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asteroid 2024 yr4

The chances of the asteroid 2024 YR4 striking Earth in 2032 have tumbled from a peak of 3.1% in February 2025 to nearly zero today, though a collision with the Moon is still possible. Brand new JWST observations give a fresh perspective on this near-Earth asteroid.

From Discovery to Today

Discovery images of asteroid 2024 YR4

Discovery images of 2024 YR4. [ATLAS]

On 27 December 2024, the Asteroid Terrestrial-impact Last Alert System (ATLAS) spotted an asteroid, cataloged as 2024 YR4, with a slim chance of a collision with Earth on 22 December 2032. As observations rolled in, the probability of 2024 YR4 slamming into Earth crept upward, reaching a peak of 3.1% on 18 February 2025. Just a few days later, revised estimates of the asteroid’s orbit reduced the odds of an Earth impact to just 0.004%. Though 2024 YR4 no longer poses a threat to Earth, there is still a small chance it will hit the Moon in 2032.

A New Look at 2024 YR4

As reported today in Research Notes of the AAS, a team led by Andrew Rivkin (The Johns Hopkins University Applied Physics Laboratory) used JWST to observe 2024 YR4 in March 2025. Using the Mid-Infrared Instrument (MIRI) and the Near-Infrared Camera (NIRCam), the team determined the size, albedo, and spectral energy distribution of the asteroid.

By modeling the asteroid’s thermal emission, Rivkin’s team estimated 2024 YR4’s diameter to be 60 ± 7 meters (197 ± 23 feet). This is consistent with previous size estimates that used measurements of the asteroid’s apparent magnitude and estimates of its albedo, or the fraction of light that is reflected from its surface. As a comparison, the asteroid that caused the Tunguska event — an air burst that toppled millions of trees, shattered windows, and sparked forest fires — is thought to have been in the range of 40–100 meters (130–328 feet).

Further Findings and Future Directions

The asteroid’s albedo is between 0.08 and 0.18, with a best-fit value of 0.13. Many classes of asteroids have albedos in this range, so this estimate doesn’t place 2024 YR4 in any particular class. However, Rivkin’s team noted that these values are compatible with previous spectra of the asteroid that suggest an S-type (stony) classification.

The team also estimated the potential impact of a 2024 YR4-like asteroid, finding that a collision with Earth would release the energy equivalent of 2–30 megatons of TNT, with a blast damage radius of up to 80 kilometers (50 miles).

Future modeling using sophisticated thermophysical models will help to define the asteroid’s properties further. And in early 2026, researchers may have another chance to observe 2024 YR4 with JWST and refine their understanding of a potential lunar impact in 2032.

Citation

“JWST Observations of Potentially Hazardous Asteroid 2024 YR4,” A. S. Rivkin et al 2025 Res. Notes AAS 9 70. doi:10.3847/2515-5172/adc6f0

30 Doradus

This Monthly Roundup covers three investigations of the high-energy universe, from a hunt for a cosmic particle accelerator in the Milky Way to an examination of a fading quasar in the distant past.

Investigating the Most Energetic Neutrino Ever Detected

In February 2023, the Cubic Kilometre Neutrino Telescope (KM3NeT) — a neutrino telescope at the bottom of the Mediterranean Sea — detected a particle called a muon with an energy of roughly 100 petaelectronvolts (a hundred quadrillion electronvolts). The muon was likely produced by an incoming neutrino with an energy of 220 petaelectronvolts — the highest-energy neutrino ever observed.

The orientation of the event suggests an astrophysical origin, but the source of this neutrino is unknown. One possibility is that the neutrino arose in a transient event that produced extremely high-energy cosmic rays: relativistic charged particles like protons, electrons, and atomic nuclei. Cosmic rays could produce neutrinos and gamma rays through interactions with photons of the cosmic microwave background. The neutrinos zip off into space, unhindered by intervening gas or magnetic fields, while the cosmic rays can be waylaid for thousands of years, caught up in the magnetic fields that lace the space between galaxies. Gamma rays fall in between the two extremes, slowed slightly by interactions with the photons of the extragalactic background. Repeated interactions between the gamma rays and background photons create a cascade of gamma rays across a range of energies.

plot of gamma-ray flux as a function of time since the neutrino's arrival

Gamma-ray flux as a function of time since the neutrino’s arrival for different intergalactic magnetic field strengths. Stronger magnetic fields lead to lower flux and later arrival times. The gray lines show the five-sigma detection limits of different instruments. Click to enlarge. [Fang et al. 2025]

Detecting this gamma-ray cascade would provide a valuable clue in the search for the origin of the ultra-high-energy neutrino detected in 2023. In a recent research article, Ke Fang (Wisconsin IceCube Particle Astrophysics Center) and collaborators estimated the flux of gamma rays that would be associated with this high-energy neutrino. The team’s estimates accounted for varying distances to the source as well as different strengths of the intergalactic magnetic field. The stronger the magnetic field, the weaker the gamma-ray flux when it arrives at Earth, and the later the arrival time at Earth.

