astrophysics

There could be “dark main sequence” stars at the galactic center

astronomy, astrophysics, Dark Matter, main sequence, Science, star evolution, stars / Mike M. / July 16, 2025

Dark matter particle and antiparticle collisions could make some stars immortal.

For a star, its initial mass is everything. It determines how quickly it burns through its hydrogen and how it will evolve once it starts fusing heavier elements. It’s so well understood that scientists have devised a “main sequence” that acts a bit like a periodic table for stars, correlating their mass and age with their properties.

The main sequence, however, is based on an assumption that’s almost always true: All of the energy involved comes from the gravity-driven fusion of lighter elements into heavier ones. However, three astrophysicists consider an alternative source of energy that may apply at the very center of our galaxy— energy released when dark matter particles and antiparticles collide and annihilate. While we don’t even know that dark matter can do that, it’s a hypothetical with some interesting consequences, like seemingly immortal stars, and others that move backward along the main sequence path.

Dark annihilations

We haven’t figured out what dark matter is, but there are lots of reasons to think that it is comprised of elementary particles. And, if those behave like all of the particles we understand well, then there will be both regular and antimatter versions. Should those collide, they should annihilate each other, releasing energy in the process. Given dark matter’s general propensity not to interact with anything, these collisions will be extremely rare except in locations with very high dark matter concentrations.

The only place that’s likely to happen is at the very center of our galaxy. And, for a while, there was an excess of radiation coming from the galactic core that people thought might be due to dark matter annihilations, although it eventually turned out to have a more mundane explanation.

At the extreme densities found within a light year of the supermassive black hole at the center of our galaxy, concentrations are high enough that these collisions could be a major source of energy. And so astronomers have considered what all that energy might do to stars that end up in a black hole’s orbit, finding that under the right circumstances, dark matter destruction could provide more energy to a star than fusion.

That prompted three astrophysicists (Isabelle John, Rebecca Leane, and Tim Linden) to try to look at things in an organized fashion, modeling a “dark main sequence” of stars as they might exist within a close proximity to the Milky Way’s center.

The intense gravity and radiation found near the galaxy’s core mean that stars can’t form there. So, anything that’s in a tight orbit had formed somewhere else before gravitational interactions had pushed it into the gravitational grasp of the galaxy’s central black hole. The researchers used a standard model of star evolution to build a collection of moderate-sized stars, from one to 20 solar masses at 0.05 solar mass intervals. These are allowed to ignite fusion at their cores and then shift into a dark-matter-rich environment.

Since we have no idea how often dark matter particles might run into each other, John, Leane, and Linden use two different collision frequencies. These determine how much energy is imparted into these stars by dark matter, which the researchers simply add as a supplement to the amount of fusion energy the stars are producing. Then, the stars are allowed to evolve forward in time.

(The authors note that stars that are thrown into the grasp of a supermassive black hole tend to have very eccentric orbits, so they spend a lot of time outside the zone where dark matter collisions take place with a significant frequency. So, what they’ve done is the equivalent of having these stars experience the energy input given their average orbital distance from the galaxy’s core. In reality, a star would spend some years with higher energy input and some years with lower input as it moves about its orbit.)

Achieving immortality

The physics of what happens is based on the same balance of forces that govern fusion-powered stars, but produces some very strange results. Given only fusion power, a star will exist at a balance point. If gravity compresses it, fusion speeds up, more energy is released, and that energy causes the star to expand outward again. That causes the density drop, slowing fusion back down again.

The dark matter annihilations essentially provide an additional source of energy that stays constant regardless of what happens to the star’s density. At the low end of the mass range the researchers considered, this can cause the star to nearly shut off fusion, essentially looking like a far younger star than it actually is. That has the effect of causing the star to move backward along the main sequence diagram.

The researchers note that even lighter stars could essentially get so much additional energy that they can’t hold together and end up dissipating, something that’s been seen in models run by other researchers.

As the mass gets higher, stars reach the point where they essentially give up on fusion and get by with nothing but dark matter annihilations. They have enough mass to hold together gravitationally, but end up too diffused for fusion to continue. And they’ll stay that way as long as they continue to get additional injections of energy. “A star like this might look like a young, still-forming star,” the authors write, “but has features of a star that has undergone nuclear fusion in the past and is effectively immortal.”

John, Leane, and Linden find that the higher mass stars remain dense enough for fusion to continue even in proximity to the galaxy’s black hole. But the additional energy kept that fusion happening at a moderate rate. They proceeded through the main sequence, but at a pace that was exceptionally slow, so that running the simulation for a total of 10 billion years didn’t see them change significantly.

The other strange thing here is that all of this is very sensitive to how much dark matter annihilation is taking place. A star that’s “immortal” at one average distance will progress slowly through the main sequence if its average distance is a light year further out. Similarly, stars that are too light to survive at one location will hold together if they are a bit further from the supermassive black hole.

