© Adriana Loureiro Fernandez for The New York Times
Exactly how the universe became reionized after cooling to form stars and galaxies remains an open question. A recent study investigates how a merging set of galaxies may have spurred an onslaught of ionizing photons into the intergalactic medium.
After its chaotic and blistering beginning, the universe cooled and began to coalesce, stars and galaxies flickering on to set the universe alight. From a few hundred million to about a billion years after the Big Bang, galaxies pumped high-energy ionizing Lyman continuum (LyC) photons into the intergalactic medium during what is known as the epoch of reionization. The exact mechanisms that produced and allowed LyC emission to escape galaxies during this era remain elusive.
Hubble Space Telescope image of the interacting galaxy group Arp 194. The blue stream connecting the galaxies is where tidally stripped gas formed millions of new stars. [NASA, ESA, and the Hubble Heritage Team (STScI/AURA)]
While observational studies have increasingly pointed to galaxy mergers as considerable drivers of LyC escape, only a small number have explicitly considered the role of mergers in the epoch of reionization due to spectral and spatial resolution limitations. Leveraging the high-resolution capabilities of JWST, Shengzhe Wang (University of Chinese Academy of Sciences; National Astronomical Observatories, Chinese Academy of Sciences) and collaborators performed detailed analyses of two LyC-leaker candidates to better understand the role of galaxy mergers in heating up the universe.
Using candidate LyC leakers first identified in the LymAn Continuum Escape Survey (LACES) carried out with the Hubble Space Telescope, Wang and team obtained high-resolution JWST spectroscopy for two galaxies at redshift z = 3.1 (about 2 billion years after the Big Bang), LACES94460 and LACES104037. After carefully identifying and measuring emission lines in each candidate, the authors confirmed LACES104037 as a merger-driven LyC leaker. However, the apparent LyC emission from LACES94460 was determined to be contamination from a low-redshift object.
Hubble and JWST observations of LACES104037. The top panels show the Hubble images (first two panels) and the JWST spectral observations (last three panels). The bottom panel shows a composite three-color image marking the location of two primary interacting galaxies and the LyC emission along with the associated spectra. Click to enlarge. [Wang et al 2026]
LACES104037-LyC represents the first confirmed LyC leaker of its kind: an external clump of star formation in a tidal tail. This discovery aids in our understanding of how mergers can drive high-energy photon emission, and highlights the critical role galaxy mergers may play in the epoch of reionization. Future high-resolution observations will hopefully reveal more knots of LyC-leaking stars within merging galaxies and provide a deeper understanding of this important evolutionary phase of the universe.
“Confirmation and Refutation of Lyman Continuum Leakers at z ~ 3 with JWST NIRSpec IFU,” Shengzhe Wang et al 2026 ApJL 1006 L17. doi:10.3847/2041-8213/ae8523
The post A Lyman-Leaking Galaxy Merger Confirmed with JWST appeared first on AAS Nova.
A star being torn apart by a black hole is already an unusual sight. In this case, the black hole was in an even stranger place: more than 30,000 light-years from the center of the galaxy that appears to host it. NASA’s Neil Gehrels Swift Observatory helped confirm the event after an automated survey spotted […]
The post A Black Hole Far from Its Galaxy’s Center Was Caught Tearing Apart a Star appeared first on Curiosmos.
The rich and famous who filed into the Kennedy Center’s opera house in December were there to enjoy one of the nation’s most exclusive celebrations of the performing arts: the center’s annual honors gala.
The black-tie event, hosted by President Donald Trump, prioritized tickets to people who donated more than $75,000 to the center. This year, it feted Hollywood icon Sylvester Stallone, the legendary glam rock band Kiss and the Grammy Award-winning disco pioneer Gloria Gaynor.
Among the attendees that evening were two lower-profile government officials whose regulatory decisions had been crucial to the future of the gala’s broadcast sponsor, CBS, and its parent company, Paramount.
