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Ursa Major III/UNIONS 1

The smallest satellite of the Milky Way, Ursa Major III/UNIONS 1, has posed a mystery since its discovery. Is this smattering of stars a dwarf galaxy or a star cluster?

The Milky Way’s Mysterious Companion

In 2024, researchers discovered a small group of stars orbiting the Milky Way at a distance of about 33,000 light-years in data from the Ultraviolet Near Infrared Optical Northern Survey (UNIONS). This collection of stars amounts to just 16 solar masses and shines with only 11 times the Sun’s luminosity, making it 4–5 times less massive than the next faintest Milky Way satellite.

Considering its small stellar population and small size (its half-light radius is 10 light-years, which is typical for a globular cluster), this satellite appeared at first glance to be an ordinary star cluster. But early data also suggested that the stars had an unusually high velocity dispersion, which could mean that they are nestled within a dense and massive dark matter halo — making the satellite not a star cluster but a dwarf galaxy.

Reflecting this uncertainty, the satellite has two names: Ursa Major III (its dwarf galaxy designation) and UNIONS 1 (its star cluster classification). Can new data clear up the confusion?

New Dispersion Data

In a recent research article, William Cerny (Yale University) and collaborators described new observations of 16 stars in Ursa Major III/UNIONS 1 with the Keck II telescope.

plots of velocity dispersion

Normalized likelihoods for the velocity dispersion based on the initial data (left) and the new data (right). The initial velocity dispersion measurement of 3.7 km/s is ruled out by the new measurements. Click to enlarge. [Cerny et al. 2026]

Crucially, these measurements enabled a new estimation of the system’s velocity dispersion. In contrast to earlier work that found a high velocity dispersion of 3.7 km/s, the new data favor a value of just 0.1 km/s and rule out values above 2.3 km/s. While these results do not unequivocally preclude the possibility of dark matter, they strongly favor little to no dark matter. This goes against the initial evidence in favor of the dwarf galaxy hypothesis.

These observations also helped to confirm one candidate binary pair and identify three new binary candidates. (This large binary fraction, though interesting, isn’t diagnostic, as both star clusters and dwarf galaxies are known to host high proportions of binary stars.)

Further Evidence

Cerny’s team conducted a further test of the Ursa Major III/UNIONS 1’s identity by measuring the metallicities of its 12 brightest stars. They found that the stars are metal poor and have little metallicity dispersion. This finding again points to a star cluster, in which stars form in a single epoch, rather than a dwarf galaxy, which may feature multiple generations of stars with different degrees of chemical enrichment.

metallicity dispersion vs velocity dispersion for faint Milky Way satellites

Upper limits on velocity dispersion and metallicity dispersion for Ursa Major III/UNIONS 1 compared to other faint Milky Way satellites. There are no confirmed dwarf galaxies for which the metallicity dispersion and velocity dispersion are both lower than for Ursa Major III/UNIONS 1. Click to enlarge. [Cerny et al. 2026]

Based on the newly collected evidence, Cerny and collaborators concluded that Ursa Major III/UNIONS 1 is likely a star cluster, though the possibility that it’s instead a dwarf galaxy could not be ruled out. If the star cluster classification holds, Ursa Major III/UNIONS 1 would be the least luminous metal-poor star cluster known. This classification may also imply the presence of a large number of compact objects — white dwarfs, neutron stars, or black holes — to keep the cluster intact, filling the role that dark matter would have played.

Going forward, further chemical abundance measurements of Ursa Major III/UNIONS 1 may still sway the argument one way or another. And as facilities like the Vera C. Rubin Observatory and the Euclid space telescope continue to collect data, other extreme Milky Way satellites may come to light.

Citation

“No Observational Evidence for Dark Matter Nor a Large Metallicity Spread in the Extreme Milky Way Satellite Ursa Major III/UNIONS 1,” William Cerny et al 2026 ApJL 999 L8. doi:10.3847/2041-8213/ae29b8

AT 2024wpp

Across the universe and across the electromagnetic spectrum, flares, bursts, and waves are making their mark on the sky. Today, we’re giving an update on three exceptional transients that have been discussed previously on AAS Nova: AT 2024wpp, GRB 250702B, and GW231123.

The Most Luminous Known Fast Blue Optical Transient, AT 2024wpp

Discovered in September 2024, AT 2024wpp burst onto the scene as the most luminous example of a fast blue optical transient (FBOT) — a growing class of events that are more luminous, more rapidly evolving, and bluer than supernovae. In October 2025, astronomers published their findings on AT 2024wpp’s X-ray and radio evolution. Just last month, a research team led by Natalie LeBaron (University of California, Berkeley) described the results of a complementary investigation of the transient’s ultraviolet, optical, and near-infrared behavior.

AT 2024wpp light curves

The ultraviolet through infrared light curve of AT 2024wpp. Results from different filters are offset vertically for clarity. [LeBaron et al. 2026]

LeBaron and collaborators collected ultraviolet through infrared photometry as well as optical and infrared spectroscopy to characterize the event. These efforts led to the first-ever collection of ultraviolet data during the rising period of an FBOT. Overall, the data reveal an almost featureless spectrum from the optical through the near-infrared and a persistent, weeks-long thermal continuum, both of which seem to be common for FBOTs.

