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merging galaxies

The galaxy NGC 4486B appears calm and collected, but its center may have been roiled by a recent merger of supermassive black holes. New modeling explores the stellar dynamics that support this hypothesis.

Strange Center

NGC 4486B double nucleus

Kinematic maps of NGC 4486B showing the locations of the two peaks of its double nucleus. Click to enlarge. [Adapted from Tahmasebzadeh et al. 2026]

Astronomers have known for 30 years that NGC 4486B, a compact elliptical galaxy near the center of the Virgo cluster, has a double nucleus. More recently, JWST observations revealed that the galaxy houses a black hole of 360 million solar masses, which is unusually large compared to the galaxy’s stellar mass of 9 billion solar masses. The two nuclei are roughly 40 light-years from the apparent center of the galaxy, and the black hole also appears to be offset from the galactic center by 20 light-years.

Now, a team led by Behzad Tahmasebzadeh (University of Michigan; Villanova University) has investigated the possibility that NGC 4486B’s double nucleus and off-center black hole can be traced to the aftermath of a supermassive black hole merger.

Simulating Kinematics

In this scenario, the black hole is displaced from the galaxy’s center because of a “kick” it received when it underwent a merger. The double nucleus is a sign of an eccentric nuclear disk: a central disk of stars on aligned elliptical orbits created when merging supermassive black holes disturb an initially orderly disk of stars.

plot of kick magnitude versus mass ratio

Estimated black hole kick magnitude as a function of the initial mass ratio of the black hole binary. Click to enlarge. [Tahmasebzadeh et al. 2026]

Tahmasebzadeh and collaborators performed dynamical modeling to test this hypothesis and understand what types of stellar orbits would be necessary to reproduce the kinematic signature of NGC 4486B’s center seen with JWST. The simulation results called for a blend of prograde and retrograde stellar orbits that closely resembled what is expected for an eccentric nuclear disk. From the properties of the simulated stellar disk, the team estimated that the mass ratio of the merging black holes was >0.15.

To explore this scenario further, the team carried out N-body simulations of the post-merger black hole’s behavior. These simulations showed that after being booted from the galactic center by the post-merger kick, the black hole returns to the center quickly — within 10–80 million years, depending on the kick strength. Because NGC 4486B’s supermassive black hole is notably off center, this suggests that the merger occurred recently.

Galaxy Merger Versus Black Hole Merger

Tahmasebzadeh’s team tested two other theories that could explain the appearance of NGC 4486B’s nucleus: dynamical buoyancy and a pre-merger supermassive black hole binary. Neither of these scenarios could reproduce the offsets seen in the center of the galaxy.

The team noted that NGC 4486B appears to be in equilibrium, with no sign of a recent merger that could have plunked a second supermassive black hole into the galaxy. How can this fact be reconciled with the evidence for a recent black hole merger? Turning again to simulations, the team found that if the black hole binary’s orbit was aligned with the galaxy’s rotation, the binary could have become trapped in a resonance that greatly delayed the merger of the black holes. This makes it possible that NGC 4486B underwent a galaxy merger in the distant past, but its central black hole merged only recently, leaving signs of a long-ago merger that has otherwise faded from view.

Citation

“JWST Observations of the Double Nucleus in NGC 4486B: Possible Evidence for a Recent Binary SMBH Merger and Recoil,” Behzad Tahmasebzadeh et al 2026 ApJL 1001 L14. doi:10.3847/2041-8213/ae52ef

gas giant orbiting white dwarf

Orbiting a white dwarf, the exoplanet WD 0806b is the subject of a recent study using JWST to measure the atmospheric conditions governing the planet.

Directly Imaged Exoplanet WD 0806b

Most of the 6,000 and counting exoplanets discovered thus far have been detected due to their impacts on their host star — passing in front of the star and causing it to dim, or gravitationally tugging on the star and causing it to wobble. On rare occasions, though, astronomers have been able to catch the exoplanet itself through direct imaging. Directly imaged exoplanets offer key insights that cannot be obtained through indirect detection methods. Luminosity measurements and spectral emission features allow astronomers to more directly measure planet mass, radius, and composition otherwise inferred from the host star.

WD 0806b system

NIRCam image showing host star WD 0806 circled in orange and the location of exoplanet WD 0806b marked by the orange arrow. [Lew et al 2026]

Included in the about 90 distant planets that have posed for a picture is WD 0806b, a rare exoplanet companion trotting around a white dwarf at an orbital distance of 2,500 au (50 times the distance from the Sun to the Kuiper belt!). Originally discovered with the Spitzer Infrared Array Camera in 2011, WD 0806b is the second-coldest directly imaged exoplanet to date. With JWST’s high-precision photometric and spectroscopic capabilities, WD 0806b provides a unique opportunity to probe the atmospheric chemistry of cold giant planets and take steps toward understanding the co-evolution of white dwarfs and their surviving exoplanets.