© Cheng Xin/Getty Images
The Bullet Cluster is best known for being considered a “smoking gun” for the existence of dark matter, but even a famous galaxy cluster like this one has its secrets. For more than two decades, different measurements of the merging clusters’ dynamical mass ratio have yielded vastly different results. In a recent research article, Boseong Young Cho (Yonsei University) and collaborators used data from JWST and the Dark Energy Camera to revisit the Bullet Cluster and measure its mass ratio using new data. The image above shows a near-infrared view of the Bullet Cluster from JWST, with an overlay of mass contours derived from gravitational-lensing measurements. The team’s analysis assigned a mass of 1.5 quadrillion solar masses for the main cluster (on the left side of the image above), and a mass of 150 trillion solar masses for the subcluster. This suggests a mass ratio of roughly 10, which matches the value supported by hydrodynamical simulations and cements the event as a minor merger. The team also identified three distinct cluster halos (two in the main cluster and one in the subcluster), providing an important constraint on the cluster configuration for future modeling. To learn more about this investigation of the Bullet Cluster, be sure to check out the full research article linked below!
“Joint JWST–DECam Lensing Reveals that the Bullet Cluster Is a Minor Merger,” Boseong Young Cho et al 2026 ApJ 1005 28. doi:10.3847/1538-4357/ae6a90
The post Featured Image: A Mass Mystery in the Bullet Cluster appeared first on AAS Nova.
Fox Corporation has agreed to buy Roku Inc. for $160 per share, an approximate enterprise value of $22 billion, the firms announced today.
The acquisition would unite Fox’s broadcast channels, including Fox, Fox News, Fox Business, and FS1, as well as its streaming businesses, including Tubi, a free ad-supported streaming television (FAST) platform that Fox bought in 2020, with Roku’s own FAST service, The Roku Channel, and Roku’s streaming hardware business, including its streaming sticks and smart TVs. Roku says it has 100 million households using its platform.
The most valuable part of Roku’s business isn't its hardware, which lost $19.1 million in the quarter ending March 31, 2026, but its the operating system (Roku OS) and advertising business. In that same quarter, Roku’s advertising and subscriptions business posted a gross profit of $584.1 million, with the advertising business pulling in $371 million in revenue. The COVID-19 pandemic helped Roku become profitable in 2021, but the company didn’t see annual profitability again until 2025.
© Getty
Paramount Skydance is accusing Netflix of maintaining a campaign against its proposed acquisition of Warner Bros. Discovery (WBD).
In a June 5 letter (PDF) addressed to Jared A. Hughes, acting section chief of the Media, Entertainment, and Communications Section of the US Department of Justice's (DOJ's) Antitrust Division, and A. Maya Kahn, a trial attorney for the Antitrust Division, and first reported on by Politico today, Paramount chief legal officer Makan Delrahim accused Netflix of trying to influence stakeholders about the merger. The letter reads:
Indeed, Netflix’s panic-level response and scorched-earth campaign to try and poison regulators and other stakeholders against the Transaction shows just how seriously Netflix takes Paramount as a scaled competitor.
The letter from Delrahim, a former assistant attorney general for the Antitrust Division, is a response to a letter that The International Brotherhood of Teamsters sent to the DOJ in March. The teamsters' letter argued that Paramount and WBD's merger would threaten film and TV workers. The union, which has 1.3 million members, asked the DOJ to block the merger "unless substantial and enforceable safeguards are put in place to increase domestic production and protect jobs," per an announcement from the group.
© Mike Cohen/Getty Images for The New York Times
Where do black hole mergers happen? Recent research finds evidence that most black hole mergers occur in triple systems containing a close inner binary and a more distant third party.
Thanks to the combined efforts of the LIGO, Virgo, and KAGRA gravitational wave detectors, we’ve now collected the subtle signals from several hundred pairs of merging stellar-mass black holes. As this collection of spacetime ripples continues to grow, researchers are seeking to understand how these merging black hole pairs were assembled.
In the simplest scenario, pairs of high-mass stars independently evolve into black holes, lose momentum by emitting gravitational waves, and meld into one another. But the population of merging black holes likely contains contributions from multiple sources, including multiple-star systems, the disks surrounding accreting supermassive black holes, and black holes that have already experienced a merger. Which of these merging populations is reflected in our catalog of gravitational wave events?
A black hole binary system with aligned spins and orbit (top) and a binary with spins tilted by 90 degrees (bottom). [AAS Nova/Kerry Hensley]
So far, the increasingly large pool of gravitational wave detections tentatively points to a feature that could illuminate the sites of black hole mergers: a peak in the spin–orbit tilt distribution around 90 degrees. This feature has been most confidently extracted from nonparametric analyses, which do not make assumptions about the population of merging black holes, but it has also been hinted at in parametric analyses, which do make population-level assumptions.