The observations point to a luminous central energy source that continuously injects energy into its surroundings, powers multiple outflows, and ionizes the ejected material. These features can be explained by the rapid accretion of material by a neutron star or a black hole, in which the accreted material forms a disk and is ejected in both a fast-moving polar outflow and a slower-moving equatorial outflow.

Though the exact nature of the central object powering AT 2024wpp’s intense radiation isn’t known — the extreme luminosity of this event points to a stellar-mass black hole as the central object, but a neutron star cannot be ruled out — the overall picture of this event matches what has been found for previous FBOTs. Upcoming instruments and surveys will discover more FBOTs and spot them earlier in their evolution, transforming our understanding of these rare transients.

An Ultra-Long X-Ray and Gamma-Ray Transient, GRB 250702B

GRB 250702B is an exceptional gamma-ray burst: an intense flash of gamma rays lasting anywhere from a few milliseconds to several hours. GRB 250702B is unprecedented among events in this class because it features several bursts arising over the course of roughly 7 hours, plus X-ray precursor emission going back 24 hours before discovery. We’ve previously covered JWST follow-up observations of GRB 250702B, and today we’re taking a look at its high-energy evolution.

Swift and Chandra observations of GRB 250702B

Swift (black circles) and Chandra (blue circles) observations of GRB 250702B. Click to enlarge. [Adapted from O’Connor et al. 2025]

Brendan O’Connor (Carnegie Mellon University) and collaborators collected observations of GRB 250702B from the Neil Gehrels Swift Observatory, the Nuclear Spectroscopic Telescope Array (NuSTAR), and the Chandra X-ray Observatory, stretching from just half a day after the transient was discovered to 65 days later. The team aimed to determine whether GRB 250702B fit the profile of an ultra-long gamma-ray burst accompanying a star’s collapse into a black hole or a relativistic tidal disruption event, in which a star is ripped apart by a massive black hole.

O’Connor’s team found that GRB 250702B didn’t fit neatly into either category. While many of the event’s properties closely resembled those of ultra-long gamma-ray bursts, the hours-long gamma-ray emission and the precursor X-ray emission were outliers. Certain features are more easily reconciled with the tidal disruption of a main-sequence star by an intermediate-mass black hole, but others are similarly hard to account for. Finding the smoking gun for a tidal disruption event — an abrupt shutoff to the X-ray emission — will require long-term X-ray monitoring.

Because GRB 250702B has features that align with both ultra-long gamma-ray bursts and tidal disruption events, O’Connor and coauthors proposed scenarios that combine aspects of both possibilities: either a stellar-mass black hole tidally disrupting a star or the merger of a helium star with a stellar-mass black hole.

The Most Massive Gravitational Wave Source, GW231123

When compact objects like neutron stars and black holes merge, their collisions produce ripples in spacetime that are accessible to detectors on Earth. One of the most exciting events so far from the fourth gravitational wave transient catalog is GW231123, which appears to be caused by the merger of two black holes, each at least 100 times the mass of the Sun. The LIGO, Virgo, and KAGRA (LVK) collaborations reported a total mass of 238 solar masses for the event, which would make it the most massive binary black hole merger yet observed.

In a recent article, Ilya Mandel (Monash University and OzGrav) urged caution when interpreting seemingly extreme events like GW231123. Mandel’s argument is a statistical one, highlighting how certain aspects of the Bayesian techniques used to estimate GW231123’s mass could have skewed that estimate toward an artificially high value.

posterior probability distributions for maximum mass of GW231123 and all gravitational wave events seen by LVK

The posterior probability distribution on the maximum mass as derived from the total LVK catalog (blue), compared to the posterior for GW231123 (orange; multiplied by 10 for visibility). Click to enlarge. [Mandel 2026]

In general, Mandel points out, events that appear extreme are simply more likely to be statistical fluctuations. As an example, consider a large group of people having their heights measured, but with very large measurement uncertainties. The highest reported height in this situation is likely to be associated with a reasonably tall person whose height had a large upward measurement error, not necessarily someone who is exceptionally tall. Similarly, the reported most massive gravitational wave source is likely not quite as massive as claimed.

Mandel also demonstrated how considering the population of black hole mergers detected by LVK as a whole can provide a different perspective on GW231123’s likely mass. By analyzing this entire sample together, Mandel showed that the maximum mass for the population of merging black holes appears to be considerably lower than the mass estimated for GW231123, again raising the possibility that the true mass of the event is lower than the current estimate.

Ultimately, Mandel pointed out that at present it’s impossible to distinguish between three possibilities: that GW231123 is a true outlier among the population of merging black holes, representing a member of a new, unexplored population; that it’s a member of the same population as other black hole mergers seen by LVK but inhabits an extreme high-mass tail; or its high mass estimate is a statistical artifact.

