“Basically what you do is you set up a bunch of particles that represent things like stars, gas, and dark matter, and you let them evolve for millions of years,” Bernet says. “Human lives are much too short to witness this happening in real time. We need simulations to help us see more than the present, which is like a single snapshot of the Universe.”

Several other groups already had galaxy simulations they were using to do other science, so the team asked one to see their data. When they found the dark matter imprint they were looking for, they checked for it in another group’s simulation. They found it again, and then in a third simulation as well.

The dark matter spirals are much less pronounced than their stellar counterparts, but the team noted a distinct imprint on the motions of dark matter particles in the simulations. The dark spiral arms lag behind the stellar arms, forming a sort of unseen shadow.

These findings add a new layer of complexity to our understanding of how galaxies evolve, suggesting that dark matter is more than a passive, invisible scaffolding holding galaxies together. Instead, it appears to react to the gravity from stars in galaxies’ spiral arms in a way that may even influence star formation or galactic rotation over cosmic timescales. It could also explain the relatively newfound excess mass along a nearby spiral arm in the Milky Way.

The fact that they saw the same effect in differently structured simulations suggests that these dark matter spirals may be common in galaxies like the Milky Way. But tracking them down in the real Universe may be tricky.

Bernet says scientists could measure dark matter in the Milky Way’s disk. “We can currently measure the density of dark matter close to us with a huge precision,” he says. “If we can extend these measurements to the entire disk with enough precision, spiral patterns should emerge if they exist.”

“I think these results are very important because it changes our expectations for where to search for dark matter signals in galaxies,” Brooks says. “I could imagine that this result might influence our expectation for how dense dark matter is near the solar neighborhood and could influence expectations for lab experiments that are trying to directly detect dark matter.” That’s a goal scientists have been chasing for nearly 100 years.

Ashley writes about space for a contractor for NASA’s Goddard Space Flight Center by day and freelances in her free time. She holds master’s degrees in space studies from the University of North Dakota and science writing from Johns Hopkins University. She writes most of her articles with a baby on her lap.

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Milky Way galaxy might not collide with Andromeda after all

Andromeda, astronomy, astrophysics, cosmology, galactic mergers, galaxies, milky way, Science / 9u50fv / June 3, 2025

100,000 computer simulations reveal Milky Way’s fate—and it might not be what we thought.

It’s been textbook knowledge for over a century that our Milky Way galaxy is doomed to collide with another large spiral galaxy, Andromeda, in the next 5 billion years and merge into one even bigger galaxy. But a fresh analysis published in the journal Nature Astronomy is casting that longstanding narrative in a more uncertain light. The authors conclude that the likelihood of this collision and merger is closer to the odds of a coin flip, with a roughly 50 percent probability that the two galaxies will avoid such an event during the next 10 billion years.

Both the Milky Way and the Andromeda galaxies (M31) are part of what’s known as the Local Group (LG), which also hosts other smaller galaxies (some not yet discovered) as well as dark matter (per the prevailing standard cosmological model). Both already have remnants of past mergers and interactions with other galaxies, according to the authors.

“Predicting future mergers requires knowledge about the present coordinates, velocities, and masses of the systems partaking in the interaction,” the authors wrote. That involves not just the gravitational force between them but also dynamical friction. It’s the latter that dominates when galaxies are headed toward a merger, since it causes galactic orbits to decay.

This latest analysis is the result of combining data from the Hubble Space Telescope and the European Space Agency’s (ESA) Gaia space telescope to perform 100,000 Monte Carlo computer simulations, taking into account not just the Milky Way and Andromeda but the full LG system. Those simulations yielded a very different prediction: There is approximately a 50/50 chance of the galaxies colliding within the next 10 billion years. There is still a 2 percent chance that they will collide in the next 4 to 5 billion years. “Based on the best available data, the fate of our galaxy is still completely open,” the authors concluded.