To investigate this feature further, a team led by Jakob Stegmann (Max Planck Institute for Astrophysics) performed a new, astrophysically motivated parametric analysis of the most recent catalog of gravitational wave detections from LIGO, Virgo, and KAGRA.
The three populations in the team’s modeling. [Stegmann et al. 2026]
Posterior predictive distribution of black hole spin–orbit tilts. Both the authors’ modeling (solid blue line) and the nonparametric modeling performed by the LIGO–Virgo–KAGRA collaboration (solid green line) find evidence for a peak in the spin–orbit tilt distribution near cosθ = 0. [Adapted from Stegmann et al. 2026]
Stegmann and collaborators pointed out that the triple-system scenario aligns well with other properties of the observed population of black hole mergers, such as the mass and effective spin distributions. This scenario may also be necessary to explain certain merging pairs that might have had slightly eccentric orbits at the moment of merging. If future gravitational wave observations strengthen the evidence for a peak in the spin–orbit tilt distribution near 90 degrees, it will have profound implications for how and where black holes merge in our universe.
“Gravitational-Wave Observations Suggest Most Black Hole Mergers Form in Triples,” Jakob Stegmann et al 2026 ApJL 1000 L59. doi:10.3847/2041-8213/ae52ec
The post Third Star’s the Charm: Most Merging Black Holes Might Be in Triple Systems appeared first on AAS Nova.
What factors impact how long it takes for a supermassive black hole binary to merge? New research investigates the influence of orbital inclination on the population of merging black holes in our universe.
This image from the Hubble Space Telescope shows NGC 6240, the result of a three-galaxy merger that contains three nuclei, two of which appear to have active supermassive black holes. [NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)]
With observational evidence for supermassive black hole mergers taking shape, theorists are exploring the details of these mergers. One aspect of this process that hasn’t yet been examined fully is how the relative inclination of the merging galaxies impacts the time it takes for their central black holes to merge — a factor that has the potential to shape the demographics of merging supermassive black holes across our universe.
Sena Ghobadi (Georgia Institute of Technology) and collaborators used 3D dynamical models to explore how the angle at which galaxies collide — and therefore how the black holes’ orbits are tilted relative to one another — impacts the time it takes the supermassive black holes at their centers to merge.
In the simulations, Ghobadi’s team placed a 106–108-solar-mass black hole at the center of a disk galaxy. Then, they sent a second, smaller black hole spiraling toward it from a distance of 1 kiloparsec (3,300 light-years).
The team varied the inclination of the incoming black hole’s orbit relative to the disk of the target galaxy from 0 to 75 degrees. They also explored the impact of changing the black hole masses and the galaxy’s central gas density, disk rotation speed, and gas mass fraction. For each simulation, they recorded how long it took for the incoming smaller black hole to get within 10 parsecs of the larger black hole.
Simulated orbits of black holes with initial inclinations of 0 (top), 25 (center), and 45 (bottom) degrees. For the 0-degree simulation, the black holes merge in 5.75 billion years. For the 25-degree simulation, the merger occurs at 7.90 billion years. The 45-degree simulation fails to merge within the lifetime of the universe. [Adapted from Ghobadi et al. 2026]
The simulations showed a clear trend with changing inclination: a black hole with an orbital inclination greater than 20 degrees took longer to merge than those with inclinations of 0–20 degrees. For black holes approaching the merger on slightly inclined orbits, dynamical friction between the black hole and the stars and gas of the galaxy worked to drag the black hole down into the disk, decreasing its inclination over time and guiding it toward a merger. Aside from the influence of inclination, the team also found that higher-mass supermassive black holes and more rapidly rotating galactic disks tended to lead to faster mergers.
For inclinations greater than roughly 45 degrees, a dramatic transition took place, with the orbits of the incoming black holes becoming more inclined over time rather than settling down into the disk. These simulated black holes failed to merge in the allotted time of 14 billion years.
With highly inclined binaries failing to merge within the lifetime of the universe, this suggests that the dual active galactic nuclei and the supermassive black hole binaries that produce gravitational waves in the universe today are the result of binaries with initial inclinations of 20 degrees or less.