For weak magnetic fields, the gamma-ray cascade should have arrived at Earth hours or days after the neutrino was detected in 2023. These gamma rays are potentially detectable as long as the magnetic field is weaker than 3 × 10-13 Gauss. For magnetic field strengths greater than 3 × 10-13 Gauss, the gamma rays wouldn’t arrive until more than a decade later, and they would likely be too faint to detect. If no gamma rays are detected, the non-detection could be used to place a lower limit on the strength of the intergalactic magnetic field.

The Hunt for a Galactic PeVatron

Across the universe, charged particles are being accelerated to near the speed of light, achieving energies in the petaelectronvolt, or PeV, range. The sources of these particles are called PeVatrons, and observations have revealed that these cosmic particle accelerators exist in the Milky Way. Supernovae, massive stars, pulsars, and pulsar wind nebulae are all candidate PeVatrons. To find out more, astronomers look to gamma rays, which can be produced when cosmic rays interact with dense matter.

Recently, the Large High Altitude Air Shower Observatory (LHAASO) collaboration investigated a possible galactic PeVatron called G35.6−0.4. G35.6−0.4 is a radio source that is thought to be associated with the gamma-ray source HESS J1858+020. Observations of this region show a supernova remnant and an H II region containing multiple X-ray point sources.

gamma-ray map of the region of interest in this study

LHAASO’s Water Cherenkov Detector Array (left) and Kilometer Square Array (right) observations of the region around the gamma-ray source HESS J1858+020. Black solid circles and black crosses represent extended and pointlike sources, respectively, resolved in this work. Dashed circles show sources resolved by LHAASO in previous work. The cyan symbols show the locations of gamma-ray sources identified by other facilities. Click to enlarge. [LHAASO Collaboration 2025]

To learn more about the origins of the gamma rays from this complex region, the collaboration used data from LHAASO, a ground-based gamma-ray and cosmic-ray observatory. Data from two of LHAASO’s detectors show five gamma-ray sources throughout the region, one of which may be associated with the previously detected gamma-ray source HESS J1858+020. The team also amassed data from other sources, pulling together a picture of the molecular and atomic gas and massive stars present in the region.

Because of the crowded nature of this area, this investigation wasn’t able to clearly point to the source of the gamma rays. The authors outlined three possible sources for the gamma rays: 1) winds from hidden massive stars or outflows from protostars within the H II region, 2) particles escaping from the supernova remnant and interacting with nearby molecular clouds, and 3) an as-yet-undetected pulsar wind nebula. While none of these scenarios is a clear front-runner, neither could any of them be ruled out (though the supernova remnant scenario faces the greatest feasibility challenges). Future searches for massive stars or pulsar wind nebulae in this region may provide further clues.

Fading Light from a Quasar at Cosmic Dawn

For the third and final article, we’re looking back into the distant past at one of the most powerful objects in the universe: a quasar. Quasars are extraordinarily luminous galactic centers in the early universe, powered by accretion of gas onto a growing black hole. Because of their extreme brightness, quasars are visible from billions of light-years away, giving researchers a glimpse into the early evolution of supermassive black holes.

Jianwei Lyu (吕建伟; University of Arizona) and collaborators investigated HSC J2239+0207, a quasar located at a redshift of z = 6.2498, when the universe was roughly 900 million years old. This redshift places the quasar near the end of the epoch of reionization, when the formation of the first stars and galaxies ionized the universe’s abundant neutral hydrogen gas. This quasar is an intriguing target because previous observations have shown that the black hole that powers it is roughly 15 times more massive than expected for the stellar mass of its host galaxy. The quasar’s accretion rate is low, indicating that the black hole may be nearing the end of its growth spurt.

JWST spectrum of the quasar HSC J2239+0207

JWST spectrum of the quasar HSC J2239+0207 (blue line). Click to enlarge. [Lyu et al. 2025]

Lyu’s team analyzed JWST spectra of this quasar, estimating the black hole’s mass to be roughly 300 million solar masses (about 75 times more massive than the Milky Way’s supermassive black hole) and its accretion rate to be just 40% of the theoretical limit. This is unusual, since quasars at this point in the universe’s history typically have accretion rates at or above the theoretical limit. The unexpectedly low accretion rate for HSC J2239+0207 could mean that the black hole’s growth is slowing down. However, the authors caution that it could be a temporary slowdown caused by a lack of fuel rather than a permanent shutdown.

The team also investigated a gas cloud located one arcsecond from the quasar. This object could be several things: an isolated high-redshift galaxy, a galaxy falling toward the quasar host galaxy, tidally disrupted material stripped from a galaxy passing nearby, or material blown out of the quasar host galaxy by the quasar itself. The authors favor this final scenario, which is indicative of black hole feedback at work.

