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Hubble, Chandra and archival radio observations revealed two likely active supermassive black holes in the center of the merging galaxy MCG-03-34-64, about 800 million light-years from Earth. The sources are separated by roughly 100 parsecs—about 300 light-years. That makes this the closest spatially resolved candidate pair reported with optical, X-ray and radio evidence, not necessarily the closest black-hole pair of any kind.
What astronomers found in MCG-03-34-64
MCG-03-34-64 is a gas-rich luminous infrared galaxy whose central region is unusually active. At its center, astronomers identified two compact sources whose emissions are best explained as two actively feeding supermassive black holes. NASA describes the galaxy as about 800 million light-years away; the research paper reports a redshift of z = 0.016.
The black holes themselves were not photographed. A black hole’s event horizon does not shine like a star. Instead, astronomers infer an active black hole from the intense radiation produced when gas and dust fall toward it, heat up and radiate across the electromagnetic spectrum.
How Hubble, Chandra and the VLA built the case
Hubble’s high-resolution optical observations showed three distinct bright spots, or emission centroids, in the galaxy’s compact nucleus. Much of the relevant optical signal comes from glowing oxygen gas, including [O III] emission. Hubble resolved structure in a region where lower-resolution observations could blend separate sources together.
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The long, cross-shaped features that may appear around compact sources in Hubble images are diffraction spikes. They are imaging artifacts created as light interacts with the telescope’s mirror structure, not physical beams or structures extending from the galaxy.
Chandra added the crucial high-energy evidence: it resolved two powerful X-ray peaks aligned with two of Hubble’s optical spots. X-rays are commonly produced by very hot material close to an accreting black hole. The research paper also reports two comparable peaks in the neutral iron K-alpha band, around 6.2–6.6 keV.
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Archival observations from the Karl G. Jansky Very Large Array (VLA) showed two radio peaks at matching positions. Those data were taken at about 8.46 GHz, in the 3.6-centimeter band. The alignment of optical, X-ray and radio sources makes the two-active-nucleus interpretation stronger than any one wavelength alone could provide.
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Why the third bright spot is not a third black hole
Hubble saw three optical spots, but Chandra and the radio data support two compact active sources. The third spot has not been identified as another black hole. It could be gas shocked by a jet from one of the active nuclei, or gas energized by radiation from them; its origin remains uncertain and needs further observations.
What “closest pair” means—and what it does not
The word “closest” needs a qualification. NASA’s September 2024 announcement called this the closest confirmed pair of supermassive black holes observed in visible light and X-rays. The peer-reviewed paper uses more cautious language: it describes a candidate dual active galactic nucleus and says that, if confirmed, its roughly 100-parsec separation would be the closest dual AGN reported with spatially resolved, multiwavelength observations.
NASA also notes that radio observations have identified at least one black-hole pair with a smaller separation, but without comparable confirmation across other wavelengths. So MCG-03-34-64 is not safely described as the closest two black holes in the universe, or as the closest binary black hole ever discovered. The record depends on what counts as confirmation and how the sources are resolved.
These terms are related but not interchangeable. A dual AGN means two active galactic nuclei in one interacting or merging galaxy system. A binary black hole specifically suggests two black holes bound in orbit around one another. A black-hole merger is their eventual coalescence, which has not occurred here. The study’s “candidate” wording reflects scientific caution: the evidence is compelling, but the black holes are inferred from emissions and their positions rather than directly imaged.
A likely merger, on a timescale far beyond human experience
The likely story is that each black hole once sat at the center of its own galaxy. As those galaxies merged or interacted, their central black holes were brought into the same galactic environment. Gas driven inward by the merger can fuel the active nuclei. The pair may eventually move closer and merge, but the roughly 100-million-year timescale mentioned in the NASA release is an estimate, not a countdown; the final stages of black-hole pairing depend on complex dynamics involving gas and stars.
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A merger of supermassive black holes would generate low-frequency gravitational waves. LIGO is designed primarily to detect much higher-frequency waves from stellar-mass compact-object mergers. A future space-based observatory such as LISA is intended to target the lower frequencies associated with massive black-hole systems. NASA’s September 2024 release described LISA as planned for the mid-2030s; that does not mean this particular pair is expected to merge soon enough to become a LISA detection target.
Why the observation matters
MCG-03-34-64 offers a relatively nearby laboratory, by extragalactic standards, for studying how galaxy mergers feed black holes and bring them together. Its significance is not just the small separation: researchers can compare optical structure, X-ray activity and radio emission from the same compact region. The case shows why combining observatories that see different wavelengths can reveal more than any single telescope can.
The result was published by Anna Trindade Falcão and colleagues as “Resolving a Candidate Dual Active Galactic Nucleus with ∼100 pc Separation in MCG-03-34-64” in The Astrophysical Journal, volume 972, article 185, on September 9, 2024. Read the peer-reviewed paper. NASA’s announcement and image explanation provide additional context.
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