WD 0806b’s Atmospheric Abundances from JWST

Aiming to characterize the physical properties and atmospheric composition of WD 0806b, Ben W.P. Lew (Bay Area Environmental Research Institute; NASA Ames Research Center) and collaborators used JWST’s Near-Infrared Camera (NIRCam) and Near-Infrared Spectrograph (NIRSpec) to obtain high-resolution imaging and spectroscopy of the exoplanet. Combining these observations, prior lower-resolution observations, and evolutionary planet models, the authors estimated the physical properties of WD 0806b including mass, radius, surface gravity, and effective temperature.

WD 0806b spectrum

NIRSpec spectrum of WD 0806b with best-fit model spectra overlaid. [Lew et al 2026]

From the derived physical properties, the authors modeled the NIRSpec spectrum to estimate molecular abundances and elemental abundance ratios in the atmosphere of WD 0806b. They obtained measurements of multiple molecules including carbon dioxide, carbon monoxide, and ammonia; these molecular abundances offer the opportunity to test chemical equilibrium and disequilibrium as well as eddy diffusion, or bulk mixing, in the planet’s atmosphere. The authors developed a novel chemical analysis framework to determine how bulk mixing varies with altitude in WD 0806b and reported the first observational evidence that mixing becomes weaker at higher altitudes in exoplanet atmospheres. This result points to the need for future studies exploring the impact of these chemical processes on spectra and photometry, which are essential to characterizing cold giant planets.

This study highlights how high-precision JWST data can reveal a rich collection of atmospheric conditions, chemical composition, and physical processes occurring in cold giant planets. With such a wide orbit from its host white dwarf, WD 0806b serves as an interesting case study for how giant planet composition may reflect the formation and evolutionary history of the overall system. Future observations of and comparisons to similarly cold giant planets will further uncover how atmospheric characterization may fit into our understanding of exoplanets and their histories.

Citation

“JWST Spectral Retrieval of Cold Directly Imaged Planet WD 0806b and the First Measurement of Altitude-dependent Kzz in Exoplanet Atmospheres,” Ben W.P. Lew et al 2026 AJ 171 227. doi:10.3847/1538-3881/ae4747

Artist's impression of a pulsar

Neutron stars are composed of some of the most extreme material in the universe, and their internal properties are challenging to determine. Recently, researchers investigated an unusual spectral feature that may help to probe the interiors of neutron stars.

Examining Extreme Matter

supernova remnant 1E 0102.2-7219

This composite X-ray and optical image shows the supernova remnant 1E 0102.2-7219. The blue source at the center of the bright red ring inside the remnant is a neutron star that was created in the supernova explosion. [X-ray (NASA/CXC/ESO/F.Vogt et al); Optical (ESO/VLT/MUSE & NASA/STScI)]

When a massive star expires in a supernova explosion, it can leave behind its condensed core in the form of a neutron star: an exceptionally dense sphere roughly as wide as Manhattan is long, composed almost entirely of neutrons. Neutron stars represent an extreme state of matter and contain some of the densest and strongest material in the universe.

By observing neutron stars, astronomers attempt to pin down their equation of state, or the relationship between their interior density and pressure. In a recent research article led by Rosario Iaria (University of Palermo) described a promising way to probe the interior of a neutron star.

This method relies on measurements of a spectral line that originates near the neutron star’s surface and is gravitationally redshifted as it escapes from the intense gravitational pull of the star. The gravitational redshift provides a direct measurement of the compactness of the neutron star, or the ratio of its radius to its mass. Despite the promise of this method, definitive identification of gravitationally redshifted absorption lines has been challenging.

Directly Measuring Compactness

Iaria’s team identified potential evidence of this phenomenon in Neutron star Interior Composition Explorer (NICER) observations of 4U 1820-30, a close binary system containing a neutron star that is accreting matter from a white dwarf companion. (4U 1820-30 is famous for having one of the most rapidly rotating neutron stars known, with a blistering rate of 716 rotations per second.)

MAXI observations of 4U 1820-30

Observations of 4U 1820-30 from the Monitor of All-Sky X-ray Image (MAXI) showing the sudden increase in counts during the superburst, which was followed by two observing epochs by NICER, marked with red and blue vertical lines. Click to enlarge. [Iaria et al. 2026]

Iaria and collaborators focused on a strong iron absorption line at 3.8 keV, which arose three hours after a carbon superburst: a rare burst of high-energy radiation from thermonuclear burning of carbon deep in the neutron star’s atmosphere. The signal persisted for nearly 17 hours before subsiding.

The team proposed that the rare superburst paved the way for the gravitationally redshifted absorption line to appear. During the superburst, a powerful radiation-driven wind swept away most or all of the neutron star’s corona, a diffuse cloud of plasma floating above and below the accretion disk. After the superburst ended and the wind subsided, but before the obscuring corona reformed, the tell-tale absorption line was visible for a short time.

