Another First for the JWST: It Detects Three Supermassive Black Holes in the Same Galaxy
The nature of supermassive black holes (SMBHs) sits at the very frontier of modern astrophysics. Virtually every large galaxy in the observable universe, including our own Milky Way, appears to harbor one of these gravitational titans at its core — yet how these behemoths grew to millions or even billions of solar masses in the relatively short span of cosmic history remains one of astronomy's most compelling open questions. Now, a stunning new discovery made possible by the James Webb Space Telescope (JWST) is offering a powerful new clue, one that could fundamentally reshape our understanding of how supermassive black holes are born and grow.
Astronomers working with the JWST have, for the first time ever, identified three simultaneously active, accreting black holes residing within a single galaxy. While instances of three interacting galaxies each hosting their own central black hole have been documented before, never has a triple black hole system been confirmed within the boundaries of one galaxy — until now. The findings not only represent a historic observational milestone but also lend compelling support to the idea that galaxy mergers played a decisive role in assembling the supermassive black holes we observe across the cosmos today.
The BlackTHUNDER Program and the Discovery
The discovery is presented in new research published in Astronomy and Astrophysics, titled "BlackTHUNDER: Evidence of three massive black holes in a z ∼ 5 galaxy." The lead author is Dr. Hannah Übler from the Max Planck Institute for Extraterrestrial Physics (MPE). The term BlackTHUNDER stands for "Black holes in the early Universe and their dense surroundings" — a dedicated JWST observing program designed specifically to probe the earliest epochs of black hole formation and growth.
The galaxy at the center of this landmark discovery is designated J0148-4214. It lies more than 12.5 billion light-years from Earth, meaning the JWST is observing it as it appeared when the universe was only about 1.2 billion years old — a mere fraction of its current estimated age of 13.8 billion years. This places the galaxy at a redshift of approximately z ∼ 5, deep within the epoch known as Cosmic Dawn, when the first large structures were rapidly assembling from primordial gas and dark matter. To study objects so ancient and so distant with this level of spectroscopic detail is a testament to the extraordinary capabilities of the JWST.
For this work, the team employed the JWST's NIRSpec Integrated Field Unit (IFU), an instrument capable of capturing both spatial and spectral information simultaneously across an extended field of view. This allowed the researchers to map the galaxy's light in extraordinary detail, disentangling the overlapping signals of three distinct black hole systems embedded within it.
Three Black Holes: Masses, Positions, and Accretion
The three actively accreting black holes detected within J0148-4214 have estimated masses of 80 million, 2 million, and 0.6 million solar masses, respectively. Their positions within the galaxy are not uniform: the most massive and the least massive black holes are located closest together, separated by a projected distance of only about 620 light-years (approximately 190 parsecs). The third black hole occupies the outer region of the galaxy, lying roughly 5,500 light-years (approximately 1.7 kiloparsecs) from the galactic center.
"The JWST data allowed us not only to identify the three black holes, but also to estimate their masses, accretion rates, and the stellar mass of the galaxy. We find a total stellar mass of about 1.3 billion suns, and the black holes represent a significant fraction of that." — Dr. Giovanni Mazzolari, second author of the study and researcher at MPE
The fact that these black holes collectively represent a significant fraction of the galaxy's total stellar mass of approximately 1.3 billion solar masses is itself remarkable. In the local universe, SMBHs typically account for only a tiny fraction — around 0.1 to 0.5 percent — of their host galaxy's bulge mass. The relatively high black-hole-to-stellar mass ratio observed in J0148-4214 may indicate that these black holes grew especially rapidly in the early universe, or that the galaxy's star formation had not yet fully caught up with the pace of black hole assembly.