Citation

“The Most Luminous Known Fast Blue Optical Transient AT 2024wpp: Unprecedented Evolution and Properties in the Ultraviolet to the Near-Infrared,” Natalie LeBaron et al 2026 ApJL 997 L10. doi:10.3847/2041-8213/ae2910

“Comprehensive X-Ray Observations of the Exceptional Ultralong X-Ray and Gamma-Ray Transient GRB 250702B with Swift, NuSTAR, and Chandra: Insights from the X-Ray Afterglow Properties,” Brendan O’Connor et al 2025 ApJL 994 L17. doi:10.3847/2041-8213/ae1741

“What Is the Most Massive Gravitational-Wave Source?” Ilya Mandel 2026 ApJL 996 L4. doi:10.3847/2041-8213/ae278d

illustration of a tidal disruption event

Where do the mysterious cosmic messengers called neutrinos come from? Researchers search for a connection between neutrinos and accretion flares from black holes.

Seeking Connections

A photograph of the IceCube Neutrino Observatory

The IceCube Laboratory at the Amundsen-Scott South Pole Station in Antarctica. [Felipe Pedreros, IceCube/NSF]

Neutrinos — neutral, nearly massless elementary particles — are generated by natural particle accelerators across the universe. Facilities like the IceCube Neutrino Observatory, which employs detectors buried a mile deep in Antarctic ice, spot neutrinos from distant astrophysical sources, but matching these neutrinos to their sources is no easy task.

Thus far, researchers have discovered a compelling connection between neutrinos and distant active galactic nuclei, which are powered by the accretion of gas onto supermassive black holes. The active galactic nucleus phase of a black hole tends to be long lived, with activity lasting hundreds of thousands of years or more, making these nuclei lasting sources of neutrinos.

Sampling Flares

Recently, researchers have found tentative evidence for coincident neutrinos from short-lived accretion events like tidal disruption events, in which a star is torn apart and accreted by a black hole. These studies have uncovered a potential connection between neutrinos and tidal disruption events with mid-infrared echoes, signaling the presence of obscuring dust.

map of sampled black hole flares and neutrinos

A map of the locations of the neutrinos (yellow diamonds) and black hole flares (blue circles) on the sky. Click to enlarge. [Wang et al. 2026]

To investigate whether there is any connection between neutrinos and short-lived accretion events, Megan Wang (Massachusetts Institute of Technology) and collaborators first compiled a sample of 99 black hole flares observed by NEOWISE. These flares were selected for their strong mid-infrared emission, location in the nuclei of galaxies, and a fast rise and slow decline. The team applied additional selection criteria to remove other bright transients, such as supernovae, and retain only tidal disruption events and active galactic nucleus accretion flares.

For the neutrino side of the equation, Wang’s team turned to IceCube’s most recent catalog of neutrino events from astrophysical sources. In total, they compiled a sample of 68 events for which the odds of the source being astrophysical was greater than 50%, and for which the directional uncertainty was below 50.

No Coincidences

For a flare and a neutrino to be considered spatially and temporally coincident, the neutrino must arrive within one year of the flare, and the flare must fall within the 90% certainty contours of the neutrino. Ultimately, Wang’s team found no neutrinos that fulfilled both criteria and only one that was spatially but not temporally coincident with a black hole flare.

This result appears to be at odds with previous studies that found several neutrinos that were coincident with tidal disruption events. Wang and collaborators noted that while this work finds no association between neutrinos and black hole flares, it’s possible that the team is studying a different population of accretion events.

infrared fluxes of black hole flares

Infrared fluxes of the flares sampled in this study (magenta), optically identified flares (gray), and the tidal disruption events thought to have coincided with neutrino detections (green and gold dashed lines). [Wang et al. 2026]

For example, while the previous neutrino-associated flares were flagged for having strong mid-infrared emission, they differ from the current sample in that they were detected at optical wavelengths. The flares sampled in this work were detected in the mid-infrared and have little to no optical emission, potentially highlighting intrinsic differences compared to previous samples.

Future work, including studies that leverage upcoming datasets from the Vera C. Rubin Observatory, the Roman Space Telescope, and the NEO Surveyor, can address this question further and search for connections with specific populations of black hole flares.

Citation

“Testing the Association of Supermassive Black Hole Infrared Flares and High-Energy Neutrinos,” Megan Wang et al 2026 ApJL 998 L29. doi:10.3847/2041-8213/ae3f90

LMC

Small galaxies are expected to have even smaller companion galaxies, according to current cosmological models. A recent study takes a look at one dwarf galaxy and finds more tiny companions than anticipated. 

Cosmology Predicts Satellite Galaxies

Critical to our understanding of dark matter and galaxy formation, satellite galaxies are small, faint galaxies surrounding a larger host galaxy. The leading cosmological model, known as lambda cold dark matter (ΛCDM), predicts dark matter halos of all masses to host smaller dark matter subhalos whose luminous counterparts make up satellite galaxies. 

While the satellite systems of Milky Way-like galaxies are well characterized, the companions of dwarf galaxies are less well studied. Despite being significantly smaller, dwarf galaxies should also host even smaller satellites galaxies around them. These satellites push the limits of observations — extremely faint and hard to distinguish from background sources, dwarf galaxy satellites require innovative imaging techniques to discover. 