Feedback may be the reason that this black hole is so massive compared to the stellar mass of its host galaxy. Powerful radiation and winds from the black hole could have suppressed the rate of star formation as the black hole grew. With the black hole’s activity winding down, star formation should have a chance to ramp up, bringing the galaxy into alignment with the expected stellar mass–black hole mass relation.

Citation

“Cascaded Gamma-Ray Emission Associated with the KM3NeT Ultrahigh-Energy Event KM3-230213A,” Ke Fang et al 2025 ApJL 982 L16. doi:10.3847/2041-8213/adbbec

“An Enigmatic PeVatron in an Area Around H II Region G35.6−0.5,” Zhen Cao et al 2025 ApJ 979 70. doi:10.3847/1538-4357/ad991d

“Fading Light, Fierce Winds: JWST Snapshot of a Sub-Eddington Quasar at Cosmic Dawn,” Jianwei Lyu et al 2025 ApJL 981 L20. doi:10.3847/2041-8213/adb613

Milky Way center

Could there have been two massive black holes in our galaxy’s center at one time? New research suggests that this scenario could explain oddities in the population of hypervelocity stars in the Milky Way’s halo.

From the Center to the Halo

stars at the center of the Milky Way

A near-infrared image from the Very Large Telescope of stars at the center of the Milky Way. [ESO/S. Gillessen et al.; CC BY 4.0]

At the heart of the Milky Way, in the neighborhood of Sagittarius A* (Sgr A*) — our galaxy’s supermassive black hole — there is a population of massive young stars. Living in such an extreme environment has its dangers, and Sgr A*’s tidal forces can tear apart stellar binaries, capturing one star into a snug orbit around the black hole and flinging the other away at high speeds.

Surveys of the Milky Way halo have indeed found massive hypervelocity stars that hastily departed the galactic center some 50–250 million years ago, cementing the idea that interactions between stellar binaries and Sgr A* can send stars careening through the galaxy.

There are two odd things about the observed population of hypervelocity stars, though: 1) their velocities top out around 700 km/s, though theory suggests roughly half of these stars should move faster than this cutoff; and 2) none of the stars remaining in orbit around Sgr A* appear to be the left-behind binary companions of these hypervelocity stars.

A Long Time Ago in a Galaxy Very, Very Close By

A second black hole in the Milky Way’s center, present long ago, may explain these oddities. In a research article published this week, Chunyang Cao (Peking University) and collaborators outlined how the existing population of hypervelocity stars could be produced with the help of an intermediate-mass black hole that entered our galaxy billions of years ago and has since merged with Sgr A*.

Cao’s team outlined how Sgr A* could gain an intermediate-mass black hole companion when our galaxy captures and incorporates a dwarf galaxy — something that is thought to have happened multiple times in the Milky Way’s history. Orbiting with Sgr A* in a binary system, the intermediate-mass black hole acts as a goalkeeper, gravitationally kicking away stars that approach the galactic center. The presence of the intermediate-mass black hole prevents most binary systems from being disrupted very close to Sgr A*, where they would score the largest gravitational boost and achieve the highest velocities.

predicted and observed radius and velocity distributions for hypervelocity stars in the Milky Way

Predicted (green and blue shaded areas) and observed (gray and black lines) velocity and galactocentric radius distributions of hypervelocity stars in the Milky Way. The two observation lines show the results from the Multiple Mirror Telescope (MMT) hypervelocity star survey and the MMT survey plus proper-motion corrections from the Gaia spacecraft. The pink and purple lines show the predictions of models that do not incorporate an intermediate-mass black hole. Click to enlarge. [Cao et al. 2025]

A Model Population

Cao’s team modeled the population of hypervelocity stars that would result from this scenario and found that the velocity and radius distributions of their model population closely matched what is observed. Crucially, the model predicts that just 2.5% of hypervelocity stars should have velocities greater than 700 km/s and can accurately reproduce the population of stars that remain in the galactic center. The model also predicts that 10% of ejected stars should be intact binary systems rather than lone binary members, which is consistent with observations.

The authors posited that the most likely source of an intermediate-mass black hole in the right time frame for this scenario to play out would be the merger with the Gaia–Sausage–Enceladus dwarf galaxy about 10 billion years ago. Modeling suggests that this dwarf galaxy’s black hole was roughly 300 times less massive than the Milky Way’s central black hole, and the two black holes likely merged 10 million years ago.

Though the intermediate-mass black hole may be long gone, its influence may still be felt: the merger of the two black holes would have caused Sgr A* to recoil, potentially accounting for the motion of Sgr A* seen today.

Citation

“A Recent Supermassive Black Hole Binary in the Galactic Center Unveiled by Hypervelocity Stars,” Chunyang Cao et al 2025 ApJL 982 L37. doi:10.3847/2041-8213/adbbf2

NGC 3603

JWST observations of distant galaxies have revealed incredibly dense and bright clusters of stars in the early universe. New simulations show how these clusters may have formed through galactic disk fragmentation.