Cracking the Mystery of Neutron Star Structure

By interpreting the 3.8 keV absorption feature as a gravitationally redshifted spectral line arising near the neutron star’s surface, Iaria and collaborators measured a redshift of 1 + z ≅ 1.72. This corresponds to a compactness of 4.46 kilometers per solar mass, which translates to a radius of 6.4 km for a typical mass of 1.4 solar masses or a mass of 2.2 solar masses for a typical radius of 10 km.

The interpretation of the 3.8 keV absorption line as a gravitationally redshifted iron line is still tentative. Further measurements with NICER, or with future facilities like the Advanced Telescope for High-ENergy Astrophysics (Athena) X-ray observatory (planned launch in 2037) or the enhanced X-ray Timing and Polarimetry (eXTP) mission (planned launch in 2030), may help to advance the study of neutron star interiors using this technique.

Citation

“A Mysterious Feature in the NICER Spectrum of 4U 1820-30: A Gravitationally Redshifted Absorption Line?” R. Iaria et al 2026 ApJ 998 58. doi:10.3847/1538-4357/ae2758

composite image of the active galaxy Centaurus A

Many galaxies, including our own, have a central supermassive black hole. In certain galaxies, gas becomes ensnared in the black hole’s gravitational pull and collects in a disk that feeds the black hole, forming an active galactic nucleus (AGN). An AGN can feast on this disk of gas and dust for millions to billions of years — glowing across the electromagnetic spectrum, brandishing relativistic jets, and creating a brilliant, variable light show visible from billions of light-years away.

Today’s Monthly Roundup investigates the connection between AGNs and neutrinos, considers the question of AGN variability, and explores modeling techniques for relativistic environments.

A Search for Neutrinos from X-Ray-Bright AGNs

Where do neutrinos — neutral, nearly massless elementary particles — come from? AGNs have emerged as one likely source; in 2022, the IceCube collaboration reported evidence for neutrinos from the nearby galaxy Messier 77, which hosts an X-ray-bright AGN, and earlier observations potentially linked a neutrino to the blazar TXS 0506+056.

NGC 7469

The spiral galaxy NGC 7469, shown here in this JWST image, contains an AGN that may be a source of neutrinos. [ESA/Webb, NASA & CSA, L. Armus, A. S. Evans; CC BY 4.0]

To investigate the possible connection between neutrinos and X-ray-bright AGNs, the IceCube collaboration performed a dedicated search for neutrinos toward a collection of X-ray-bright AGNs in the northern sky. The team’s initial analysis confirmed that Messier 77 is the strongest source of neutrinos in that area. Focusing specifically on X-ray-bright AGNs with hard X-ray fluxes at least 20% of Messier 77’s flux — 47 AGNs in total — the team found evidence for a possible neutrino excess toward 11 of the galaxies, with NGC 7469 being the strongest candidate.

This result suggests that X-ray-bright AGNs are indeed a plausible source of neutrinos. As for the specific physical origin of these AGN-generated neutrinos, the data provide a few clues. The X-ray emission of an AGN is thought to arise from a billion-degree cloud of plasma called the corona. The interaction of coronal X-rays with high-energy protons is thought to produce neutrinos in the energy range of 1–10 teraelectronvolts — roughly the energies of the neutrinos associated with Messier 77 in this study. However, NGC 7469 was associated with neutrinos more energetic than this range, suggesting that not all AGN-produced neutrinos have a coronal origin. Thus, though the body of evidence suggesting that X-ray-bright AGN are a source of neutrinos, the precise mechanism through which these neutrinos are produced remains unknown.

diagram of the unified model of active galactic nuclei

A diagram of the unified model of active galactic nuclei, showing an accretion disk, dusty torus, and jets. Click to enlarge. [B. Saxton NRAO/AUI/NSF; CC BY 4.0]

Disk or Jet: Which Varies More?

If there’s one constant when it comes to AGNs, it’s change; observations show that the emission from active galaxies varies on timescales from minutes to decades. Though both the accretion disk and the jet contribute to the overall multiwavelength behavior of an AGN, it’s not yet clear which of these structures is a greater contributor to an AGN’s variability.

Vineet Ojha (Peking University) and coauthors considered AGN variability at optical and mid-infrared wavelengths. Ojha’s team collected a sample of AGN that lie at similar redshifts and fall into one of three categories: narrow-line Seyfert 1 galaxies detected in gamma rays, narrow-line Seyfert 1 galaxies not detected in gamma rays, and broad-line Seyfert 1 galaxies detected in gamma rays. These three classes are distinguished by the presence or absence of relativistic jets as well as their accretion rate; gamma-ray-detected Seyfert 1s likely host jets, and narrow-line Seyfert 1s likely have higher accretion rates than broad-line Seyfert 1s.

Using optical data from the Zwicky Transient Facility and mid-infrared data from the Wide-field Infrared Survey Explorer, the team separated statistical wiggles from true variability and determined which sample of AGNs was most variable. They found that broad-line AGNs detected in gamma rays are the most variable, suggesting that jets are major contributors to AGN variability. Narrow-line AGNs detected in gamma rays are next in line, likely because these AGNs have jets but also have a strong thermal emission component from their accretion disks due to their high rate of accretion. Narrow-line AGNs not detected in gamma rays are the least variable, lacking a jet and dominated by thermal emission. Taken together, these results suggest that AGN variability is mostly jet driven, with some contribution from accretion disk instabilities.