“Evolution of Supermassive Black Hole Pairs on Inclined Orbits in Postmerger Galaxies,” Sena Ghobadi et al 2026 ApJ 999 131. doi:10.3847/1538-4357/ae40bc
The post How Tilted Orbits Impact Supermassive Black Hole Collisions appeared first on AAS Nova.
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.
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]
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.
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.
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]
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.
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.
“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
The post Signs of a Supermassive Black Hole Merger in NGC 4486B appeared first on AAS Nova.
Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.
Title: Fast Radio Bursts from White Dwarf Binary Mergers: Isolated and Triple-Induced Channels
Authors: Cheyanne Shariat et al.
First Author’s Institution: California Institute of Technology
Status: Published in ApJL
Since the discovery of the Lorimer Burst in 2007, astronomers have studied fast radio bursts (FRBs) in extreme detail. However, these extragalactic millisecond-duration radio transients have remained an enigma. Like many astrophysical phenomena, their discovery was serendipitous. The most prolific FRB-discovery instrument, CHIME, was not originally built to study them; the acronym stands for “Canadian Hydrogen Intensity Mapping Experiment,” reflecting its original (and ongoing) mission to map the cosmological distribution of hydrogen in the primordial universe. Since its construction, CHIME has discovered several thousand FRBs and even identified a smaller fraction that are known to repeat, some with consistent periods and others that are sporadic and seemingly random. (Check out their live catalog of repeating sources.)
Despite having thousands of sources to study, the only thing we know fairly confidently about FRBs is that many of them are extragalactic, and some others are even at cosmological distances. If we don’t understand how FRBs are produced, how can we know their distance? FRBs display a unique signature in their radio spectra, where higher frequencies arrive at the telescope first and lower frequencies arrive later, delayed by an inverse frequency-squared relationship. In a “dynamic spectrum,” this makes the pulse actually appear like a sweep from high to low frequency that follows the shape of a parabola; for example, the brightest pixels in Figure 1 of this Bite display this sweeping shape. This effect results from the FRB propagating through an ionized plasma with a specific density. Since the FRB is ultimately an electromagnetic wave, this slows down the speed of the FRB as a function of frequency. Measuring this effect, called dispersion, allows us to infer that the FRB signal passed through more plasma than can be accounted for within our own Milky Way galaxy, indicating that the FRB must have originated from much farther away. So far, there is only one example of an FRB from within our own galaxy, which was identified as originating from a magnetar, a highly magnetic pulsar.
Although we know that most FRBs are located at extragalactic or even cosmological distances, it’s still not clear what objects that can generate them. Since magnetars and other compact objects are often considered reliable progenitors of FRBs, understanding their populations and comparing them with observed FRBs may reveal the origin of FRBs. Candidate FRB progenitor systems are usually divided into “prompt” channels — like the collapse of a star into a magnetar during a core-collapse supernova — and “delayed” channels where the FRB engine is formed on longer timescales, like accretion-induced collapse of white dwarfs or compact-object mergers. Figure 1 displays several of these channels, particularly those considered in today’s article, in which stellar evolution results in the transfer of material from one star to another (accretion-induced collapse) or gravitational radiation allows two objects to inspiral and merge. These events could create highly magnetized white dwarfs or magnetars that may produce FRBs.
Figure 1: Three different pathways and their associated fractions for producing an FRB candidate. Depending on the mass and composition of the merging objects, an FRB candidate may be produced instead of causing the white dwarf to explode in a Type Ia supernova. These are the systems of interest in today’s article. [Shariat et al. 2026]
In today’s article, we’re considering the role of hierarchical triple star systems in producing FRB-emitting objects. A hierarchical triple consists of an inner binary and a distant, tertiary star; see the first panel of Figure 1. These systems may play an important role because many stars that become white dwarfs of at least 0.8 solar mass exist in hierarchical triples: 35% of 2-solar-mass stars and 50% of 8-solar-mass stars are in triples. Stars of both masses will end their lives as white dwarfs.
The tertiary star plays an essential role in increasing the likelihood of the inner binary interacting: the eccentric Kozai–Lidov (EKL) mechanism causes the inner orbit to oscillate between highly inclined (with respect to the orbit of the tertiary star) and highly eccentric. Unlike isolated binaries that aren’t excited in this way, the eccentric orbit phase causes the inner stars to pass much closer to each other, increasing the likelihood of interaction or even a merger that could produce an FRB-emitting remnant.