From the Early Universe to the Present Day

While JWST observations were expected to shed light on the earliest stages of galaxy evolution, certain discoveries raised more questions than answers. How structures like galactic disks, supermassive black holes, and massive star clusters developed so rapidly is an open question.

simulation results showing gas and stellar density

The gas density (left) and stellar density (right) for the main galaxy, labeled “0,” and its two galactic companions. Click to enlarge. [Adapted from Mayer et al. 2025]

In a recent research article, a team led by Lucio Mayer (University of Zurich) tackled the question of how dense star clusters like those spotted by JWST formed in the early universe. Mayer and collaborators used a hydrodynamical simulation with a resolution of just 6.5 light-years to study the aftermath of a merger of two massive gas-rich galaxies at a redshift of z = 7.6, when the universe was about 700 million years old. The resulting galaxy has a stellar mass of 80 billion solar masses, placing it on the extreme high-mass end of galaxies during this time period. The galaxy also has two galactic companions that are 8 and 400 times less massive.

In the wake of the galaxy collision, the resulting galaxy forms a dense, gas-rich disk. Because of the high density of this gas disk, fragmentation of the disk into massive star clusters occurs quickly, on timescales of a million years. In less extreme environments, the rate of star formation is moderated by feedback — for example, from supernova explosions that inject energy and momentum into star-forming clouds, heating and disrupting the clouds and slowing the rate of star formation. Here, however, fragmentation happens too fast for supernova feedback to stop it, resulting in massive, dense clumps of gas that collapse further to form star clusters.

Cosmic Gems arc

The gravitationally lensed Cosmic Gems arc, imaged by JWST at a redshift of z = 10.2 (about 500 million years after the Big Bang) bears considerable similarity to the smallest of the three galaxies in this simulation. [ESA/Webb, NASA & CSA, L. Bradley (STScI), A. Adamo (Stockholm University) and the Cosmic Spring collaboration; CC BY 4.0]

Cluster Assemblage

The central galaxy and its two smaller companions all formed star clusters through this method. The mass of the clusters appears to scale with the mass of the galaxy, with the largest galaxy forming the largest number of clusters and the most massive clusters. In total, roughly 20–30% of the total stellar mass of each galaxy forms through fragmentation of the disk, creating clusters with masses of 105–108 solar masses. The large fraction of galactic stellar mass belonging to massive star clusters, as well as the rapid time frame of their formation — the simulation spans just 6 million years — are both consistent with JWST’s observations of star clusters in the early universe.

These star clusters may provide a clue to another early universe mystery: the development of supermassive black holes. The extreme density of the simulated star clusters provides a natural avenue for the formation of intermediate-mass black holes with masses up to 105 solar masses. These black holes could sink to the center of the galaxy and coalesce to form a single supermassive black hole with a mass of 10 million solar masses — comparable in mass to many of the black holes seen with JWST.

Intriguingly, in the smallest galaxy explored in the team’s simulation, the black hole’s mass would be only a factor of a few less massive than the total mass of stars in the galaxy, potentially also explaining why some galaxies appear to have over-massive black holes.

Citation

“In Situ Formation of Star Clusters at z > 7 via Galactic Disk Fragmentation: Shedding Light on Ultracompact Clusters and Overmassive Black Holes Seen by JWST,” Lucio Mayer et al 2025 ApJL 981 L28. doi:10.3847/2041-8213/adadfe

galaxies containing green seeds

68 galaxies containing green seeds

JWST images of the full sample of 68 hydrogen-alpha emitters that contain green seeds. [Chen et al. 2025]

The peak of cosmic star formation, also called cosmic noon, took place when the universe was 2–3 billion years old. During this time, roughly half of the stellar mass present in galaxies today was formed, and the structures that existed during cosmic noon provide important clues to how galaxies evolved to their current forms. Recently, Nuo Chen (The University of Tokyo and National Astronomical Observatory of Japan) and collaborators used JWST to look back to cosmic noon and study hydrogen-alpha emitters — galaxies with prominent hydrogen-alpha emission, signaling active star formation. Within these star-forming galaxies, Chen’s team discovered compact regions of bright green emission, which they named green seeds. The images above and to the right show 68 hydrogen-alpha emitters hosting a total of 128 green seeds. The team also found 59 red seeds, which are similarly compact regions that are redder in color, indicating an older or dustier population of stars. Green seeds appear to be sites of intense star formation that has been triggered either by galaxy mergers or gravitational instabilities. What these green seeds evolve into is still unknown, but Chen and collaborators suspect that massive green seeds might migrate to the centers of their galaxies, morph into red seeds, and eventually form the central bulges of their galaxies. On the other hand, low-mass green seeds might dissipate over time. To learn more about green seeds and galaxies at cosmic noon, be sure to check out the full research article linked below.