More on AGN Jets: The Slow-Light Effect

The galaxy Messier 87 (M87) hosts one of the most studied AGN jets. M87’s jet is visibly structured, and it exhibits superluminal motion, in which the relativistic plasma appears to move faster than the speed of light. Because the plasma accelerated by an AGN moves so quickly, researchers attempting to model this plasma may opt to use the slow-light approach, in which the evolution of the ambient plasma is modeled simultaneously with the propagation of light through the medium. Though more computationally intensive than the fast-light approach, in which photons zip through a static medium, slow-light techniques are needed in regions of relativistic flows or strong acceleration.

comparison of slow light and fast light results for M87's jet

Comparison of the jet morphology from the slow-light (top row) and fast-light (bottom row) methods. [Adapted from Tsunetoe et al. 2026]

Yuh Tsunetoe (Chinese Academy of Sciences; University of Tsukuba) and collaborators demonstrated their use of the slow-light approach in models of M87’s jet-launching region, where plasma is accelerated to relativistic velocities. The team performed general relativistic magnetohydrodynamics simulations of the accretion disk and jet from M87’s supermassive black hole and compared the results from slow-light and fast-light methods.

The team found that slow-light images tended to be smoother, lacking the looping, helical structures within the jet seen in fast-light images. Slow-light images show more evidence for the superluminal motion that is observed in relativistic jets like M87’s, as well as greater limb brightening and less wobbling motion. The team also investigated the effects of changing the black hole spin, finding greater wobbling in the jet for slow black hole spins and a straighter, wider jet for rapid black hole spins. Overall, Tsunetoe and coauthors found that the slow-light approach generated images that are more consistent with the properties of M87’s jet, demonstrating that the slow-light approach is necessary to capture the behavior of relativistic jets.

Citation

“Evidence for Neutrino Emission from X-Ray-Bright Active Galactic Nuclei with IceCube,” R. Abbasi et al 2026 ApJL 1000 L26. doi:10.3847/2041-8213/ae4aad

“The Relative Contributions of Accretion Disk Versus Jet to the Optical and Mid-Infrared Variability of Seyfert Galaxies,” Vineet Ojha et al 2025 ApJ 994 84. doi:10.3847/1538-4357/ae0a38

“Slow-Light Effect in the Jet-Launching Region of M87,” Yuh Tsunetoe et al 2026 ApJ 1000 29. doi:10.3847/1538-4357/ae43e7

A rendering of two purple jets streaming away from a central explosion.

Over many centuries of observing the night sky, astronomers have found only a single visible afterglow of a collision between neutron stars. For a few days in the summer of 2025, it seemed like observers may have found another of these treasured but elusive prizes; unfortunately, the promising candidate turned out to be a supernova in disguise.

A Rare Prize

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)]

When two neutron stars (the ultra-dense remnants of massive stellar explosions) spiral together and collide, the cataclysm is energetic enough to release strong gravitational waves, forge heavy elements, and briefly glow across the electromagnetic spectrum. These events are called kilonovae, and they’re quite rare; while we may have detected a handful at high energies, there is only one event for which astronomers managed to record both gravitational waves and an optical transient.

That one kilonova, found back in 2017, taught astronomers much about how heavy elements are formed and left the scientific community hungry for more data. Since then, each time a gravitational wave detector like LIGO reports that it may have spotted a neutron star merger, telescopes across the world scramble to look in the probable region of the sky, hoping to find the short-lived electromagnetic counterpart.

Too Good to Be True

On 18 August 2025, the LIGO/Virgo/KAGRA collaboration sent out an alert that it may have detected a neutron star–neutron star merger. The odds that their signal was real weren’t great, and the researchers gave it just a 29% chance of being a genuine astrophysical signal. Still, given the potential payoff of finding the next kilonova, several telescopes quickly began searches for the optical counterpart. The gravitational wave signal suggested that the event likely came from a curved patch of sky delightfully referred to as the “northern banana,” and after trawling that region for a few nights, astronomers hauled in 47 new transients. Any one of these could have been the kilonova, and all of them received extensive follow-up observations.

A team led by James Gillanders (University of Oxford) recently summarized some of these follow-up observations carried out with the Pan-STARRS and ATLAS telescopes. In the initial exciting few days after the alert went out, one candidate transient stood out as the most promising. Named AT2025ulz, it was first spotted by the Zwicky Transient Facility and initially started fading rapidly and changing colors, just as models of kilonovae predict. For four days, it seemed like the world may have witnessed its second-ever kilonova. But then the telescopes began their fifth night of observations.

Supernova Unmasked

A scatter plot showing a rapidly fading, then reversing and spiking, light curve.