The authors of today’s article produced a realistic population of triple systems by sampling from empirically measured distributions of masses, periods, and orbital eccentricities. These distributions were derived from observations collected by the Gaia satellite, an all-sky optical telescope that has now observed more than one billion stars in the Milky Way. The authors also created a “control” sample of isolated binaries by removing the tertiary star from the same sample of systems. The systems were then evolved for an age randomly selected between 0 and 12.5 billion years to simulate “constant” star formation similar to that in the Milky Way. Finally, they reapply these simulations at three different “metallicities” — the fraction of elements other than hydrogen or helium relative to that of the Sun.
Figure 2 displays these two populations: the isolated binaries are shown as filled histograms, and the hierarchical triples are shown as histograms with dashed lines. Each column represents the three different progenitor classes considered in Figure 1. At solar metallicity, the delays to merging are greater in triples than in secular binaries, leading to more mergers overall (top left panel). At lower metallicities, there are fewer mergers in all progenitor pathways in triples than in isolated binaries due to changes in stellar evolution attributed to weaker winds and larger pre-white dwarf masses.
Figure 2: The number of mergers as a function of time from the hierarchical triple’s formation, given different three metallicities (rows) and the three primary formation pathways for FRB-emitting merged remnants considered in Figure 1. Filled histograms are isolated binaries, dashed histograms are triples. [Shariat et al. 2026]
Since we can’t realistically track individual star systems from their formation to the end, we need to convert the delay-time distributions shown in Figure 2 to an observable. To do this, the authors combine the results of their simulations with measurements of the cosmic star formation rate — how many stars form per unit time as a function of the universe’s age — and the evolution of the universe’s metallicity as stars fuse hydrogen into heavier elements and return this material to the interstellar medium as these stars end their lives in shedding red giants or supernovae. This analysis produces the number of FRBs that should be observed as a function of distance or lookback time into the universe’s history, which is displayed in Figure 3.
Figure 3: The number of FRBs expected as a function of cosmological redshift. The present is to the left, the distant and early universe is on the right. The top panel shows the three formation channels, with binary and triple systems separated. The bottom panel displays the composite rate and compares it with empirical estimates from existing FRB observations. [Shariat et al. 2026]
Original astrobite edited by Catherine Slaughter.
About the author, Will Golay:
I am a graduate student in the Department of Astronomy at Harvard University and the Center for Astrophysics | Harvard & Smithsonian, advised by Edo Berger. I study radio emission from transient astrophysical objects like tidal disruption events.
The post Triple Double: Do Merging White Dwarfs Produce Fast Radio Burst–Emitting Remnants? appeared first on AAS Nova.
This image from JWST shows the galaxy cluster XLSSU J021744.1-034536. The cluster is at a redshift of z = 1.98, placing it at cosmic noon, when the universe’s star formation was at its peak. To study the formation and evolution of this cluster, Zachary Scofield (Yonsei University) and collaborators collected wide-ranging clues, including signs of weak gravitational lensing, the glow of gas in between the galaxies of the cluster, and an analysis of the cluster members. The image above (click for the full view) indicates the galaxies that are members of the cluster. The brightest galaxy in the cluster is marked with a yellow square, and the remaining cluster members are circled, with the color indicating whether the galaxy’s cluster membership was assigned photometrically (magenta) or spectroscopically (green). Combining all available lines of evidence, Scofield’s team found that XLSSU J021744.1-034536 is undergoing a merger, giving a rare glimpse into this stage of cluster formation. To learn more about this work and what it tells us about galaxy clusters at cosmic noon, be sure to check out the full article linked below.
“An Active Galaxy Cluster Merger at Cosmic Noon Revealed by JWST Weak Lensing and Multiwavelength Probes,” Zachary P. Scofield et al 2026 ApJL 999 L1. doi:10.3847/2041-8213/ae447a
The post Featured Image: An Active Galaxy Cluster Merger at Cosmic Noon appeared first on AAS Nova.
Editor’s Note: For the remainder of 2025, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume January 2nd.