Citation

“Compact [O iii] Emission-line Regions (“Green Seeds”) in Hα Emitters at Cosmic Noon from JWST Observations,” Nuo Chen et al 2025 ApJ 981 96. doi:10.3847/1538-4357/adad69

circumstellar disk

What happens when a newly formed star captures material from a nearby cloud? A new study provides a glimpse into how the planet-forming system AB Aurigae is under renovation.

Late Infall and Planet Formation

Gas, dust, and rocky materials swirl around recently formed stars, creating a circumstellar disk that seeds the formation of planets. Recent observations of some of these disks reveal filamentary structures of material falling into the disk, feeding the planet-forming environment with new ingredients in a process known as “late-stage infall.” According to models, this infall can significantly impact the chemical composition and dynamics within the system — shock-heating materials, forming of pressure bumps or spiral arms, and inciting gravitational instability — which complicates the picture of planet formation. 

One such system is AB Aurigae, a 3.9–4.4 million-year-old young stellar object whose circumstellar disk hosts several candidate protoplanets. Observations revealing spiral arm structures that extend outside of the disk and motions suspected to be associated with gravitational instability have led researchers to believe that AB Aurigae may be experiencing late-stage infall. However, the nature of the infall and its potential consequences remain unclear.

Finding Infall in AB Aurigae

ALMA observations of AB Aur

ALMA observations showing intensity maps of the gas and dust in the circumstellar disk of AB Aurigae. Each panel shows a different wavelength with the distribution of the material changing depending on what emission is being observed. Click to enlarge. [Speedie et al 2025]

In order to determine if AB Aurigae is truly experiencing late-stage infall and if this infall is the cause of the gravitational instability in the disk, Jessica Speedie (University of Victoria) and collaborators acquired high-resolution observations of the system with the Atacama Large Millimeter/submillimeter Array (ALMA). From their imaging, the authors find that the disk extends up to ~1,600 au in radius with spiral structure present throughout, which is consistent with the conditions of gravitational instability. 

Through modeling the motions within the disk, the authors successfully separate the regularly rotating disk component from an infalling “exo-disk” component that shows distinct motions consistent with material falling into the disk. The team identified three exo-disk streams, with S1 and S2 being in front of the disk moving away from us into the system, and S3 being smaller and behind the disk moving toward us into the system. S1 and S2 seem to meet the disk in a “merging zone” where their emission becomes indistinguishable from the main disk. The authors note that this merging zone aligns with where planet candidates have been identified. These findings support the idea that AB Aurigae is undergoing a late-stage infall event.

streams

The paths of the three identified infalling streams overlaid on the dust emission intensity maps. The merging zone is shown where S1 and S2 meet the disk, which aligns with an intensity peak in the ring of dusty emission. Click to enlarge. [Speedie et al 2025]

Implications and Impacts for AB Aurigae

Where does this infalling material come from, and how will it impact AB Aurigae? Previous optical observations show a reflection nebula of dusty material nearby, and the ALMA observations of this study show a faint emission structure that appears to align with the reflection nebula. Though further kinematic analysis is required, this suggests that the infall is coming from a small cloud that has been gravitationally captured by the star.

As the infalling material enters the disk, it can cause perturbations that create vortices and pressure bumps within the disk that trap material, facilitating the growth of protoplanets. Additionally, as this late-stage infall dumps gas and dust into the system, more material is available for planet formation. Continued studies of objects like AB Aurigae will reveal more about the dynamical impacts of late-stage infall on circumstellar disks and how those changes influence planet formation.

Citation

“Mapping the Merging Zone of Late Infall in the AB Aur Planet-forming System,” Jessica Speedie et al 2025 ApJL 981 L30. doi:10.3847/2041-8213/adb7d5

Illustration of stellar-mass black holes embedded within the accretion disk of a supermassive black hole

Gravitational-wave detectors have captured the chirps of dozens of merging black holes. Could any of these mergers have happened in the disk around a supermassive black hole?

Black Holes Around Black Holes

diagram of a black hole binary orbiting a supermassive black hole

Diagram of a binary black hole system orbiting within the disk of a supermassive black hole. The observer is located at N in this diagram. [Leong et al. 2025]

At the centers of galaxies across the universe, the disks surrounding accreting supermassive black holes — known as active galactic nuclei — provide an extreme ecosystem for stars and stellar-mass black holes. When a pair of black holes within an active galactic nucleus disk merges, the collision produces gravitational waves that can be picked up by detectors on Earth. If, from our perspective, that merger takes place behind the supermassive black hole, the gravitational-wave signal will be gravitationally lensed: split into two “images” of the same wave with slightly different properties.