The light curve of SN2025ulz. Note that the source appeared to grow brighter again after about 5 days. Click to enlarge. [Gillanders et al. 2025]

The light curve of AT2025ulz, after fading steadily in the preceding days, suddenly turned upwards; in other words, whatever was causing the transient got brighter. Models of kilonova evolution predict no such brightening, but Type IIb supernovae are known to follow just this behavior. AT2025ulz showed itself to be another run-of-the-mill stellar explosion, not the sought-after fireworks of two neutron stars slamming together.

Frustrating as this particular result may be, the effort was far from wasted. The team could use their non-detection of the true kilonova to place limits on its timing and peak magnitude, assuming it existed in the first place. And, as gravitational wave detectors grow more sensitive and alerts like this more common, hindsight will likely frame this scramble as a dress rehearsal for an ultimately successful kilonova recovery effort. Until then, astronomers will keep searching, and will keep their guard up against cosmic impostors.

Citation

“Pan-STARRS Follow-Up of the Gravitational-Wave Event S250818k and the Light Curve of SN2025ulz,” J. H. Gillanders et al 2025 ApJL 995 L27. doi:10.3847/2041-8213/ae2125

NIRCam JADES image

JWST has uncovered a peculiar population of very distant objects known as “little red dots.” The mechanisms powering these objects’ peculiar properties — high luminosities, compact radii, and “V”-shaped spectra — remain uncertain. Could little red dots be the result of supermassive stars, galaxy mergers, or, as a recent study explores, black hole stars?

Direct-Collapse Black Holes and Little Red Dots

Since their discovery, little red dots have been the subject of many studies, but a complete explanation for their unique combination of properties remains elusive. Multiple studies have explored the growing population of peculiar dots, with some postulating that they are the result of stars thousands of times the mass of the Sun, supermassive stars entrenched in gas from a galaxy merger, or black holes embedded in dense accretion disks. 

Even more recently, studies have proposed that little red dots may be the remnants of direct-collapse black holes. In this scenario, a large cloud of gas in the early universe directly collapses into a stellar-mass black hole, and the remaining gas envelops the black hole, forming what is known as a quasi-star (or black hole star). Researchers have suggested that little red dots may be the late stage of this process when the black hole has accreted at least 10% of the total mass of the system. Direct comparisons between quasi-star evolutionary models and JWST observations of little red dots have yet to be explored but are necessary to confirm the potential of quasi-star origins.

Evolutionary Models of Quasi-Stars

Quasi-star HR diagram

Evolution of a theoretical quasi-star model on a Hertzsprung–Russell diagram. [Modified from Santarelli et al 2026]

Seeking to compare quasi-star evolutionary models to JWST observations of little red dots, Andrew D. Santarelli (Yale University) and collaborators developed a modeling framework to simulate quasi-star evolution. In this framework, a quasi-star is modeled as a black hole at the core of a massive star that accretes material from the stellar envelope over its lifetime. The authors computed models for a range of initial masses and tracked the evolution of the quasi-stars, predicting luminosities, temperatures, surface gravities, and lifetimes of these objects.

After a short initial phase of contraction lasting around 10,000 years, the quasi-star spends the rest of its approximately 20–40-million-year lifetime in the “late stage” as the central black hole continues to eat up the surrounding envelope. In the end, the quasi-star becomes a supermassive black hole a million times the mass of the Sun.

SED comparison of quasi-star + LRDs

Predicted spectral energy distributions of 1 million solar mass, late-stage quasi-stars both alone (black dashed line) and embedded in a host galaxy (solid blue line). Each panel compares the quasi-star to a little red dot observed by JWST. [Modified from Santarelli 2026]

From these models, the authors extracted synthetic spectral energy distributions for late-stage quasi-stars. In comparing the simulated quasi-star spectra to those of three observed little red dots, the authors found that the synthetic quasi-star spectra generally reproduce the main continuum features seen in all three of the little red dots. While the models do not account for specific emission lines, the continuum slopes of the ultraviolet and near-infrared parts of the spectral energy distributions that form the signature “V” shape generally agree with those seen in the three comparison little red dots.

Plausible Progenitors

The results of this study indicate that late-stage quasi-stars can naturally produce the defining continuum features of little red dot spectra. While further modeling is required to trace specific emission line features, the initial modeling presented in this study establishes quasi-stars as plausible progenitors of not only little red dots but also the universe’s first supermassive black holes. 

As the authors noted, the short lifetimes of quasi-stars shown in their models in conjunction with the observed number density of little red dots implies that a significant fraction of supermassive black holes formed through this mechanism. Continued observations of little red dots and advanced modeling of direct-collapse black holes will aid in determining if these peculiar dots are showing off the birth of supermassive black holes in the universe.

Citation

“Evolutionary Tracks and Spectral Properties of Quasi-stars and Their Correlation with Little Red Dots,” Andrew D. Santarelli et al 2026 ApJL 998 L4. doi:10.3847/2041-8213/ae3713

Dinkinesh and Selam

The Lucy spacecraft, en route to its primary mission targets, zipped past the main-belt asteroid Dinkinesh and revealed its tiny satellite, Selam. New research examines the surfaces and histories of these small rocky worlds.