In recent years, new surveys have identified a number of satellites around dwarf galaxies that appear broadly consistent with ΛCDM predictions, though the sample size is still small. Continuing the search for little galaxies’ little galaxies is critical to testing the ΛCDM framework and will further our understanding of galaxy formation on the smallest scales. 

DDO 161 satellite map

Satellite candidates around DDO 161 as they appear on the sky. Click to enlarge. [Li et al 2026]

Searching for Satellites Around DDO 161

Diving into a specific system, a team led by Jiaxuan Li (李嘉轩) from Princeton University began an in-depth search for satellites around the dwarf galaxy DDO 161. Prior to this study, DDO 161 was known to have one companion, UGCA 319, that is about 10 times smaller than its host. 

The authors used data from the Legacy Surveys to search for satellites within about 400,000 light-years of DDO 161 — the expected reach of DDO 161’s gravitational influence. After removing bright sources and smoothing the image to bring out very faint objects, they found eight satellite galaxy candidates including UGCA 319.

Images of the satellite candidates of DDO 161 from the Legacy Surveys. The top row shows the confirmed satellites, and the bottom row shows the rejected satellites. Click to enlarge. [Li et al 2026]

To confirm if these candidates were actual satellites of DDO 161, the team performed follow-up observations of the seven unconfirmed sources with the Magellan 6.2-meter telescope. Using the surface brightness fluctuation technique — measuring how a galaxy’s light fluctuates from pixel to pixel to determine its distance — the authors confirmed that three of the candidates are located at a similar distance to DDO 161 and are likely satellites. With the previously confirmed companion UGCA 319, DDO 161 has four satellite galaxies with stellar masses above 250,000 solar masses, making it the most satellite-rich dwarf galaxy known to date. 

Too Many Satellites

How does DDO 161 compare to theoretical predictions for galaxies of its size? The authors ran cosmological simulations to quantify the expected satellite populations around galaxies of similar size to DDO 161. Just as it is an outlier observationally, DDO 161 is also an outlier when compared to the simulation results that predicted fewer satellite galaxies in similarly massive host systems. 

DDO 161 Satellite Population Comparison

Number of satellites versus host galaxy stellar mass for DDO 161 (red circle) compared to other observed low-mass galaxy satellite systems (data points) and predicted ranges from simulations (blue regions). DDO 161 is a clear outlier, having a high number of satellites for the host galaxy’s stellar mass. Click to enlarge. [Modified from Li et al 2026]

Making sure to cover all bases, the authors considered their observational uncertainties, limitations of the simulations, different relationships between stellar and dark matter halo mass, and possible environmental effects. None of these checks could, at this point, sufficiently explain the satellite overabundance in DDO 161, introducing a possible “too-many-satellites” problem for this galaxy. A larger sample of satellites around dwarf galaxies is necessary to better understand this discrepancy, test our current cosmological framework, and give new insight into galaxy formation on small scales. 

Citation

“A Possible ‘Too-many-satellites’ Problem in the Isolated Dwarf Galaxy DDO 161,” Jiaxuan Li et al 2026 ApJL 998 L24. doi:10.3847/2041-8213/ae3ddd

GD 362 and its debris disk

Roughly 25–50% of the white dwarfs in the solar neighborhood are “polluted,” showing signs of accretion of metal-rich planetary material. New JWST observations reveal the contents of the debris disk surrounding one of the most polluted white dwarfs in the solar neighborhood.

Heavy-Metal Pollution

Hubble Space Telescope images of stars and a white dwarf

A white dwarf is faintly visible at the center of this Hubble Space Telescope image, tucked between the two brightest stars. [NASA, ESA and H. Richer (University of British Columbia); CC BY 4.0]

White dwarfs are the remnants of low- to intermediate-mass stars that have ceased core fusion and evolved off the main sequence. The presence of metals in white dwarf atmospheres shows that it’s common for white dwarfs to accrete rocky materials from tidally disrupted bodies in their surroundings, allowing for a post-mortem investigation of the composition of rocky worlds in far-off planetary systems.

One of the most impressively metal-polluted white dwarfs known is GD 362, which is surrounded by a dusty debris disk and shows evidence for at least 16 accreted elements. GD 362 is a helium-atmosphere white dwarf, but previous observations have also shown that it has an unusually large amount of hydrogen in its atmosphere — a possible sign that the star has consumed one or more bodies containing water or ice.

Disk Investigation

spectrum of GD 362

Mid-infrared JWST spectrum and photometry of GD 362, with additional spectral and photometric data from the Spitzer Space Telescope. Click to enlarge. [Reach et al. 2025]

Now, a team led by William Reach (Space Science Institute) has obtained JWST spectra of GD 362 to examine the material in its disk directly. Using both the Near-Infrared Camera (NIRCam) and the Mid-Infrared Instrument (MIRI), the team collected spectra of the white dwarf from 0.6 to 17 microns, with additional photometric measurements at 18, 21, and 25.5 microns. The JWST spectra show bright emission from the white dwarf at wavelengths shorter than 2.5 microns, with a clear signature of disk material at longer wavelengths, including a strong emission feature at 9–11 microns.

