Astronomers find evidence for three subpopulations of merging black holes - Astronomy & Space - Astronomy - UNIVERSE

Artist's impression of a pair of black holes merging, involving one with unusual spin. Credit: Carl Knox, OzGrav, Swinburne University of Technology

Astronomers analyzing gravitational-wave data from the LIGO-Virgo-KAGRA Collaboration have reported that merging binary black holes fall into three distinct categories. The study shows that the three subpopulations have their own characteristic masses, spin behavior, and merger rate that may be linked to different dominant formation mechanisms. The paper outlining their results was submitted to the preprint server arXiv on March 18.

A mix of three

The data in the fourth gravitational-wave catalog (GWTC-4), released by the LIGO-Virgo-KAGRA collaboration, included more than 150 detected black hole mergers. The analysis of this dataset revealed that the overall population of merging binary black holes may not have the same origins.

When researchers analyzed how the masses of black holes are distributed across the population, they saw prominent peaks around 10 solar masses and 35 solar masses. Similar features were also seen in how the systems' spins and mass ratios behave, with noticeable changes around 20 and 40 solar masses. If the same process drove all mergers, a smoother distribution would be expected. These features suggested that black hole mergers may be produced by multiple formation channels.

In the new study, researchers simulated key properties, such as masses, spin behavior, and merger rates, to reproduce the observed features of the overall population, and found that it is best explained as a mixture of three distinct groups of binary black holes. They then linked the parameter values that represent the properties of each group with theoretical predictions to identify the most likely pathway that formed them.

Distributions of primary masses of the three simulated components: first (in blue), second (yellow), and third (green) subpopulations. The first subpopulation shows a peak around 10 solar masses and the second one shows a peak around 35 solar masses. Credit: arXiv (2026). DOI: 10.48550/arxiv.2603.17987

Heavy, heavier, heaviest

The first group of binary black holes, which makes up 79% of the overall population, shows a sharp peak around 10 solar masses. These low-mass black holes are slowly spinning systems with very little wobbling. Their spins are also aligned with the orbit.

All these features point to the fact that these binary black holes likely originated through an isolated binary evolution. That is, when two stars born as a pair evolve, exchange mass, and collapse into black holes that merge, without external influence.

The second subpopulation of binary black holes makes up nearly 14.5% of all detected binaries and it explains the prominent feature around 35 solar masses seen in the observations.

The team found that these binaries have black holes of nearly equal masses and equal fractions of black holes' spins aligned and misaligned with the orbit, with greater wobbling compared to the first group. These intermediate-mass black holes show signs of a more chaotic origin compared to the first group.

Researchers suggest that these binaries likely formed in a crowded environment, such as a globular cluster. They say that a pair of black holes influenced by a third distant object could also produce these binary black holes.

Finally, the third population, which makes up only 2.5% of the overall population, falls at the higher end of the mass distribution. These systems have black holes of unequal mass and show complex spin behavior, with noticeable wobbling. Researchers suggest that they likely formed through hierarchical mergers, with at least one of the black holes being a remnant of an earlier merger.

However, researchers note that these formation channels are what likely dominated each subpopulation, but there may be other processes at work.

"While these conclusions are reasonably robust, the direct association of subpopulations with single channels remains elusive," they write in their paper. With the upcoming data releases of the LIGO-Virgo-KAGRA collaboration, they aim to produce more conclusive results about how these different merging black hole populations form. 

by Shreejaya Karantha, Phys.org

edited by Sadie Harley, reviewed by Robert Egan

Source: Astronomers find evidence for three subpopulations of merging black holes 

One-step CO₂ system triples capture, ditches silver for zinc, and turns emissions into industrial fuel feedstock - Engineering - Energy & Green Tech

University of Chicago Pritzker School of Molecular Engineering researcher Reginaldo Gomes, Ph.D.'25, is the first author on a new study from the lab of Asst. Prof. Chibueze Amanchukwu that modeled a system that can simultaneously capture and convert CO₂. Credit: UChicago Pritzker School of Molecular Engineering / John Zich

Every year, power plants and factories release billions of tons of carbon dioxide (CO₂) into the atmosphere. Methods exist to capture that CO₂ using chemical solutions and, separately, to convert pure CO₂ into useful fuels and chemicals. But doing both steps at once, in a cost-efficient and scalable way, has been difficult.

Now, researchers at the University of Chicago Pritzker School of Molecular Engineering (UChicago PME) and the U.S. Department of Energy's Argonne National Laboratory have developed a system that can simultaneously capture and convert CO₂. The approach, they reported in Nature Energy, offers a more efficient and potentially lower-cost approach than carrying out each step separately.