A Stop Along the Way

In 2021, the Lucy spacecraft embarked on a 12-year mission to study several Jupiter trojan asteroids, which share Jupiter’s orbit. The spacecraft won’t have its first encounter with a Jupiter trojan until 2027, but it hasn’t been idle during its long journey. In November 2023, Lucy performed a dress rehearsal of its trojan-encounter maneuvers during a flyby of a 738-meter-wide main-belt asteroid called (152830) Dinkinesh.

Dinkinesh and Selam

Two images of Dinkinesh from the Lucy mission. The second image features its satellite, Selam. [Bierhaus et al. 2025]

During this flyby, Lucy flew within 430 km (267 mi) of the asteroid and made a surprise discovery: Dinkinesh has a tiny companion, which has been named Selam. (The name Dinkinesh comes from the Amharic name for the Lucy fossil discovered in 1974; Selam is similarly named after a hominid fossil that is sometimes called “Lucy’s baby.”) Remarkably, Selam is the first known asteroid satellite that is a contact binary, made up of two separate bodies that have gently merged into one.

Craters and Boulders

In a recent research article, Edward Bierhaus (Lockheed Martin Space) and collaborators analyzed Lucy flyby images to study the surfaces of Dinkinesh and Selam. The team analyzed global features like troughs and ridges and measured visible craters and boulders.

craters on Dinkinesh and Selam

Locations of craters on Dinkinesh and Selam. The blue lines indicate the most confidently identified craters; orange and green lines mark craters identified less confidently. Click to enlarge. [Bierhaus et al. 2025]

While the small numbers of craters detected — 29 for Dinkinesh and 3 for Selam — makes further analysis difficult, the team determined that Dinkinesh’s crater size–frequency distribution is shallower than expected, though similar to that of small near-Earth asteroids (e.g., Ryugu). The seemingly shallow distribution may be an effect of the phase angle and resolution of the observations, or it could be a sign of impact armoring or other surface processes.

The size–frequency distribution of Dinkinesh’s boulders, on the other hand, is somewhat steeper. This discovery may mean that S-type (stony) asteroids like Dinkinesh have different boulder size distributions than C-type (carbonaceous) asteroids.

Possible Histories

An analysis of the collisional timescales in the main asteroid belt suggests that Dinkinesh is a fragment of a larger asteroid that was split apart at least once, though it’s not yet possible to say which parent body it might have come from. After splitting off from its parent asteroid, Dinkinesh appears to have been subjected to the Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect. This effect arises when sunlight falls on a small asymmetric asteroid, creating a torque that can alter its spin rate. In extreme cases, the YORP effect can crank up an asteroid’s spin rate so much that it sheds some of its surface material or flies apart altogether.

Sumak Fossa and Fab Dorsum on Dinkinesh

Locations of Sumak Fossa (top) and Fab Dorsum (bottom) indicated with arrows. [Adapted from Bierhaus et al. 2025]

Dinkinesh seems to show clear signs of YORP-driven structural failure. Sumak Fossa (a large trough) and Fab Dorsum (a ridge running along the asteroid’s equator) may have formed through this process. If Sumak Fossa was excavated by a YORP-induced mass-shedding event, both Fab Dorsum and Selam could have formed out of the cast-off material.

With Lucy en route to its primary objective, there’s still much for us to learn from this mission about the small bodies of our solar system. In the meantime, be sure to check out the full article linked below, as there are far more details about Dinkinesh and Selam than could be included in this short summary!

Citation

“The Geology of a Small Main-Belt S-Class Binary Asteroid System: Dinkinesh and Its Contact Binary Satellite Selam as Observed by the Lucy Mission,” E. B. Bierhaus et al 2025 Planet. Sci. J. 6 299. doi:10.3847/PSJ/ae1968

simulation of a supermassive black hole binary system

Researchers have undertaken a new search for a supermassive black hole binary, placing limits on its properties and exploring a promising technique for simultaneously analyzing electromagnetic and gravitational wave data.

Narrowing the Search for Supermassive Black Hole Binaries

In 2023, astronomers announced the discovery of compelling evidence for the gravitational wave background: the collective murmurs and rumblings of distant supermassive black hole binaries. The time may now be ripe for the natural next phase of discovery: the detection of gravitational waves from an individual supermassive black hole binary.

predicted frequency evolution of gravitational waves from 3C 66B

The predicted frequency evolution of gravitational waves from the black hole binary candidate in 3C 66B. Click to enlarge. [Cardinal Tremblay et al. 2026]

One of the most promising candidates for this search is 3C 66B, a poetically named elliptical galaxy roughly 300 million light-years away. More than 20 years ago, researchers discovered that the radio source at the heart of this galaxy wobbles to and fro in a way that’s consistent with the elliptical paths of supermassive black holes in a close binary. Further work predicted that the gravitational waves produced by this binary would have a frequency of about 60 nanohertz and potentially be within the observational reach of current methods.