Reach’s team modeled the JWST spectra to constrain the mineralogy of the disk and its physical attributes. They found that the disk likely stretches from 140 to 1,400 times the white dwarf’s radius and contains both silicates and carbon, with the prominent emission feature pointing to olivine and pyroxene silicate minerals. The elements seen in the white dwarf’s atmosphere are generally similar to those observed in the disk, strengthening the connection between accretion from the disk and the metals present in the white dwarf’s spectrum.

Hydrogen Mismatch

The new JWST data furthered the investigation of GD 362’s atmospheric hydrogen and the possibility that it’s accreting water- or ice-rich material. Though water molecules have spectral features in the range studied, none of these features appeared in the JWST spectra of GD 362. This isn’t necessarily a surprise; the team showed that water molecules in the disk would quickly be split apart by GD 362’s high-energy radiation.

JWST images of GD 362

Left column: Three JWST images of GD 362. Right: The same images with the white dwarf’s point spread function subtracted. [Reach et al. 2025]

Curiously, though, Reach’s team found little evidence for hydrogen-rich material within the disk that could be the source of the white dwarf’s atmospheric hydrogen. Because accreted hydrogen would remain in the star’s atmosphere indefinitely, the presence of atmospheric hydrogen could be evidence that the white dwarf accreted water-rich material from its debris disk in the past, though the materials currently present in the disk are dry.

In addition to investigating GD 362’s disk, Reach and collaborators searched for planets orbiting the white dwarf. Though they found none, they were able to rule out the presence of companions with masses greater than 25 times the mass of Jupiter.

Citation

“Composition of Planetary Debris Around the White Dwarf GD 362,” William T. Reach et al 2025 ApJ 994 195. doi:10.3847/1538-4357/ae11a9

Taurus Molecular Cloud

To probe the chemistry of cold pre-stellar cores, researchers investigated Lynds 1544 and made the first-ever spatially resolved map of methanimine — a simple molecule that may be involved in the formation of amino acids in space — in this environment.

Astrochemistry at Work

chemical structure of methanimine

The chemical structure of methanimine, CH2NH. [Ben Mills]

Peering across the galaxy, astronomers have discovered a broad and growing array of molecules, revealing the rich chemistry taking place everywhere from molecular clouds to protoplanetary disks. A central question in astrochemistry is where, when, and how molecules form in space, and how molecules might survive the various phases of star formation and be incorporated into newborn planetary systems, where they could play a role in the construction of complex organic molecules necessary for life.

Today’s research article, led by Yuxin Lin (Max Planck Institute for Extraterrestrial Physics), focuses on a molecule called methanimine (CH2NH). CH2NH is the simplest molecule in a class of nitrogen-containing compounds called imines, and it’s thought to be an important stepping-stone in the creation of amino acids.

Taurus Molecular Cloud

A Herschel Space Observatory view of the Taurus Molecular Cloud, where L1544 resides. This star-forming region is about 450 light-years away. [ESA/Herschel/PACS, SPIRE/Gould Belt survey Key Programme/Palmeirim et al. 2013]

First Spatially Resolved Map

Lin and collaborators searched for CH2NH within Lynds 1544 (L1544), a pre-stellar core in the nearby Taurus Molecular Cloud. Pre-stellar cores — condensed, gravitationally bound clumps of cold gas within a molecular cloud — represent an early stage in the process of star formation, just before the gas collapses to form a protostar.

L1544 has been the subject of many astrochemical investigations, and previous studies have revealed that it contains a complex, chemically differentiated envelope surrounding a dense core in which many chemical species have frozen out onto dust grains, leaving the gas depleted of these species.

Using the Institute for Radio Astronomy in the Millimetre Range 30-meter radio telescope, Lin’s team mapped the distribution of CH2NH across L1544. This isn’t the first time that CH2NH has been detected in a cold pre-stellar core, but it’s the first time that the molecule has been mapped in a spatially resolved way in such an environment. This allows researchers to compare the distribution of CH2NH to other molecules in the core and build an understanding of the chemistry behind its formation.

The Chemistry of CH2NH

abundance of methanimine in L1544

Abundance of CH2NH in L1544. The contours indicate the column density of molecular hydrogen. The crosses show where the emission from other chemical species is at its peak. Click to enlarge. [Adapted from Lin et al. 2026]

The new observations show that CH2NH can be found throughout L1544, with clear spatial trends in the molecule’s abundance: higher near the outer layers of the core, lower near the center. Using chemical models to aid their analysis, Lin’s team showed that the observed distribution suggests formation pathways driven by moderate amounts of ultraviolet radiation.

In the outer layers of the core, where the gas is less dense, ultraviolet light creates reactive chemical species called radicals that can spur the formation of CH2NH. In the interior of the core, dense gas prevents the incursion of ultraviolet light. This means that few radicals are created that can help form CH2NH molecules, and the molecules that do form tend to freeze out onto dust grains, disappearing from the gas.

The discovery of CH2NH throughout L1544 suggests that the precursors to complex organic molecules can form during the cold, calm pre-stellar core phase of star formation, allowing these molecules to be inherited by nascent planetary systems.