- Black Hole 1: ~80 million solar masses — located in the central region of the galaxy
- Black Hole 2: ~0.6 million solar masses — located in the central region, ~620 light-years from BH1
- Black Hole 3: ~2 million solar masses — located in the galactic outskirts, ~5,500 light-years from center
- Host galaxy stellar mass: ~1.3 billion solar masses
- Galaxy redshift: z ∼ 5 (observed ~1.2 billion years after the Big Bang)
"This is the first evidence of three active black holes in a single galaxy in the distant Universe. It suggests that processes in the early Universe were efficient at bringing massive black holes together, setting the stage for the massive black hole mergers we expect to detect with future gravitational wave observatories." — Dr. Hannah Übler, lead author, MPE
The Spectroscopic Fingerprint: How the Black Holes Were Identified
Identifying black holes at such extreme distances requires exquisitely precise spectroscopic analysis. The researchers detected three distinct broad-line regions (BLRs) — compact zones of rapidly moving gas that exist in the immediate vicinity of actively accreting black holes. In a BLR, hydrogen gas orbits the central black hole at velocities of thousands of kilometers per second. This rapid motion causes the Doppler effect to broaden the characteristic Hydrogen-alpha (Hα) emission line, producing a spectral signature that is recognizably distinct from other astrophysical sources.
However, broad Hα emission alone is not a definitive diagnostic. Other energetic phenomena — including supernovae, shock fronts, and stellar winds from massive stars — can also produce broadened hydrogen lines. The key to unambiguous identification lies in a complementary measurement: the width of the [O III] (doubly-ionized oxygen) emission line. In an Active Galactic Nucleus (AGN), the [O III] line remains characteristically narrow, because the forbidden transition that produces it cannot occur in the dense, high-velocity gas of the BLR. Therefore, the combination of broad Hα and narrow [O III] is effectively a unique spectroscopic fingerprint of an AGN-powered black hole, ruling out stellar or supernova origins.
As the research team reports in the paper:
"The black holes are revealed through broad Hα emission (FWHM = 430–2920 km/s) without a forbidden-line counterpart in the bright [O III] doublet. Channel maps of the asymmetric central Hα profile isolate two spatially distinct broad-line regions (BLRs), separated by 190 ± 40 pc, while a third BLR is found in the galaxy outskirts with a projected separation of 1.7 kpc."
This rigorous multi-line spectroscopic approach, made possible by the sensitivity and spatial resolution of the James Webb Space Telescope, gives astronomers high confidence that what they are seeing are indeed three simultaneously active, accreting black holes — not artifacts of star formation or other energetic events within the galaxy.
Two Pathways to Black Hole Growth: Accretion and Mergers
Understanding black hole growth requires appreciating the two fundamental mechanisms available to them. The first is accretion — the gradual consumption of surrounding gas and dust that spirals inward through an accretion disk, releasing enormous amounts of energy in the process and causing the black hole to grow incrementally over time. Over the vast sweep of cosmic history, accretion is widely considered the dominant driver of SMBH mass growth, and the AGN activity observed in J0148-4214 confirms that all three black holes are actively doing exactly this.
The second pathway is mergers — the coalescence of two black holes following the gravitational inspiral and eventual collision of their host galaxies. Mergers can deliver large, discrete jumps in black hole mass on relatively short timescales compared to accretion alone. Theorists have long suspected that mergers must have been especially common and impactful in the early universe, when galaxies were smaller, denser, and more frequently colliding in the crowded environment of the young cosmos. The detection of a triple black hole system in J0148-4214 offers direct observational evidence that this violent, merger-driven history was indeed playing out in earnest just over a billion years after the Big Bang.
"These results are extremely exciting. They suggest that black hole merging may be an additional, fast route for their rapid growth in the early Universe." — Professor Roberto Maiolino, University of Cambridge, co-author of the study
The observational evidence from J0148-4214 captures both mechanisms in action simultaneously — active accretion confirmed through AGN spectroscopy, and impending mergers inferred from the close proximity and dynamical trajectories of the black hole pair.
A Future of Mergers: What Happens Next?