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Have we finally solved mystery of magnetic moon rocks?

asteroids, astrophysics, core dynamo, geology, magnetism, moon rocks, paleomagnetism, Science, the Moon / Mike M. / May 24, 2025

NASA’s Apollo missions brought back moon rock samples for scientists to study. We’ve learned a great deal over the ensuing decades, but one enduring mystery remains. Many of those lunar samples show signs of exposure to strong magnetic fields comparable to Earth’s, yet the Moon doesn’t have such a field today. So, how did the moon rocks get their magnetism?

There have been many attempts to explain this anomaly. The latest comes from MIT scientists, who argue in a new paper published in the journal Science Advances that a large asteroid impact briefly boosted the Moon’s early weak magnetic field—and that this spike is what is recorded in some lunar samples.

Evidence gleaned from orbiting spacecraft observations, as well as results announced earlier this year from China’s Chang’e 5 and Chang’e 6 missions, is largely consistent with the existence of at least a weak magnetic field on the early Moon. But where did this field come from? These usually form in planetary bodies as a result of a dynamo, in which molten metals in the core start to convect thanks to slowly dissipating heat. The problem is that the early Moon’s small core had a mantle that wasn’t much cooler than its core, so there would not have been significant convection to produce a sufficiently strong dynamo.

There have been proposed hypotheses as to how the Moon could have developed a core dynamo. For instance, a 2022 analysis suggested that in the first billion years, when the Moon was covered in molten rock, giant rocks formed as the magma cooled and solidified. Denser minerals sank to the core while lighter ones formed a crust.

Over time, the authors argued, a titanium layer crystallized just beneath the surface, and because it was denser than lighter minerals just beneath, that layer eventually broke into small blobs and sank through the mantle (gravitational overturn). The temperature difference between the cooler sinking rocks and the hotter core generated convection, creating intermittently strong magnetic fields—thus explaining why some rocks have that magnetic signature and others don’t.

Or perhaps there is no need for the presence of a dynamo-driven magnetic field at all. For instance, the authors of a 2021 study thought earlier analyses of lunar samples may have been altered during the process. They re-examined samples from the 1972 Apollo 16 mission using CO₂ lasers to heat them, thus avoiding any alteration of the magnetic carriers. They concluded that any magnetic signatures in those samples could be explained by the impact of meteorites or comets hitting the Moon.

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A star has been destroyed by a wandering supermassive black hole

astronomy, astrophysics, black holes, Science, spaghettification, supermassive black hole, tidal disruption / DJ Henderson / May 9, 2025

But note the phrasing there: “in most cases” and “eventually.” Even in the cases where a merger takes place, the process is slow, potentially taking millions or even billions of years. As a result, a large galaxy might have as many as 100 extremely large black holes wandering about, with about 10 of them having masses of over 10⁶ times that of the Sun. And the galaxy that AT2024tvd resides in is very large.

One consequence of all these black holes wandering about is that not all of them will end up merging. If two of them approach the central black hole at the same time, then it’s possible for gravitational interactions to eject the smallest of them at nearly the velocity needed to escape the galaxy entirely. As a result, for millions of years afterwards, these supermassive black holes may be found at quite a distance from the galaxy’s core.

At the moment, it’s not possible to tell which of these explanations account for AT2024tvd’s location. The galaxy it’s in doesn’t seem to have undergone a recent merger, but there is the potential for this to be a straggler from a far-earlier merger.

It’s notable that all of the galaxies where we’ve seen an off-center tidal disruption event are very large. The paper that describes AT2024tvd suggests this is no accident: larger galaxies mean more mergers in the past, and thus more supermassive black holes floating around the interior. They also suggest that off-center events will be the only ones we see in large galaxies. That’s because larger galaxies will have larger supermassive black holes at their center. And, once a supermassive black hole gets big enough, its event horizon is so far out that stars can pass through it before they get disrupted, and all the energetic release would take place inside the black hole.

Presumably, if you were close enough to see this happen, the star would just fade out of existence.

The arXiv. Abstract number: 2502.17661 (About the arXiv). To be published in The Astrophysical Journal Letters.