Detecting a gravitationally lensed gravitational-wave signal from merging black holes would provide valuable information about the population of black holes that reside in active galactic nucleus disks, as well as the properties of the disks themselves.

plot of the constraining power of gravitational wave observations to determine the fraction of binary black hole mergers happening in active galactic nucleus disks

Constraints able to be placed on the fraction of binary black hole mergers happening in active galactic nucleus disks as a function of the number of observations, Nobs, and the distance between the binary system and the central supermassive black hole, indicated by the fill pattern. The filled area shows the values that are ruled out. This plot assumes that no gravitationally lensed gravitational waves are observed. [Adapted from Leong et al. 2025]

Lensing Likelihood

So far, no gravitationally lensed gravitational waves have been detected — but luckily, even this non-detection contains valuable information. To explore the implications of this non-detection, Samson Leong (The Chinese University of Hong Kong) and collaborators developed an analytical model that describes a binary black hole pair orbiting and merging within the disk of an active galactic nucleus. The team calculated the probability that gravitational waves from the merger of these black holes would be gravitationally lensed from the perspective of a distant observer. This probability is dependent upon the orientation of the disk relative to the viewer, as well as the distance from the binary system to the central supermassive black hole.

Then, given the fact that none of the dozens of mergers detected so far have had gravitationally lensed signals, Leong’s team constrained the fraction of observed mergers happening in active galactic nucleus disks. With only about 100 binary black hole merger observed to date, the constraining power of the non-detection is limited. For now, all that can be said is that no more than 47% of the observed mergers took place in the disks around active galactic nuclei. As the number of detected black hole mergers grows, the constraint will grow more stringent; if no lensed events have been observed after roughly 1,000 mergers have been detected, that would mean that no more than 5% of the mergers took place within an active galactic nucleus disk.

To Be Constrained

plot of the constraining power of gravitational wave observations to determine the fraction of binary black hole mergers happening in active galactic nucleus disks

Similar to the previous figure, but this time emphasizing the impact of the orbital distance of the merging black holes. The vertical dotted lines indicate the locations of potential migration traps. [Adapted from Leong et al. 2025]

This estimate is based on the assumption that all black holes in active galactic nucleus disks merge within the migration trap nearest the central supermassive black hole. Several migration traps — particular orbital radii within the disk where black holes are expected to collect — are predicted to exist. If the black holes instead merge within a migration trap at a much larger radius, many more observations will be needed to narrowly constrain the number of mergers happening within accretion disks.

Future observations may yield new information about active galactic nucleus accretion disks. In particular, it may be possible to discern the minimum size of an accretion disk, as well as where within the disk binary black holes are most likely to merge.

Citation

“Constraining Binary Mergers in Active Galactic Nuclei Disks Using the Nonobservation of Lensed Gravitational Waves,” Samson H. W. Leong et al 2025 ApJL 979 L27. doi:10.3847/2041-8213/ad9ead

Andromeda Galaxy in X-rays

The Chandra X-ray Observatory has been keeping tabs on the Andromeda Galaxy for years. What do these observations tell us about the supermassive black hole at the center of our nearest major galactic neighbor?

Examining a Moderate Black Hole

Comparison of the sizes of two black holes: M87* and Sagittarius A*

Illustration of the relative sizes of the supermassive black holes at the center of the Milky Way (Sgr A*) and Messier 87 (M87*). Click to enlarge. [EHT collaboration (acknowledgment: Lia Medeiros, xkcd); CC BY 4.0]

Essentially all massive galaxies are expected to harbor a supermassive black hole. The properties of supermassive black holes are varied, spanning a wide range of masses and activity levels. The two best-studied black holes fall at opposite ends of the mass spectrum: the Milky Way’s, at 4 million solar masses, and Messier 87’s, at 6 billion solar masses.

The Milky Way’s nearest massive galactic neighbor, the Andromeda Galaxy, provides an opportunity to study a supermassive black hole that falls between these two mass extremes. Andromeda’s central supermassive black hole has a moderate mass of roughly 100 million solar masses and is known to exhibit flaring behavior. The Chandra X-ray Observatory has provided a wealth of X-ray observations of Andromeda’s center since 2000. What do these data tell us about what Andromeda’s black hole has been up to recently?

Journey to Center of Andromeda

In a recent publication, Stephen DiKerby (Michigan State University) and collaborators examined the behavior of Andromeda’s central supermassive black hole over the past 15 years. The team amassed a large sample of Chandra observations of Andromeda’s center from 2009 to the present, including all of the observations in the public archive. These data were stitched into an extensive light curve to investigate the supermassive black hole’s behavior.

X-ray sources near the center of the Andromeda Galaxy

Stacked image from Chandra’s High Resolution Camera showing the locations of the four X-ray sources near Andromeda’s center. [DiKerby et al. 2025]

This task is easier said than done, as the supermassive black hole isn’t the only source of X-rays in Andromeda’s center. Andromeda’s center contains a crowded cluster of four X-ray sources named P2, N1, S1, and SSS. P2 is associated with the supermassive black hole, and the nature of the remaining three sources is unknown. Because these sources are so close to one another, their emission overlaps, and each source varies on its own individual timescale. DiKerby’s team simultaneously modeled the flux from each source, reconstructing their individual light curves and extracting the X-ray behavior of the black hole.