By swapping the water usually used in carbon capture and conversion systems for a different solvent, the team was able to capture CO₂ more efficiently and convert it into carbon monoxide, an industrially relevant building block for the chemical industry used to make a wide range of fuels and chemicals today. They also turned to zinc, rather than the usual silver, to catalyze the conversion reaction, bringing costs for the process down further.

"The concept of being able to integrate capture and conversion into a single step is a relatively new one, and we've made significant headway in not only showing that this is possible but that it can be done under conditions that are relevant for industrial deployment," said Chibueze Amanchukwu, Neubauer Family Assistant Professor of Molecular Engineering at UChicago PME and senior author of the new study.

Insights into catalytic activity and selectivity. Credit: Nature Energy (2026). DOI: 10.1038/s41560-026-02035-4

One process instead of two

In conventional carbon capture, amines—nitrogen-based compounds that bind readily to CO₂—are dissolved in water. Releasing the captured CO₂ for later use requires heating the solution to temperatures as high as 150°C and compressing the CO₂. Meanwhile, if that captured CO₂ was converted in water, water carries out unwanted side reactions, ultimately leading to hydrogen gas.

Amanchukwu, whose lab focuses on electrochemistry in non-aqueous solvents, was brought together with scientists at Argonne National Laboratory through the University of Chicago Joint Task Force Initiative, a program designed to foster collaboration between the two institutions. About four years ago, the group formed a team and asked themselves what big problem was worth tackling together. They landed on reactive capture—the idea that CO₂ could be converted directly into a useful product while still bound to the amine.

"The challenge with current capture methods comes when you need to recover that CO₂. You need to boil the solution, which requires significant energy," said first author of the study, Reginaldo Gomes, who completed his Ph.D. at UChicago PME and is now a postdoctoral researcher at Argonne. "We asked whether, instead of going through those costly steps, we could use electricity to convert the captured CO₂ directly into something valuable."

Changing the solvent changes the chemistry

Many of the challenges around combining current capture and conversion methods revolve around water's unwanted chemical reactions. So the team began by replacing water with DMSO—a widely used industrial solvent.

In water, two amines must come together to bind each captured CO₂ molecule. Amanchukwu, Gomes, and their colleagues showed that in DMSO, the same amines form a different arrangement and can capture one CO₂ for every amine, doubling the system's capture capacity. At the same time, no CO₂ is lost to the competing chemical pathways that occur in water. Overall, the team observed nearly three times higher CO₂ uptake per amine molecule in DMSO compared to water.

With fewer hydrogen-forming side reactions, the group realized they could also make another change to the system. Silver catalysts, used in water-based capture approaches because they are resistant to making hydrogen, could be swapped for zinc—an earth-abundant metal far less expensive than the silver.

"We didn't anticipate how removing water would open up all these other new ways to make capture and conversion more efficient," said Amanchukwu. "It worked better than we had even hoped for."

Under lab conditions with pure CO₂, the zinc catalyst achieved 78% efficiency in converting captured CO₂ to carbon monoxide, a key industrial feedstock. Computational work by collaborator Cong Liu at Argonne revealed exactly why the zinc outperformed the silver in the DMSO system, requiring less energy.

Performing under real-world conditions

A critical test for any carbon capture technology is whether it works under actual industrial exhaust conditions rather than only with pure CO₂ in the lab. The team tested their system using simulated flue gas mixtures containing oxygen, which typically interferes with chemical reactions and can lower the efficiency of carbon capture and conversion.

The new approach still achieved up to 43% efficiency in converting CO₂ to carbon monoxide over multiple capture-and-conversion cycles. That figure matches what state-of-the-art water-based systems achieve using silver under pure CO₂, a far less challenging condition.

Collaborators at Argonne, led by Dr. Chukwunwike Iloeje, carried out a techno-economic analysis to estimate the cost of using DMSO instead of water. They found that the improved performance of the system, particularly higher CO₂ conversion, can substantially offset the higher solvent cost. Replacing silver with zinc in the DMSO system could further reduce costs by using a more active and abundant catalyst.

The researchers are candid that significant work remains before the system can be scaled up. It must be able to run for thousands of hours rather than days, and reaction rates must increase roughly tenfold to reach commercial viability. New reactor designs better suited to industrial scale will also be required. Still, a patent disclosure has been filed, and the team has already been contacted by industry.

"We established the scientific foundation for this system," said Gomes. "We're not just working with a pure, controlled CO₂ stream in the lab—we have developed something that can start to handle the complexity of real-world challenges." 

Provided by University of Chicago 

by Sarah C.P. Williams, University of Chicago

edited by Gaby Clark, reviewed by Robert Egan 

Source: One-step CO₂ system triples capture, ditches silver for zinc, and turns emissions into industrial fuel feedstock