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3D map of exoplanet atmosphere shows wacky climate

astronomy, astrophysics, exoplanet atmospheres, exoplanets, Science, spectroscopy / Mike M. / February 19, 2025

Last year, astronomers discovered an unusual Earth-size exoplanet they believe has a hemisphere of molten lava, with its other hemisphere tidally locked in perpetual darkness. And at about the same time, a different group discovered a rare small, cold exoplanet with a massive outer companion 100 times the mass of Jupiter.

Meet Tylos

The different layers of the atmosphere on WASP-121b.

This latest research relied on observational data collected by the European South Observatory’s (ESO) Very Large Telescope, specifically, a spectroscopic instrument called ESPRESSO that can process light collected from the four largest VLT telescope units into one signal. The target exoplanet, WASP-121b—aka Tylos—is located in the Puppis constellation about 900 light-years from Earth. One year on Tylos is equivalent to just 30 hours on Earth, thanks to the exoplanet’s close proximity to its host star. Since one side is always facing the star, it is always scorching, while the exoplanet’s other side is significantly colder.

Those extreme temperature contrasts make it challenging to figure out how energy is distributed in the atmospheric system, and mapping out the 3D structure can help, particularly with determining the vertical circulation patterns that are not easily replicated in our current crop of global circulation models, per the authors. For their analysis, they combined archival ESPRESSO data collected on November 30, 2018, with new data collected on September 23, 2023. They focused on three distinct chemical signatures to probe the deep atmosphere (iron), mid-atmosphere (sodium), and shallow atmosphere (hydrogen).

“What we found was surprising: A jet stream rotates material around the planet’s equator, while a separate flow at lower levels of the atmosphere moves gas from the hot side to the cooler side. This kind of climate has never been seen before on any planet,” said Julia Victoria Seidel of the European Southern Observatory (ESO) in Chile, as well as the Observatoire de la Côte d’Azur in France. “This planet’s atmosphere behaves in ways that challenge our understanding of how weather works—not just on Earth, but on all planets. It feels like something out of science fiction.”

Nature, 2025. DOI: 10.1038/s41586-025-08664-1

Astronomy and Astrophysics, 2025. DOI: 10.1051/0004-6361/202452405 (About DOIs).

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Fast radio burst in long-dead galaxy puzzles astronomers

astronomy, astrophysics, fast radio bursts, magnetars, Science / DJ Henderson / January 22, 2025

A surprising source

FRBs are of particular interest because they can be used as probes to study the large-scale structure of the universe. That’s why Calvin Leung, a postdoc at the University of California, Berkeley, was so excited to crunch data from Canada’s CHIME instrument (Canadian Hydrogen Intensity Mapping Experiment). CHIME was built for other observations but is sensitive to many of the wavelengths that make up an FRB. Unlike most radio telescopes, which focus on small points in the sky, CHIME scans a huge area, allowing it to pick out FRBs even though they almost never happen in the same place twice.

Leung was able to combine data from several different telescopes to narrow down the likely position of a repeating FRB, first detected in February 2024, located in the constellation Ursa Minor. When he and his CHIME collaborators further refined the accuracy of the location by averaging many bursts from the FRB, they discovered that this FRB originated on the outskirts of a long-dead distant galaxy. That throws a wrench into the magnetar hypothesis because why would a dead galaxy in which no new stars are forming host a magnetar?

It’s the first time an FRB has been found in such a location, and it’s also the furthest away from its galaxy. CHIME currently has two online outrigger radio arrays in place—companion telescopes to the original CHIME radio array in British Columbia. A third array comes online this week in Northern California, and according to Leung, it should enable astronomers to pinpoint FRB sources much more accurately—including this one. Data has already been incorporated from an outrigger in West Virginia, confirming the published position with a 20-times improvement in precision.

“This result challenges existing theories that tie FRB origins to phenomena in star-forming galaxies,” said co-author Vishwangi Shah, a graduate student at McGill University. “The source could be in a globular cluster, a dense region of old, dead stars outside the galaxy. If confirmed, it would make FRB 20240209A only the second FRB linked to a globular cluster.”