Red ellipses overplotted on the first eight rings identified with Hubble in the Bullseye galaxy. Click to enlarge. [Pasha et al 2025]
Galaxy mergers and interactions are commonplace in the universe, but it is very rare for a dwarf galaxy to strike right through a large galaxy’s core. When this does happen, the collision sends shock waves through the impaled galaxy, sweeping gas and dust outward and forming rings where star formation can occur (hence the name, collisional ring galaxy). These galaxies are valuable sites to explore galactic structure and evolution as well as provide a unique case to study galaxy mergers and interactions. Previously discovered collisional ring galaxies have at most two or three rings, so when Pasha and collaborators found the stunning nine in the Bullseye, it was clear they caught something significant.
Why does the Bullseye have so many rings? Theoretical studies of these head-on collisions predict that many rings will form and travel outward after the initial crash; however, the rings tend to dissolve after only a few hundred million years. Catching this collision only 50 million years after it happened means we are witnessing the earlier stages of the Bullseye’s evolution. This lucky discovery not only confirms theoretical predictions for collisional ring galaxies, it also may be a clue to how another unique galaxy type, giant low-surface brightness galaxies, originate. Given the wispy ring material found at very large radii from the Bullseye, the authors suggested that collisional ring galaxies may evolve into giant low-surface brightness galaxies as they expand outward and fade. Further investigation and a larger sample of collisional ring galaxies are necessary to confirm this hypothesis, but the Bullseye provides interesting and critical observational evidence of these predictions for the first time.
Imad Pasha et al 2025 ApJL 980 L3. doi:10.3847/2041-8213/ad9f5c
The post Selections from 2025: The Nine-Ringed Bullseye Galaxy appeared first on AAS Nova.
The gravitational wave background — the constant, low-frequency hum of colliding supermassive black holes across the universe — seems to have a larger amplitude than expected. Can preferential accretion explain why?
The peculiar galaxy Arp 122, shown here in a Hubble Space Telescope image, is in the process of merging. When two galaxies merge, the supermassive black holes at their centers eventually merge as well. [ESA/Hubble & NASA, J. Dalcanton, Dark Energy Survey/DOE/FNAL/DECam/CTIO/NOIRLab/NSF/AURA; Acknowledgement: L. Shatz; CC BY 4.0]
Though some researchers have invoked new physics to explain this discrepancy, all that may be required is an adjustment to our understanding of the supermassive black hole binary population in our universe. In a recent research article, Julia Comerford (University of Colorado Boulder) and Joseph Simon (University of Colorado Boulder; Oregon State University) tackled one of the underlying assumptions about the masses of merging supermassive black holes.
When supermassive black holes merge, the strength of the gravitational wave signal depends on how similar the black holes were in mass; merging equal-mass black holes produce stronger signals than mismatched black holes do. What if, Comerford and Simon proposed, the masses of merging supermassive black holes scattered across the universe are more similar than previously thought?
The initial and final black hole mass ratio, q. The blue and orange lines show a 10% and 20% increase in the total mass of the binary, respectively. The dotted line divides major mergers (q ≥ 0.25) and minor mergers. [Comerford & Simon 2025]
What’s more, it appears that the smaller of the two black holes tends to pack on more mass, meaning that the masses of the black holes grow more similar as they wind toward a merger. (This somewhat counterintuitive result arises because the smaller of the two black holes encounters more gas as it spirals inward.)
If accretion tends to even out the masses of the binary members, is the resulting increase in signal strength large enough to explain the observed amplitude of the gravitational wave background?
Probability distribution functions for the calculated gravitational wave background amplitude for different accretion conditions. The brown diamond shows the median amplitude from the NANOGrav data set. [Adapted from Comerford & Simon 2025]
The signal-boosting effect from preferential accretion may be especially important if the binary evolution timescale — the time it takes for black holes approaching a merger to reach a separation at which their gravitational waves become accessible to pulsar timing arrays — is longer than the estimated 1.8 billion years. At longer timescales, models without this accretion become entirely inconsistent with observations. Future research that examines the masses and accretion rates of supermassive black holes as they approach make their way toward a merger can provide further clues to the role of preferential accretion.
“Preferential Accretion onto the Secondary Black Hole Strengthens Gravitational-Wave Signals,” Julia M. Comerford and Joseph Simon 2025 ApJ 994 168. doi:10.3847/1538-4357/ae1133
The post Boosting the Gravitational Wave Background appeared first on AAS Nova.
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