The two central black holes, separated by just 620 light-years, are gravitationally bound and appear destined to merge. Applying standard models of dynamical friction — the process by which a massive body moving through a field of other masses gradually loses momentum and spirals inward — the research team estimates that this merger will likely occur within approximately 700 million years. In cosmological terms, this is a remarkably short timescale, suggesting that these early-universe mergers may have been a rapid and efficient channel for building the most massive black holes observed in the present-day universe.
The third, more distant black hole appears to be slowly sinking toward the galactic center, dragged inward by the gravitational potential of the system. If this trend continues, it too will eventually participate in a merger, creating what would be a sequential double-merger event — a process that could dramatically accelerate the growth of the central SMBH.
As the authors note in the paper:
"We argue that the two central black holes will likely rapidly merge, with a simple dynamical friction time estimate of the order of ≲700 Myr. Assuming that the third off-nuclear black hole is also in the process of sinking towards the centre, it will likely lead to a second merger."
However, the researchers also acknowledge an intriguing alternative explanation for the third black hole's off-center position. It is possible that this object is actually the result of a previous three-body gravitational interaction or a gravitational recoil event — a process in which the anisotropic (asymmetric) emission of gravitational waves during a black hole merger can impart a powerful "kick" that ejects the merged remnant from the galactic center at high velocity. Crucially, the JWST's observations reveal that even if this is the case, the black hole has retained its accretion disk and BLR — indicating that these structures can survive even highly energetic dynamical events, an important constraint for theoretical models of black hole evolution.
Implications for Gravitational Wave Astronomy
The broader implications of this discovery extend well beyond the study of J0148-4214 itself. The impending merger of the central black hole pair would produce a cataclysmic burst of gravitational waves — ripples in spacetime predicted by Einstein's General Theory of Relativity and first directly detected by LIGO in 2015. However, black hole mergers involving objects millions of times more massive than the sun emit gravitational waves at much lower frequencies than LIGO can detect. These signals fall squarely in the detection range of next-generation space-based observatories such as the Laser Interferometer Space Antenna (LISA), which the European Space Agency is developing for launch in the 2030s.
Discoveries like this one are therefore invaluable for predicting the frequency and nature of the gravitational wave signals that LISA and similar instruments will eventually detect. If triple black hole systems were indeed common in the early universe — as this and other recent findings suggest — then future gravitational wave observatories may detect a rich and complex background of overlapping signals from SMBH mergers across cosmic time.
- Future LISA observations may directly detect gravitational waves from SMBH mergers in the frequency range relevant to J0148-4214-type systems
- The Pulsar Timing Array experiments have already detected hints of a gravitational wave background, potentially from such early-universe SMBH mergers
- Multiple triple black hole systems have now been identified at high redshift, suggesting these configurations were not rare exceptions but common features of early galaxy assembly
JWST: Rewriting the Story of the Early Universe
This discovery is yet another chapter in a rapidly accumulating body of JWST results that are reshaping our understanding of the early universe. Since its first science observations in 2022, the James Webb Space Telescope has repeatedly surprised astronomers with the complexity and maturity of galaxies and black holes at extremely high redshifts. The existence of galaxies with multiple massive black holes just over a billion years after the Big Bang challenges earlier models that assumed SMBH assembly was a slower, more sequential process.
The results from J0148-4214 suggest that, far from being exceptional, multiple black hole systems were likely commonplace in the early universe, and that their mergers contributed substantially to the growth of the supermassive black holes that now anchor galaxies across the cosmos — including the 4-million-solar-mass Sgr A* at the heart of our own Milky Way, imaged for the first time by the Event Horizon Telescope in 2022.
"The possible discovery of a black hole triplet at high redshifts, together with other recent results on distant black hole pairs, indicates that multiple massive black hole systems were likely common in the early Universe." — Übler et al., 2026, Astronomy and Astrophysics
Each new revelation from the JWST pushes the boundaries of what astronomers thought possible to observe. With programs like BlackTHUNDER continuing to probe the universe's earliest epochs, the coming years promise to deliver an ever more detailed and nuanced portrait of how the cosmos — and its most extreme inhabitants — came to be.
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