Pulsar Timing Array

Jacob Cardinal Tremblay (Max Planck Institute for Gravitational Physics and Leibniz University Hannover) and collaborators conducted a search for gravitational waves from the candidate supermassive black hole binary in 3C 66B using the Parkes Pulsar Timing Array (PPTA).

artist's impression of the gravitational wave background from a supermassive black hole binary sweeping across an array of pulsars

Artist’s impression of a supermassive black hole binary generating gravitational waves that sweep across an array of pulsars. [Aurore Simonnet / NANOGrav; CC BY 4.0]

A pulsar timing array is a collection of pulsars monitored for signs of passing gravitational waves. Pulsars are the condensed, rapidly spinning cores of high-mass stars that exploded as supernovae. Named for their characteristics radio pulses, these extreme stars spin with exceptional regularity, and the passage of a gravitational wave can shrink or expand spacetime enough to speed up or delay the arrival of a pulsar’s pulses. By searching for coordinated changes in pulse arrival times from a collection of pulsars, researchers hope to detect low-frequency gravitational waves that are inaccessible to observatories like LIGO.

Researchers have previously searched for gravitational waves from 3C 66B in data from other pulsar timing arrays, such as the North American Nanohertz Observatory for Gravitational Waves, and no black hole binary has been detected. This is the first search of the third PPTA data release, which contains measurements of 32 pulsars over 18 years, providing a long baseline to search for the slow undulations of low-frequency gravitational waves.

Placing Limits

constraints on chirp mass of 3C 66B

Constraints placed on the chirp mass by this work (blue and green histograms) compared to constraints from electromagnetic observations (gray and peach shaded areas). Click to enlarge. [Cardinal Tremblay et al. 2026]

Using Bayesian statistical methods to analyze their pulsar timing data, the team was unable to confirm or rule out the presence of a supermassive black hole binary in 3C 66B. However, they were able to place limits on its properties, such as the chirp mass and the amplitude of the signal, and certain limits were more stringent than those placed by existing electromagnetic data.

While this analysis didn’t result in the first-ever detection of gravitational waves from a single supermassive black hole binary, it did allow the team to test a new method that could someday play a role in precision cosmology. This method simultaneously analyzes electromagnetic and gravitational wave data from known supermassive black hole binaries, establishing these sources as “standard sirens” that can complement standard candles like Type Ia supernovae for measurements of the expansion rate of the universe.

Citation

“A Multimessenger Search for the Supermassive Black Hole Binary in 3C 66B with the Parkes Pulsar Timing Array,” Jacob Cardinal Tremblay et al 2026 ApJL 998 L42. doi:10.3847/2041-8213/ae3c98

Small Magellanic Cloud

Researchers show that the highly disrupted state of the Small Magellanic Cloud can be attributed to a long history of interactions with its neighbor, the Large Magellanic Cloud.

A Disrupted Dwarf Galaxy?

The Small Magellanic Cloud (SMC) is one of the largest and nearest satellites of the Milky Way. At a distance of just 200,000 light-years, the SMC has long been used as an accessible analog for low-metallicity star-forming galaxies in the early universe.

Small Magellanic Cloud photometric and HI kinematic centers

Locations of the SMC’s photometric center and hydrogen gas kinematic center. The two locations are separated by several thousand light-years. [Adapted from Rathore et al. 2026]

However, observations increasingly suggest that the SMC may not be an ideal analog for high-redshift galaxies. The SMC appears to have been disrupted: its gas seems to rotate while its stars do not, it has an unusually large line-of-sight depth compared to its extent on the sky, and its gas distribution has two peaks.

The source of this disequilibrium might be the Milky Way’s largest satellite, the Large Magellanic Cloud (LMC). As the SMC inches along its orbit around the Milky Way, it also engages in a gravitational dance with the LMC. These galaxies may be clumsy dance partners, with data suggesting a potential direct collision between the SMC and LMC within the past 200 million years.

Modeling the Magellanic Clouds

To understand how a potential collision between the Magellanic Clouds might have impacted the present-day properties of the SMC, Himansh Rathore (University of Arizona) and collaborators used hydrodynamical simulations to explore the interconnected histories of the SMC, the LMC, and the Milky Way.

SMC–LMC separation in the authors' models

SMC–LMC separation over time in the control model (orange dot-dashed line) and the collision model (purple solid line). Click to enlarge. [Rathore et al. 2026]

At the beginning of the simulations, the SMC is in equilibrium, with its stellar and gaseous components neatly rotating in unison. From there, its fate diverged: in a control simulation, the SMC and LMC maintained a respectful distance, never getting less than about 100,000 light-years from one another. In the collision simulation, the galaxies collided 100 million years ago.

Both of these model scenarios reproduced certain features of the SMC–LMC system, such as the bridge of gas that spans the distance between them. Only the collision scenario, though, can explain the apparent discrepancy between the SMC’s stars and gas.