Citation

“First Mapping of Prebiotic Molecule CH2NH in a Prestellar Core,” Yuxin Lin et al 2026 ApJL 996 L32. doi:10.3847/2041-8213/ae2c76

galaxy cluster MACS J0416.1-2403

JWST images of galaxy clusters are rich with gravitationally lensed galaxies. Recent work examines a source in a lensed galaxy that may offer a glimpse into the lives of massive stars billions of years ago.

Cosmic Gems arc

JWST image of the gravitationally lensed Cosmic Gems arc, in which multiple individual star clusters are visible. [ESA/Webb, NASA & CSA, L. Bradley (STScI), A. Adamo (Stockholm University) and the Cosmic Spring collaboration; CC BY 4.0]

Glimpses of Single Stars

Galaxy clusters, the largest gravitationally bound structures in the universe, create the conditions necessary for astronomers to perform an extraordinary feat: examine individual massive stars and star clusters at far greater distances than our telescopes can typically achieve. This is possible thanks to gravitational lensing, the bending of spacetime by an immense mass, which warps and magnifies the light from more distant objects.

These glimpses of single stars and star clusters offer a rare chance to study massive stars in our universe’s distant past directly. In particular, these observations allow us to probe whether factors like the multiplicity fraction — how many massive stars are in binary or multiple systems — have changed over cosmic time.

Peering Through a Gravitational Lens

In a recent research article, a team led by Hayley Williams (University of Minnesota) reported on their examination of an intriguing source in a gravitationally lensed galaxy called the “Warhol arc.” This galaxy, located at a redshift of z = 0.94 (when the universe was roughly 6 billion years old), is gravitationally lensed by the massive galaxy cluster MACS J0416.1−2403. The cluster is located at a redshift of z = 0.396, corresponding to when the universe was about 9.4 billion years old.

Using data from the JWST Prime Extragalactic Areas for Reionization and Lensing Science (PEARLS) program and the Canadian NIRISS Unbiased Cluster Survey (CANUCS), Williams’s team analyzed a source in the Warhol arc called W2, which previous work suggests is either a binary star system or a small star cluster.

Multiplicity and Microlensing

Warhol arc

Top row: The Warhol arc during four epochs of JWST observations. Bottom row: On the left, a magnified image of W2 during the first epoch. The remaining images show the difference in brightness between subsequent epochs and the first epoch. Click to enlarge. [Williams et al. 2026]

Across four epochs spanning 126 days, the JWST observations show the source W2 within the Warhol arc. Williams and collaborators performed spectral fitting of the JWST light curves to investigate the multiplicity of the source. They found that the data are best matched by a binary system containing stars with temperatures of 3500K and 12600K.

W2 varies between observations in both brightness and color, a fact that the authors suggested is due to microlensing by a star within the lensing galaxy cluster, rather than variability within the binary system itself. Under this hypothesis, the orbital motions of the binary bring the stars across the microlensing caustic — a region in which the magnification is exceptionally high — and the brightness and color of W2 vary as the components of the binary approach and recede from the caustic.

Williams and collaborators also performed stellar population modeling to explore the binary configurations that could match the observations. They found that the stars likely have masses of 21–24 solar masses, with one being a cool red supergiant and the other a hot, main-sequence companion. Depending on the precise evolutionary stage of the binary, it’s possible that one of the stars is nearing a supernova explosion. Lending more support to the binary system hypothesis, the microlensing measurements constrain W2 to be no larger than 90 au — too small for even a compact star cluster.

The team closed by proposing further observations of W2’s position to rule out the possibility that the microlensing rate there is unusually high, an outcome that may suggest that microlensing of two unrelated stars, rather than a binary system, is responsible for these observations.

Citation

“JWST’s PEARLS: A Candidate Massive Binary Star System in a Lensed Galaxy at Redshift 0.94,” Hayley Williams et al 2026 ApJ 997 292. doi:10.3847/1538-4357/ae2003

'Oumuamua

Traversing the galaxy from places yet known, a few interstellar objects have taken a quick dip into our solar system. These objects have inspired a frenzy of questions regarding their origins and paths that led them here. A recent study took to nearby planet-forming stellar systems as possible launching posts.

Interstellar Object Origins

As 3I/ATLAS was first spotted entering the solar system in July 2025, all eyes turned to the curious interstellar visitor. Just the third interstellar object astronomers have observed, 3I/ATLAS joins 1I/ʻOumuamua (2017) and 2I/Borisov (2019) in capturing our attention and igniting many questions regarding their origins and journeys to our corner of the Milky Way. While astronomers have characterized these objects’ compositions, sizes, and paths through the solar system, the mechanisms by which these objects leave their origin systems and travel the galaxy before arriving here are not well constrained. 

These three large bodies are just part of the interstellar material visiting the solar system — spacecraft have detected an influx of small interstellar dust particles, and while not yet directly confirmed, interstellar meteoroids burning up in Earth’s atmosphere are possible. This loose material likely originates in dusty, planet-forming disks (debris disks) around stars — rubble kicked out into interstellar space, sending chemicals, organic compounds, and perhaps even the precursors of life to other nearby systems. Understanding the origins and trajectories of such material will allow us to investigate planet formation and how developing systems may impact their environments.