V. Shah et al., Astrophysical Journal Letters, 2025. DOI: 10.3847/2041-8213/ad9ddc (About DOIs).

T. Eftekhari et al., Astrophysical Journal Letters, 2025. DOI: 10.3847/2041-8213/ad9de2 (About DOIs).

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Fast radio bursts originate near the surface of stars

astronomy, astrophysics, fast radio burst, magnetar, neutron star, Science / 9u50fv / January 4, 2025

One of the two papers published on Wednesday looks at the polarization of the photons in the burst itself, finding that the angle of polarization changes rapidly over the 2.5 milliseconds that FRB 20221022A lasted. The 130-degree rotation that occurred follows an S-shaped pattern, which has already been observed in about half of the pulsars we’ve observed—neutron stars that rotate rapidly and sweep a bright jet across the line of sight with Earth, typically multiple times each second.

The implication of this finding is that the source of the FRB is likely to also be on a compact, rapidly rotating object. Or at least this FRB. As of right now, this is the only FRB that we know displays this sort of behavior. While not all pulsars show this pattern of rotation, half of them do, and we’ve certainly observed enough FRBs we should have picked up others like this if they occurred at an appreciable rate.

Scattered

The second paper performs a far more complicated analysis, searching for indications of interactions between the FRB and the interstellar medium that exists within galaxies. This will have two effects. One, caused by scattering off interstellar material, will spread the burst out over time in a frequency-dependent manner. Scattering can also cause a random brightening/dimming of different areas of the spectrum, called scintillation, and somewhat analogous to the twinkling of stars caused by our atmosphere.

In this case, the photons of the FRB have had three encounters with matter that can induce these effects: the sparse intersteller material of the source galaxy, the equally sparse interstellar material in our own Milky Way, and the even more sparse intergalactic material in between the two. Since the source galaxy for FRB 20221022A is relatively close to our own, the intergalactic medium can be ignored, leaving the detection with two major sources of scattering.

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Supermassive black hole binary emits unexpected flares

astronomy, astrophysics, black hole binary, black holes, Science, supermassive black hole / 9u50fv / December 1, 2024

“In addition to stars, gas clouds can also be disrupted by SMBHs and their binaries,” they said in the same study. “The key difference is that the clouds can be comparable to or even larger than the binary separation, unlike stars, which are always much smaller. “

Looking at the results of a previous study that numerically modeled this type of situation also suggested a gas cloud. Just like the hypothetical supermassive black hole binary in the model, AT 2021hdr would accrete large amounts of material every time the black holes were halfway through orbiting each other and had to cross the cloud to complete the orbit—their gravity tears away some of the cloud, which ends up in their accretion disks, every time they cross it. They are now thought to take in anywhere between three and 30 percent of the cloud every few cycles. From a cloud so huge, that’s a lot of gas.

The supermassive black holes in AT 2021hdr are predicted to crash into each other and merge in another 70,000 years. They are also part of another merger, in which their host galaxy is gradually merging with a nearby galaxy, which was first discovered by the same team (this has no effect on the BSMBH tidal disruption of the gas cloud).

How the behavior of AT 2021hdr develops could tell us more about its nature and uphold or disprove the idea that it is eating away at a gaseous cloud instead of a star or something else. For now, it seems these black holes don’t just get gas from what they eat—they eat the gas itself.

Astronomy & Astrophysics, 2024. DOI: 10.1051/0004-6361/202451305

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Researchers spot black hole feeding at 40x its theoretical limit

astronomy, astrophysics, black holes, Chandra telescope, cosmology, eddington limit, Science, supermassive black holes, Webb telescope / DJ Henderson / November 4, 2024

Similar feeding events could explain the rapid growth of supermassive black holes.

How did supermassive black holes end up at the center of every galaxy? A while back, it wasn’t that hard to explain: That’s where the highest concentration of matter is, and the black holes had billions of years to feed on it. But as we’ve looked ever deeper into the Universe’s history, we keep finding supermassive black holes, which shortens the timeline for their formation. Rather than making a leisurely meal of nearby matter, these black holes have gorged themselves in a feeding frenzy.