Staying Active

X-ray count rate for the source P2 over the last 15 years

Count rate for the X-ray source P2 over the past 15 years. An X-ray flare is highlighted in red. Click to enlarge. [DiKerby et al. 2025]

The light curve for Andromeda’s supermassive black hole shows a roughly constant count rate punctuated by a flare in 2013. The count rate from 2009 to 2016 is a continuation of the elevated flux state that began after a flare in 2006. The gap in observations between 2016 and 2021 makes it difficult to say for sure, but observations from 2022 to the present suggest that the black hole may still be in an elevated flux state today. Given the gaps in the data, it’s impossible to know if the elevated flux observed today is due to the flare seen in 2006, or if it’s due to subsequent flares.

Another aspect of the black hole’s X-ray emission is its hardness ratio: a measure of whether more high-energy or low-energy X-rays were emitted. The team found that the hardness ratio was about the same during the 2013 flare as it was during non-flaring times. This suggests that the emission mechanism is the same for both flaring and non-flaring states.

Interestingly, the emission from Andromeda’s black hole has a similar hardness ratio to the Milky Way’s black hole when both black holes are quiescent, but the flares from Andromeda’s black hole are significantly softer (i.e., they have a greater proportion of low-energy X-rays) than those from the Milky Way’s black hole. This suggests that more investigation of the flaring mechanisms of the two black holes is needed, as is continued monitoring of the black hole at the heart of our neighboring galaxy.

Citation

“Fifteen Years of M31* X-Ray Variability and Flares,” Stephen DiKerby et al 2025 ApJ 981 50. doi:10.3847/1538-4357/adb1d5

Artist's impression of the view from one of the planets orbiting Barnard's Star

Following decades of disproven claims, four small exoplanets have been confirmed to orbit Barnard’s Star, the second-closest star system to Earth after Alpha Centauri.

System Under Scrutiny

illustration of the location of Barnard's Star and other nearby stars

Illustration of the nearest star systems to Earth. Barnard’s Star is the nearest single star and second-nearest star system; the triple-star Alpha Centauri system is closer. The remaining two systems shown here contain brown dwarfs. [NASA/Penn State University]

Just 6 light-years away, Barnard’s Star is a well-studied 10-billion-year-old M dwarf with a mass of 0.16 solar mass. Finding exoplanets around Barnard’s Star has been something of a white whale for astronomers for more than half a century; starting in the 1960s, researchers have claimed to have spotted various planets around Barnard’s Star, from distant Jupiter-mass companions to close-in super-Earths. Each of these claims has been refuted.

Now, the white whale appears to have been caught at last. Just last November, researchers reported the discovery of a planet orbiting Barnard’s Star with a period of 3.154 days. The data hinted at the presence of three other planets, but these candidates could not be confirmed. In a new research article published today, Ritvik Basant (University of Chicago) and collaborators leveraged years of data to confirm that Barnard’s Star hosts not just one, but four planets.

Continuing the Search

The team observed Barnard’s Star from 2021 to 2023 with the M dwarf Advanced Radial velocity Observer Of Neighboring eXoplanets (MAROON-X), a spectrograph tailored to the properties of M dwarfs. Basant and coauthors searched the MAROON-X data for subtle shifts in spectral lines indicating the change in the star’s radial velocity from planets tugging on the star as they orbit.

phase-folded fits to radial velocity data of Barnard's Star

Phase-folded fits to the MAROON-X data from this work (red triangles) and the data from other authors (beige and gray squares). Click to enlarge. [Adapted from Basant et al. 2025]

This search was complicated by periodic signals related to the star’s activity cycle and rotation period, which the authors modeled and removed from the data. With these interfering signals removed, two clear signals emerged from the previously detected planet Barnard b and the planet candidate Barnard d, as well as a less-robust signal from the candidate Barnard c. The team then iteratively modeled and removed statistically significant signals until no significant signals remained in the data. This analysis showed that the fit statistics improved with each successive planet, confirming the presence of planets b, c, and d.

After removing the signals from the three confirmed planets, a tiny amount of signal remained at the period of the planet candidate e — but not enough to confirm its presence. To investigate further, Basant’s team jointly modeled their MAROON-X data with the data that led to the discovery of Barnard b last November. This improved the statistics and strengthened the evidence for planet e, confirming its existence.