Deliveries from Debris Disks

Debris disks offer ideal testbeds for tracing small bodies and dusty material through the galaxy. Interactions with large planets in debris disks can launch loose material like planetesimals and dust into interstellar space. To explore how debris disks may drive interstellar visitors to our solar system, Cole R. Gregg and Paul A. Wiegert (The University of Western Ontario) simulated how much material the 20 nearest debris disks would contribute to the interstellar object population of our solar system.

sky projection for particle arrival directions

Arrival direction on the sky for particles that reach the solar system from the 20 debris disks studied here. The arrival locations for 1I/ʻOumuamua, 2I/Borisov, and 3I/ATLAS are shown for comparison. Click to enlarge. [Gregg and Wiegert 2025]

The authors modeled the trajectories of ejected material from each debris disk within a simulated Milky Way, running the simulations backward and forward in time to trace the ejecta over 100 million years. These simulations show that material from each of the 20 systems is currently expected to be within our solar system. For interstellar objects (≥100 meters), the simulations predict that about two objects from these debris disks are currently within the inner solar system.

Trickier to spot, smaller meteoroids (≥200 microns) that would appear as meteors in Earth’s atmosphere are also expected. The only way to observationally confirm interstellar origins for meteors is based on how fast they are moving relative to the material in our solar system, or their excess velocities. Based on the simulations, many of these smaller particles have low excess velocities, making them observationally difficult to distinguish from bound solar system objects.

Observational Connections and Expectations

'oumuamua zoom in plot

Zoomed-in view focusing on the direction where 1I/ʻOumuamua entered the solar system. The interstellar object falls near the arrival directions of particles from HD 38858 and HD 166. Click to enlarge. [Gregg and Wiegert 2025]

How do these simulations compare to observations? From their analysis, the authors identified where the interstellar material from each debris disk would first enter the solar system. Comparing these to the entry locations of 1I/ʻOumuamua, 2I/Borisov, and 3I/ATLAS, ʻOumuamua appears near the entry locations for debris from HD 166 and HD 38858. (2I/Borisov and 3I/ATLAS had no such match up in this sample of debris disks.) However, tracing ʻOumuamua’s travel backward, its closest approaches to these systems are about 32 and 18 light-years away, respectively — likely too far to have originated from either system but still a clear proof of concept that we can place interstellar objects near potential systems of origin. 

Where are the expected interstellar meteoroids? Current meteor detectors including the Canadian Meteor Orbit Radar (CMOR) and the Global Meteor Network (GMN) search the sky for incoming material. While the simulations predict hundreds of meteoroid-sized particles from each debris disk currently in the inner solar system, the small collecting area of CMOR and the limiting size of grains detectable by GMN statistically require decades of observations to detect even a single interstellar meteoroid. Thus, the lack of detections thus far is not surprising, but with advanced instrumentation, discoveries of more interstellar particles and objects are imminent.

Citation

“A Catalogue of Interstellar Material Delivery from Nearby Debris Disks,” Cole R. Gregg and Paul A. Wiegert 2025 PSJ 6 309. doi:10.3847/PSJ/ae284f

Vera Rubin Observatory

The detection of gravitational waves and light from a single source was one of the most important discoveries of the gravitational wave era. Can upcoming data from Vera C. Rubin Observatory help astronomers repeat that feat?

kilonova as seen by Hubble

These images from the Hubble Space Telescope show the fading light of the kilonova associated with the gravitational wave event GW170817. [NASA and ESA Acknowledgment: A. Levan (U. Warwick), N. Tanvir (U. Leicester), and A. Fruchter and O. Fox (STScI)]

Not Yet Repeated

When two neutron stars merge, the collision sends ripples through spacetime and generates an electromagnetic signal called a kilonova. In 2017, researchers detected gravitational waves and kilonova emission from colliding neutron stars — the first and, to date, only electromagnetic signal definitively associated with a gravitational wave event.

In a research article published this week, a team led by Simon Stevenson (Swinburne University of Technology; OzGrav) considered how the much-anticipated Legacy Survey of Space and Time (LSST), carried out by Rubin Observatory, can enhance our ability to detect kilonovae and pair them with their gravitational wave counterparts.

Survey Simulations

When a new gravitational wave signal is detected, researchers rush to search for an electromagnetic counterpart to the signal. While Rubin Observatory can aid this type of reactive search, Stevenson’s team focused on a different strategy: spotting kilonovae during routine survey operations and using these detections to trigger a targeted hunt through gravitational wave data, searching for signals missed by automated detection algorithms.