With the advent of the Webb Space Telescope, the problem has pushed up against theoretical limits. The matter falling into a black hole generates radiation, with faster feeding meaning more radiation. And that radiation can drive off nearby matter, choking off the black hole’s food supply. That sets a limit on how fast black holes can grow unless matter is somehow fed directly into them. The Webb was used to identify early supermassive black holes that needed to have been pushing against the limit for their entire existence.

But the Webb may have just identified a solution to the dilemma as well. It has spotted a black hole that appears to have been feeding at 40 times the theoretical limit for millions of years, allowing growth at a pace sufficient to build a supermassive black hole.

Setting limits

Matter falling into a black hole generally gathers into what’s called an accretion disk, orbiting the body and heating up due to collisions with the rest of the disk, all while losing energy in the form of radiation. Eventually, if enough energy is lost, the material falls into the black hole. The more matter there is, the brighter the accretion disk gets, and the more matter that gets driven off before it can fall in. The point where the radiation pressure drives away as much matter as the black hole pulls in is called the Eddington Limit. The bigger the black hole, the higher this limit.

It is possible to exceed the Eddington Limit if matter falls directly into the black hole without spending time in the accretion disk, but it requires a fairly distinct configuration of nearby clouds of gas, something that’s unlikely to persist for more than a few million years.

That creates a problem for supermassive black holes. The only way we know to form a black hole—the death of a massive star in a supernova—tends to produce them with only a few times the mass of the Sun. Even assuming unusually massive stars in the early Universe, along with a few black hole mergers, it’s expected that most of the potential seeds of a supermassive black hole are in the area of 100 times the Sun’s mass. There are theoretical ideas about the direct collapse of gas clouds that avoid the intervening star formation and immediately form a black hole with 10,000 times the mass of the Sun or more, but they remain entirely hypothetical.

In either case, black holes would need to suck down a lot of matter before reaching supermassive proportions. But most of the early supermassive black holes spotted using the Webb are feeding at roughly 20 percent of the Eddington limit, based on their lack of X-ray emissions. This either means that they fed at well beyond the Eddington Limit earlier in their history or that they started their existences as very heavy black holes.

The object that’s the focus of this new report, LID-568, was first spotted using the Chandra X-ray Telescope (an observatory that was recently threatened with shutdown). LID-568 is luminous at X-ray wavelengths, which is why Chandra could spot it, and suggests the possibility that it is feeding at an extremely high rate. Imaging in the infrared shows that it appears to be a point source, so the research team concluded that most of the light we’re seeing comes directly from the accretion disk, rather than from the stars in the galaxy it occupies.

But that made it difficult to determine any details about the black hole’s environment or to figure out how old it was relative to the Big Bang at the time we’re viewing it. So, the researchers pointed the Webb at it to capture details that other observatories couldn’t image.

A fast eater

Use of spectroscopy revealed that we were viewing LID-568 as it existed about 1.5 billion years after the Big Bang. The emissions from gas and dust in the area were low, which suggests that the black hole resides in a dwarf galaxy. Based on the emission of hydrogen, the researchers estimate that the black hole is roughly a million times the mass of the Sun—nothing you’d want to get close to, but small compared to many supermassive black holes.

It’s actually similar in mass to a number of black holes the Webb was used to identify in galaxies that are considerably older. But it’s much, much brighter (as bright as something 10 times heavier) and includes the X-ray emissions that those lack. In fact, it’s so bright compared to its mass that the researchers estimate that it could only produce that much radiation if it were feeding at well above the Eddington Limit. Ultimately, they estimate that it’s exceeding the Eddington Limit by a factor of over 40.

Critically, the Webb was able to identify two lobes of material that were moving toward us at high velocities, based on the blue shifting of hydrogen emissions lines. These suggest that the material is moving at over 500 kilometers a second and stretched for tens of thousands of light years away from the black hole. (Presumably, these obscured similar blobs of material moving away from us.) Given their length and apparent velocity, and assuming they represent gas driven off by the black hole, the researchers estimated how long it was emitting this intense radiation.

Working back from there, they estimate the black hole’s original mass was about 100 times that of the Sun. “This lifetime suggests that a substantial fraction of the mass growth of LID-568 may have occurred in a single, super-Eddington accretion episode,” they conclude. For that to work, the black hole had to have ended up in a giant molecular cloud and stayed there feeding for over 10 million years.