Is there anything to this?

The big caution is that this work only looks at the average input from dark matter annihilation. In reality, a star that might be immortal at its average distance will likely spend a few years too hot to hold together, and then several years cooling off in conditions that should allow fusion to reignite. It would be nice to see a model run with this sort of pulsed input, perhaps basing it on the orbits of some of the stars we’ve seen that get close to the Milky Way’s central black hole.

In the meantime, John, Leane, and Linden write that their results are consistent with some of the oddities that are apparent in the stars we’ve observed at the galaxy’s center. These have two distinctive properties: They appear heavier than the average star in the Milky Way, and all seem to be quite young. If there is a “dark main sequence,” then the unusual heft can be explained simply by the fact that lower mass stars end up dissipating due to the additional energy. And the model would suggest that these stars simply appear to be young because they haven’t undergone much fusion.

The researchers suggest that we could have a clearer picture if we were able to spend enough time observing the stars at our galaxy’s core with a large enough telescope, allowing us to understand their nature and orbits.

Physical Review D, 2025. DOI: Not yet available (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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Merger of two massive black holes is one for the record books

astronomy, astrophysics, black hole mergers, black holes, gravitational waves, LIGO/VIRGO/KAGRA, Physics, Science / Kelly Newman / July 15, 2025

Physicists with the LIGO/Virgo/KAGRA collaboration have detected the gravitational wave signal (dubbed GW231123) of the most massive merger between two black holes yet observed, resulting in a new black hole that is 225 times more massive than our Sun. The results were presented at the Edoardo Amaldi Conference on Gravitational Waves in Glasgow, Scotland.

The LIGO/Virgo/KAGRA collaboration searches the universe for gravitational waves produced by the mergers of black holes and neutron stars. LIGO detects gravitational waves via laser interferometry, using high-powered lasers to measure tiny changes in the distance between two objects positioned kilometers apart. LIGO has detectors in Hanford, Washington, and in Livingston, Louisiana. A third detector in Italy, Advanced Virgo, came online in 2016. In Japan, KAGRA is the first gravitational-wave detector in Asia and the first to be built underground. Construction began on LIGO-India in 2021, and physicists expect it will turn on sometime after 2025.

To date, the collaboration has detected dozens of merger events since its first Nobel Prize-winning discovery. Early detected mergers involved either two black holes or two neutron stars. In 2021, LIGO/Virgo/KAGRA confirmed the detection of two separate “mixed” mergers between black holes and neutron stars.

A tour of Virgo. Credit: EGO-Virgo

LIGO/Virgo/KAGRA started its fourth observing run in 2023, and by the following year had announced the detection of a signal indicating a merger between two compact objects, one of which was most likely a neutron star. The other had an intermediate mass—heavier than a neutron star and lighter than a black hole. It was the first gravitational-wave detection of a mass-gap object paired with a neutron star and hinted that the mass gap might be less empty than astronomers previously thought.

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New evidence that some supernovae may be a “double detonation”

astronomy, astrophysics, Science, supernova, Type Ia supernova, white dwarf / Shannon Garcia / July 3, 2025

Type Ia supernovae are critical tools in astronomy, since they all appear to explode with the same intensity, allowing us to use their brightness as a measure of distance. The distance measures they’ve given us have been critical to tracking the expansion of the Universe, which led to the recognition that there’s some sort of dark energy hastening the Universe’s expansion. Yet there are ongoing arguments over exactly how these events are triggered.

There’s widespread agreement that type Ia supernovae are the explosions of white dwarf stars. Normally, these stars are composed primarily of moderately heavy elements like carbon and oxygen, and lack the mass to trigger additional fusion. But if some additional material is added, the white dwarf can reach a critical mass and reignite a runaway fusion reaction, blowing the star apart. But the source of the additional mass has been somewhat controversial.

But there’s an additional hypothesis that doesn’t require as much mass: a relatively small explosion on a white dwarf’s surface can compress the interior enough to restart fusion in stars that haven’t yet reached a critical mass. Now, observations of the remains of a supernova provide some evidence of the existence of these so-called “double detonation” supernovae.

Deconstructing white dwarfs

White dwarfs are the remains of stars with a similar mass to our Sun. After having gone through periods during which hydrogen and helium were fused, these tend to end up as carbon and oxygen-rich embers: hot due to their history, but incapable of reaching the densities needed to fuse these elements. Left on their own, these stellar remnants will gradually cool.

But many stars are not left on their own; they exist in binary systems with a companion, or even larger systems. These companions can provide the material needed to boost white dwarfs to the masses that can restart fusion. There are two potential pathways for this to happen. Many stars go through periods where they are so large that their gravitational pull is barely enough to hold on to their outer layers. If the white dwarf orbits closely enough, it can pull in material from the other star, boosting its mass until it passes a critical threshold, at which point fusion can restart.

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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).

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