Over the 10-year course of LSST, Rubin will scan the visible southern sky every few days, uncovering a wide variety of transient sources such as supernovae, novae, and tidal disruption events. Because Rubin will amass a huge amount of data each night, astronomers worldwide will use data brokers to pass along the most promising signals. Stevenson’s team used for their analysis a bespoke and modular data broker called Fink. Fink currently handles 200,000 alerts per night from the Zwicky Transient Facility and will be scaled up to handle 10 million alerts per night as Rubin requires.

simulated kilonova light curves

Examples of simulated kilonova light curves for different redshifts and models (Kasen and Bulla). Click to enlarge. [Adapted from Stevenson et al. 2026]

Searching for Signals

Though Rubin is a transient-tracking powerhouse, it’s not ideally suited to discovering kilonovae specifically; these events evolve so rapidly that Rubin may only be able to collect a few data points before the event fades from its view. To gauge Rubin’s efficacy as a kilonova detector, Stevenson and collaborators simulated kilonova light curves and identified detections with a signal-to-noise ratio greater than 5; these detections will be passed to the data broker. They found that Rubin will detect about 42 kilonovae per year, but only a few will have a strong enough signal to be passed to the broker.

For the handful of detections that are passed to the broker, what are the prospects for tracking down the associated gravitational wave signals? Due to the survey cadence, most kilonova detections occurred 1–2 days after the gravitational waves from the neutron star collision would have reached Earth, but delays of up to 5 days were possible. This means that searches for gravitational wave signals must sift through multiple days of data — a computationally intensive prospect that might require the development of new search techniques. The computational requirements of this search are increased by the possibility of contaminants (signals misclassified as kilonovae and passed on by the data broker), which will require additional followup.

Despite these challenges, it’s clear that Rubin’s upcoming detections of kilonovae will advance our study of these rare events. With LSST expected to get underway early this year, the next multi-messenger signal may be just around the corner.

Citation

“Strategy for Identifying Vera C. Rubin Observatory Kilonova Candidates for Targeted Gravitational-Wave Searches,” Simon Stevenson et al 2026 ApJ 998 8. doi:10.3847/1538-4357/ae244e

illustration of the Milky Way's dark matter halo

New research suggests that the mysterious “little red dots” spotted in the early universe could be supermassive black holes birthed in the collapse of dark matter halos.

Another Theory for Little Red Dots

little red dots

JWST images of six very distant galaxies dubbed “little red dots.” [NASA, ESA, CSA, STScI, Dale Kocevski (Colby College)]

Many theories exist for the origins of little red dots: compact, reddish objects spotted mainly in the first billion years after the Big Bang. The leading theory states that that little red dots are growing black holes encased in dense gas. The black holes at the centers of these little red dots appear to be tens or even hundreds of millions of times the mass of the Sun, raising questions about how they grew so massive so quickly. One possibility is that they started out quite massive, arising from black holes born from the collapse of massive gas clouds in the early universe.

A recent research article explores that theory, with a twist: that the black holes at the heart of little red dots were born not out of regular, baryonic matter, but out of dark matter.

A Dark (Matter) Past

In the early days of the universe, tiny fluctuations in the density of dark matter began to grow, eventually collapsing into vast spheroidal structures called dark matter halos. In the leading theory of cosmology, ΛCDM, dark matter interacts with itself and normal matter only through gravity. This leads to the creation of stable dark matter halos that act as the invisible scaffolding within which the first stars and galaxies grow.

In today’s article, Fangzhou Jiang (Peking University) and collaborators investigated the evolution of halos containing self-interacting dark matter. Particles of self-interacting dark matter, as the name suggests, can interact with one another through collisions and exchange heat. This small adjustment, sometimes invoked in alternatives to ΛCDM to explain the properties of dwarf galaxies, can lead to the collapse of dark matter halos into black holes.

Seeding Black Holes

Jiang and coauthors examined whether this process could explain the observed population of little red dots. First, the team examined whether the timescales are feasible: is it possible for dark matter to assemble itself into massive halos and collapse into black holes, all within the first billion years of the universe?

The answer appears to be yes — in this framework, dark matter halos with masses between 106.5 and 108.5 solar masses were able to form black holes between 104.5 and 106.5 solar masses by a redshift of z = 8.5, when the universe was just 600 million years old.

plot comparing observed and modeled black hole populations

Comparison of the inferred black hole population in little red dots (red diamonds) to models of black holes formed in dark matter halo collapse (shaded areas) at two redshifts. The red area shows the fiducial model, while the blue and green areas show the effects of changing the dark matter self-interaction cross-section. The dashed red lines show an extreme case in which the presence of baryons accelerates halo collapse. Click to enlarge. [Jiang et al. 2026]

Jiang’s team then performed semianalytic modeling to explore how these “seed” black holes grow and evolve through accretion and mergers. This analysis generated a distribution of black hole masses consistent with what has been observed in little red dots. The team cautioned that their results are sensitive to the interaction cross-section of the dark matter particles, as well as various other parameters like the black hole accretion duty cycle. Their modeling also assumes that the halo collapse takes place before any baryonic matter collects within it. Intriguingly, the authors found that the presence of baryons accelerates the collapse of the dark matter halo, speeding the seeding process.

To close, Jiang and collaborators noted that this process is not mutually exclusive with other black hole seeding mechanisms, and as studies of self-interacting dark matter continue, its ability to create black holes in the early universe should be examined further.

Citation

“Formation of the Little Red Dots from the Core Collapse of Self-Interacting Dark Matter Halos,” Fangzhou Jiang et al 2026 ApJL 996 L19. doi:10.3847/2041-8213/ae247a

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