The researchers suspect that this intense activity interfered with star formation in the galaxy, which is one of the reasons that it is relatively star-poor. That may explain why we see some very massive black holes at the center of relatively small galaxies in the present Universe.

So what does this mean?

In some ways, this is potentially good news for cosmologists. Forming supermassive black holes as quickly as the size/age of those observed by Webb would seemingly require them to have fed at or slightly above the Eddington Limit for most of their history, which was easy to view as unlikely. If the Eddington Limit can be exceeded by a factor of 40 for over 10 million years, however, this seems to be less of an issue.

But, at the same time, the graph showing mass versus luminosity of supermassive black holes the research team generated shows that LID-568 is in a class by itself. If there were a lot of black holes feeding at these rates, it should be easy to identify more. And it’s a safe bet that these researchers are checking other X-ray sources to see if there are additional examples.

Nature Astronomy, 2024. DOI: 10.1038/s41550-024-02402-9 (About DOIs).

John is Ars Technica’s science editor. He has a Bachelor of Arts in Biochemistry from Columbia University, and a Ph.D. in Molecular and Cell Biology from the University of California, Berkeley. When physically separated from his keyboard, he tends to seek out a bicycle, or a scenic location for communing with his hiking boots.

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What do planet formation and badminton have in common?

astronomy, astrophysics, badminton, dust, planet-forming disk, planetary formation, protoplanetary disks, Science / Mike M. / October 15, 2024

It might not come as a surprise to learn that Lin is a badminton player. “The experience of playing badminton is really the thing that kick-started the idea and led me to ask the right questions,” he said.

Previous explanations attribute the dust alignment to the magnetic influence of the central star, the physics of which can be complicated and not always intuitive. The beauty of the proposed birdie mechanism is its simplicity. “It’s a very good first step,” said Bing Ren, an astronomer at France’s Côte d’Azur Observatory who wasn’t involved in the study.

Still, the birdie-alignment hypothesis is just that—a hypothesis. To confirm whether it holds water, scientists will need to throw their full observational arsenal at protoplanetary disks, such as viewing them at different wavelengths, to sniff out the finer details of particle-gas interactions.

Tracing invisible gas

Real-life protoplanetary disks are likely more complicated than a uniform squadron of space potatoes suspended in thin air. Ren suspects that the grains come in various shapes, sizes, and speeds. Nevertheless, he says Lin’s study is a good foundation for computer models of interstellar clouds, onto which scientists can tack layers of complexity.

The new research points a way forward for probing protoplanetary disks, particularly gas behavior. Given that the grains trace the gas direction, studying dust organization using existing tools like polarized light can allow scientists to map a disk’s aerodynamic flow. Essentially, these grains are tiny flags that signal where the wind blows.

As granular as the details are, the dust alignment is a small but key step in a grand journey of particle-to-planet progression. The nitty-gritty of a particle’s conduct will determine its fate for millions of years—perhaps the primordial seed will hoover up hydrogen and helium to become a gas giant or amass dust to transform into a terrestrial world like Earth. It all starts with it flailing or keeping steady amid a sea of other specks.

Monthly Notices of the Royal Astronomical Society, 2024. DOI: 10.1093/mnras/stae2248 (About DOIs)

Shi En Kim is a DC-based freelance journalist who writes about health, the environment, technology, and the physical sciences. She and three other journalists founded Sequencer Magazine in early 2024. Occasionally, she creates art to accompany her writings or does it simply for fun. Follow her on Twitter at @goes_by_kim, or see more of her work on her personal website.

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Black hole jet appears to boost rate of nova explosions

astronomy, astrophysics, black hole jets, novas, Science, supermassive black holes / DJ Henderson / September 27, 2024

Image of a bright point against a dark background, with a wavy, lumpy line of material extending diagonally from the point to the opposite corner of the image. — Enlarge / One of the jets emitted by galaxy M87’s central black hole.