An Impactful Collision

SMC stellar density and viewing schematic

Left: Simulated stellar density 200 million years post-collision. Right: A schematic of our likely perspective on the post-collision SMC–LMC system. Click to enlarge. [Rathore et al. 2026]

Post-collision, the SMC’s stellar population is significantly elongated and features a tidal tail. If this tidal tail is oriented along our line of sight, it would explain why the galaxy’s depth in that direction is unexpectedly large. The presence of a gas tidal tail may also help to explain the two peaks in the galaxy’s gas density.

In terms of the galaxy’s rotation, only the stars nearest the SMC’s center rotate, while the rest move radially outward, swayed by the tidal pull of the LMC. This radial motion is also present in the galaxy’s gas. While previous work has interpreted the observed velocity dispersion of the SMC’s gas as a sign of rotation, Rathore’s team showed that radial expansion, viewed from an inclined angle, can mimic a rotational signature. 

The team also showed that the gas is far more disturbed than the stars, reflecting the pressure exerted by the LMC’s gas when the two galaxies collided. This type of interaction can transform an irregular dwarf galaxy into an ellipsoidal or spheroidal dwarf galaxy — a transition that Rathore’s team argues is underway for the SMC.

Ultimately, these simulations demonstrate that the SMC is a profoundly disrupted galaxy, likely still reeling from a recent collision with its larger neighbor. Viewing the SMC through this lens is critical to understanding its role as an analog to early universe galaxies.

Citation

“A Galactic Transformation — Understanding the SMC’s Structural and Kinematic Disequilibrium,” Himansh Rathore et al 2026 ApJ 1000 50. doi:10.3847/1538-4357/ae4507 

A close-up photograph of Jupiter. Dramatic cloud bands encircle the planet, and a large dark shadow of a nearby moon appears at left.

Jupiter’s upper atmosphere is hundreds of degrees hotter than it has any right to be, and for decades astronomers could only study it in brief, seemingly chaotic glimpses. New global maps built from years of observations have finally revealed the big picture — and it is surprisingly calm.

Order from Chaos

Although we’re well acquainted with Jupiter, having sent eight spacecraft on close passages and lived with it dominating the night sky for millennia, some of its simplest characteristics continue to puzzle us. For example, high above Jupiter’s cloud bands, temperatures exceed 800 K — far hotter than we’d expect given the meager sunlight that reaches that distance. Astronomers have long suspected that this excess is deposited by energy from the planet’s brilliant polar auroras, but previous observations provided only localized, sporadic snapshots that collectively painted a confusing picture of an ever-changing landscape.

A photograph of Jupiter seen in red/orange false color with a dark stripe down the middle.

Jupiter as seen by the slit camera on the Keck telescope during one of the team’s observations. The spectrograph slit is the dark vertical band stretching from the equator to the north pole, and the Great Red Spot appears in black at the lower right. [Adapted from Roberts et al. 2026]

To truly understand how Jupiter’s upper atmosphere got so hot and whether it was actually changing over time, astronomers needed to construct a global, comprehensive map — and then do so repeatedly over several years and look for any changes. A team led by Kate Roberts (Boston University) has now assembled just this sequence using the NIRSPEC instrument at the Keck Observatory. The researchers collected over 175,000 high-resolution infrared spectra across 14 nights from 2022 to 2025; they then processed the massive dataset to extract the emission from a charged form of hydrogen, H3+. This species is a standard natural thermometer that glows at infrared wavelengths, and by mapping the spectra to their corresponding specific locations on Jupiter, the team could construct the first true global maps of Jupiter’s upper atmospheric temperature, density, and radiance.

Consistency is Key

A look at how the maps evolved over time revealed striking order: the spatial patterns remained nearly constant, and the temperatures barely changed even over years. In general, the team noted that temperature decreases smoothly from the auroral regions near the poles (~1,150 K) to the equator (~750 K) at every longitude, and that although there is some night-to-night temperature variation, things stay remarkably steady over the years-long baseline.

A 2D heatmap with longitude on the bottom axis and latitude on the vertical axis. There is a general trend towards higher temperatures at higher latitudes.

A map of the average temperature of Jupiter’s upper atmosphere, as measured by the new Keck NIRSPEC data. Click to enlarge. [Adapted from Roberts et al. 2026]

What once “appeared stochastic,” the authors write, “exhibits predominantly spatial rather than temporal variability.” Earlier observations were simply sampling different longitudes at different times, mistaking geography for weather.

The conclusion that the upper atmosphere is complex but steady has implications beyond just Jupiter, as the team notes that JWST has already observed spatial patterns in the temperatures of the upper atmospheres of both Uranus and Neptune. As Roberts and colleagues note, “similar physical mechanisms could give rise to comparably stable structures” on those distant ice giants, meaning this analysis could ground our understanding of the upper atmospheres of giant worlds across the solar system.

Citation

“A Global View of Jupiter’s Upper Atmosphere Through H3+,” Kate Roberts et al 2026 ApJL 998 L13. doi:10.3847/2041-8213/ae3c9b

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