Three supermassive black holes discovered in a single galaxy for the first time

An international team of astronomers led by the Max Planck Institute for Extraterrestrial Physics has identified three actively accreting supermassive black holes in the galaxy J0148-4214. Matter is falling into the black holes from accretion disks surrounding them.

The galaxy is located more than 12.5 billion light-years from Earth. At this distance, the expansion of space itself becomes apparent. The galaxy therefore appears to be moving away from us, causing its light to shift toward longer wavelengths, resulting in a so-called redshift of z = 5.02.

Using that redshift, the researchers calculated the galaxy’s enormous distance. It is so far away that its light has traveled for 12.5 billion years. We are therefore seeing the galaxy as it appeared only about 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,” says Hannah Übler, research group leader at MPE and lead author of the study.

The research is published in the journal Astronomy & Astrophysics.

Two are located in the galactic center and are separated by only 620 light-years in projection. A third black hole is located in the outer region of the galaxy, at a distance of approximately 5,500 light-years from the center. “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,” says Übler.

Theories of galaxy evolution—based on observations—suggest that, early in the history of the universe, galaxies came very close to one another and merged. In the process, black holes at their centers also merged, giving rise to even more massive black holes at the centers of the merged galaxies.

Hydrogen emission as a tracer

The researchers identified the black holes through their spectral fingerprints: the signatures of hydrogen atoms moving at high velocity in the gravitational potential of the black holes. In the central region, the spectrum exhibits a complex structure best explained by two black holes in close proximity.

To disentangle the two central sources, the team applied spectro-astrometry, a technique that precisely measures spatial shifts in line emission across the galaxy. This made it possible to determine the positions of the black holes, even though they cannot be spatially resolved as separate point sources. A third black hole was detected in the outer region.

Masses and growth

The analysis yields black hole masses of approximately 80 million, 0.6 million and 2 million solar masses. The most massive black hole is accreting at a lower rate than the nearby black hole with a mass of 0.6 million solar masses, which is actively feeding and even exceeding the maximum accretion rate predicted by basic theories of black hole growth (the Eddington limit).

“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,” says Dr. Giovanni Mazzolari, second author of the study and researcher at MPE. “We find a total stellar mass of about 1.3 billion solar masses, and the black holes represent a significant fraction of that.”

The central black hole pair is expected to merge within the next few hundred million years. “These results are extremely exciting,” adds Roberto Maiolino, professor at the University of Cambridge and co-author of the study. “They suggest that black hole merging may be an additional, fast route for their rapid growth in the early universe.”

The third black hole, located off-nucleus, may be the remnant of a previous merger, be displaced from the center by a gravitational recoil kick, or be currently migrating inward.

Implications for research

These observations demonstrate that integral field spectroscopy is an important tool for identifying multiple active black holes in distant galaxies. Without the spatially resolved information provided by NIRSpec-IFS, only one of the three black holes would likely have been detected.

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Swati Mestri

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Robert Egan

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