The intense electromagnetic environment near a black hole can accelerate particles to a large fraction of the speed of light and sends the speeding particles along jets that extend from each of the object’s poles. In the case of the supermassive black holes found in the center of galaxies, these jets are truly colossal, blasting material not just out of the galaxy, but possibly out of the galaxy’s entire neighborhood.

But this week, scientists have described how the jets may be doing some strange things inside of a galaxy, as well. A study of the galaxy M87 showed that nova explosions appear to be occurring at an unusual high frequency in the neighborhood of one of the jets from the galaxy’s central black hole. But there’s absolutely no mechanism to explain why this might happen, and there’s no sign that it’s happening at the jet that’s traveling in the opposite direction.

Whether this effect is real, and whether we can come up with an explanation for it, may take some further observations.

Novas and wedges

M87 is one of the larger galaxies in our local patch of the Universe, and its central black hole has active jets. During an earlier period of regular observations, the Hubble Space Telescope had found that stellar explosions called novas appeared to be clustered around the jet.

This makes very little sense. Novas occur in systems with a large, hydrogen-rich star, with a nearby white dwarf in orbit. Over time, the white dwarf draws hydrogen off the surface of its companion, until it reaches a critical mass on its surface. At that point, a thermonuclear explosion blasts the remaining material off the white dwarf, and the cycle resets. Since the rate of material transfer tends to be fairly stable, novas in a stellar system will often repeat at regular intervals. And it’s not at all clear why a black hole’s jet would alter that regularity.

So, some of the people involved in the first study got time on the Hubble to go back and have another look. And for a big chunk of a year, every five days, Hubble was pointed at M87, allowing it to capture novas before they faded back out. All told, this picked up 94 novas that occurred near the center of the galaxy. Combined with 41 that had been identified during earlier work, this left a collection of 135 novas in this galaxy. The researchers then plotted these relative to the black hole and its jets.

The area containing the jet (upper right) experiences significantly more novas than the rest of the galaxy's core. — The area containing the jet (upper right) experiences significantly more novas than the rest of the galaxy’s core.

Lessing et. al.

Dividing the area around the center of the galaxy into 10 equal segments, the researchers counted the novas that occurred in each. In the nine segments that didn’t include the jet on the side of the galaxy facing Earth, the average number of novas was 12. In the segment that included the jet, the count was 25. Another way to look at this is that the highest count in a non-jet segment was only 16—and that was in a segment immediately next to the one with the jet in it. The researchers calculate the odds of this arrangement occurring at random as being about one in 1,310 (meaning less than 0.1 percent).

To get a separate measure of how unusual this is, the researchers placed 8 million novas around the center of the galaxy, with the distribution being random but biased to match the galaxy’s brightness under the assumption that novas will be more frequent in areas with more stars. This was then used to estimate how often novas should be expected in each of these segments. They then used a wide variety of wedges: “In order to reduce noise and avoid p-hacking when choosing the size of the wedge, we average the results for wedges between 30 and 45 degrees wide.”

Overall, the enhancement near the jet was low for either very narrow or very wide wedges, as you might expect—narrow wedges crop out too much of the area affected by the jet, while wide ones include a lot of space where you get the normal background rate. Things peak in the area of wedges that are 25 degrees wide, where the enrichment near the jet is about 2.6-fold. So, this appears to be real.

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Astronomers think they’ve found a plausible explanation of the Wow! signal

aliens, astronomy, astrophysics, Physics, Science, WOW! signal / DJ Henderson / August 21, 2024

“I’m not saying it’s aliens…” —

Magnetars could zap clouds of atomic hydrogen, producing focused microwave beams.

Jennifer Ouellette – Aug 21, 2024 8: 14 pm UTC

The Wow! signal represented as — Enlarge / The Wow! signal, represented as “6EQUJ5,” was discovered in 1977 by astronomer Jerry Ehman.

Public domain

An unusually bright burst of radio waves—dubbed the Wow! signal—discovered in the 1970s has baffled astronomers ever since, given the tantalizing possibility that it just might be from an alien civilization trying to communicate with us. A team of astronomers think they might have a better explanation, according to a preprint posted to the physics arXiv: clouds of atomic hydrogen that essentially act like a naturally occurring galactic maser, emitting a beam of intense microwave radiation when zapped by a flare from a passing magnetar.