More to Learn

Ultimately, Basant’s team confirmed the presence of four planets with minimum masses between 19% and 34% of Earth’s mass. Barnard e is possibly the lowest-mass planet to be detected using the radial-velocity method.

compactness of the Barnard's Star system compared to other compact M-dwarf systems

Compactness of the Barnard’s Star system compared to other compact M-dwarf planetary systems. Click to enlarge. [Adapted from Basant et al. 2025]

These planets are in remarkably close quarters, with periods of just 2.34, 3.15, 4.12, and 6.74 days. Is such a compact setup likely to be stable? Using a machine-learning algorithm, the team showed that if the planets have perfectly circular orbits, the system is stable long term. However, using the best-fitting orbital parameters — which are consistent with circular orbits within 1.5 sigma — the system became unstable within just 2,000 years. More work is needed to understand the orbits of the newfound planets and the long-term stability of the system.

Now, for the million-dollar question: could any of these planets be habitable? None of the newly discovered planets lie within Barnard’s Star’s habitable zone, which spans orbital periods from 10 to 42 days. The current data also rule out the presence of habitable-zone planets with masses greater than 0.57 Earth mass, though smaller planets are still possible.

Citation

“Four Sub-Earth Planets Orbiting Barnard’s Star from MAROON-X and ESPRESSO,” Ritvik Basant et al 2025 ApJL 982 L1. doi:10.3847/2041-8213/adb8d5

JWST image of the Flame Nebula, NGC 2024

For the first time, researchers have identified a turnover in the initial mass function of a star cluster. The new finding suggests that newborn stars and sub-stellar objects become more prevalent with decreasing mass down to the turnover at 12 Jupiter masses. The study also suggests that the fundamental lower limit of the process through which stars and brown dwarfs form may lie around 3 Jupiter masses.

How Low Does the Mass Function Go?

Take a cloud of turbulent hydrogen gas and give it time to coalesce into a cluster of stars. How many stars do you get, and how massive are they? These questions are answered by the initial mass function, which describes the mass distribution of newborn objects in a cluster. The initial mass function decreases from just a few stars at high masses to myriad stars, brown dwarfs, and planetary-mass objects at low masses.

However, the increase in number with decreasing mass doesn’t go on forever; star-formation theories predict that the initial mass function turns over, with objects becoming less prevalent below the turnover mass. The turnover of the initial mass function has never been observed — until now.

Searching for the Turnover

NGC 2024, the Flame Nebula, in visible light

This Hubble Space Telescope view of NGC 2024 shows the thick clouds of dust throughout the region. [NASA, ESA, and N. Da Rio (University of Virginia); Processing: Gladys Kober (NASA/Catholic University of America)]

In a research article published today, a team led by Matthew De Furio (University of Texas at Austin) presented their investigation of the low-mass end of the initial mass function in a nearby star cluster. The target of this search was NGC 2024 (the Flame Nebula), a star-forming region that is less than 1 million years old, just 1,300 light-years away, and suffused with thick clouds of dust.

NGC 2024’s extreme dustiness was actually a plus for this investigation. Though dust is often a hindrance for astronomers, in this case it helped to screen out background sources that might contaminate the exploration of the cluster. Using JWST’s Near-Infrared Camera (NIRCam), De Furio’s team picked out the faint point sources indicating low-mass objects in the star cluster. Existing star-finding algorithms were stymied by the dusty nebula, so the team developed a new automated routine, which detected 100 point sources in their images.

De Furio’s team trimmed this initial sample by removing background objects and sources that saturated the detector, were too heavily obscured by dust, or were likely more massive than the tail end of the initial mass function being studied here. This left 28 objects, for which the team used evolutionary models to calculate the masses. The least-massive object in the sample weighed in around 3 Jupiter masses.

histogram of object masses studied in NGC 2024

Histogram showing the masses of the 28 cluster members used in this study. The vertical red dashed line shows the breakpoint mass, or where the initial mass function turns over. [Adapted from De Furio et al. 2025]

Sampling Sub-Stellar Sources

Using Bayesian statistics, the team constructed the initial mass function for the cluster given the masses of the detected objects and the varying sensitivity across the field of view; NIRCam can detect objects down to 0.5–2.0 Jupiter masses, depending on the amount of obscuring dust. The resulting initial mass function shows a clear turnover around 12 Jupiter masses — the first time this feature has been identified for any cluster.

Although NIRCam is sensitive to objects down to 0.5–2.0 Jupiter masses, the team found no objects below 3 Jupiter masses. This suggests that the end of the initial mass function — the minimum-mass of an object that a turbulent cloud can create through fragmentation — may lie around this mass. Previous work predicted the end of the initial mass function to be between 1 and 10 Jupiter masses.

De Furio and coauthors plan to continue their investigation of the initial mass function in NGC 2024, using JWST spectra to confirm the masses of the objects in the sample, as well as to investigate whether the mass function varies from the center of the cluster, where this study was performed, to its outskirts.

Citation

“Identification of a Turnover in the Initial Mass Function of a Young Stellar Cluster Down to 0.5 MJ,” Matthew De Furio et al 2025 ApJL 981 L34. doi:10.3847/2041-8213/adb96a

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