As previously reported, the Wow! signal was detected on August 18, 1977, by The Ohio State University Radio Observatory, known as “Big Ear.” Astronomy professor Jerry Ehman was analyzing Big Ear data in the form of printouts that, to the untrained eye, looked like someone had simply smashed the number row of a typewriter with a preference for lower digits. Numbers and letters in the Big Ear data indicated, essentially, the intensity of the electromagnetic signal picked up by the telescope over time, starting at ones and moving up to letters in the double digits (A was 10, B was 11, and so on). Most of the page was covered in ones and twos, with a stray six or seven sprinkled in.

But that day, Ehman found an anomaly: 6EQUJ5 (sometimes misinterpreted as a message encoded in the radio signal). This signal had started out at an intensity of six—already an outlier on the page—climbed to E, then Q, peaked at U—the highest power signal Big Ear had ever seen—then decreased again. Ehman circled the sequence in red pen and wrote “Wow!” next to it. The signal appeared to be coming from the direction of the Sagittarius constellation, and the entire signal lasted for about 72 seconds. Alas, SETI researchers have never been able to detect the so-called “Wow! Signal” again, despite many tries with radio telescopes around the world.

One reason for the excited reaction is that such a signal had been proposed as a possible communication from extraterrestrial civilizations in a 1959 paper by Cornell University physicists Philip Morrison and Giuseppe Cocconi. Morrison and Cocconi thought that such a civilization might use the 1420 megahertz frequency naturally emitted by hydrogen, the universe’s most abundant element and, therefore, something an alien civilization would be familiar with. In fact, the Big Ear had been reassigned to the SETI project in 1973 specifically to hunt for possible signals. Ehman himself was quite skeptical of the “it could be aliens” hypothesis for several decades, although he admitted in a 2019 interview that “the Wow! signal certainly has the potential of being the first signal from extraterrestrial intelligence.”

Several other alternative hypotheses have been suggested. For instance, Antonio Paris suggested in 2016 that the signal may have come from the hydrogen cloud surrounding a pair of comets, 266P/Christensen and 335P/Gibbs. This was rejected by most astronomers, however, in part because comets don’t emit strongly at the relevant frequencies. Others have suggested the signal was the result of interference from satellites orbiting the Earth, or a signal from Earth reflected off a piece of space debris.

Space maser!

Astrobiologist Abel Mendez of the University of Puerto Rico at Arecibo and his co-authors think they have the strongest astrophysical explanation to date with their cosmic maser hypothesis. The team was actually hunting for habitable exoplanets using signals from red dwarf stars. In some of the last archival data collected at the Arecibo radio telescope (which collapsed in 2020), they noticed several signals that were remarkably similar to the Wow! signal in terms of frequency—just much less intense (bright).

Mendez admitted to Science News that he had always viewed the Wow! signal as just a fluke—he certainly didn’t think it was aliens. But he realized that if the signals they were identifying had blazed brighter, even momentarily, they would be very much like the Wow! signal. As for the mechanism that caused such a brightening, Mendez et al. propose that a magnetar (a highly magnetic neutron star) passing behind a cloud of atomic hydrogen could have flared up with sufficient energy to produce stimulated emission in the form of a tightly focused beam of microwave radiation—a cosmic maser. (Masers are akin to lasers, except they emit microwave radiation rather than visible radiation.)

Proving their working hypothesis will be much more challenging, although there have been rare sightings of such naturally occurring masers from hydrogen molecules in space. But nobody has ever spotted an atomic hydrogen cloud with an associated maser, and that’s what would be needed to explain the intensity of the Wow! signal. That’s why other astronomers are opting for cautious skepticism. “A magnetar is going to produce [short] radio emissions as well. Do you really need this complicated maser stuff happening as well to explain the Wow! signal?” Michael Garrett of the University of Manchester told New Scientist. “Personally, I don’t think so. It just makes a complicated story even more complicated.”

arXiv, 2024. DOI: 10.48550/arXiv.2408.08513 (About DOIs).

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