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Traditionally, black holes are usually thought to "reside" at the centers of galaxies. However, a research team led by Dr. An Tao from the Shanghai Astronomical Observatory of the Chinese Academy of Sciences has discovered a wandering black hole in a dwarf galaxy located about 230 million light-years away (redshift z = 0.017).

Unlike typical black holes, this one is not located at the galactic center, but rather, it is offset by nearly one kiloparsec and is launching radio jets. This off-nuclear, in-situ, accreting black hole with jets, found in a nearby dwarf galaxy, is one of the closest and most convincing cases confirmed thus far.

The discovery, in the Science Bulletin on September 4, strengthens the idea that black hole growth is not limited to galactic centers, offering a new perspective on how supermassive black holes could have grown so rapidly in the early universe.

Black holes are not always in galactic centers

In the common picture of the cosmos, black holes are often regarded as the "hearts" of galaxies. However, an increasing number of observations show that some black holes do not remain obediently at the galactic center. Instead, they are displaced, roaming through the galactic disk or outskirts. These displaced black holes are called wandering black holes, like lost travelers drifting across the universe.

Why look for them in dwarf galaxies? Dwarf galaxies have smaller masses and relatively simpler evolutionary histories. They act as "cosmic fossils" that preserve clues about the early growth of black holes.

Theory predicts that gravitational recoil after galaxy mergers, or interactions involving multiple bodies, can easily kick black holes out of the shallow gravitational wells of dwarf galaxies, leaving them wandering thousands of light-years away. Some simulations even suggest that a considerable fraction of dwarf-galaxy black holes may be displaced by nearly a kiloparsec (about 3,000 light-years). Yet for a long time, direct and unambiguous observational evidence has been lacking.

New discovery: A 'radio-loud wandering black hole' in a dwarf galaxy

The researchers led by Dr. An focused on a dwarf galaxy named MaNGA 12772-12704, located only about 230 million light-years away (z ≈ 0.017) from Earth. Using integral field unit data from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey, they found that this galaxy exhibits weak active galactic nucleus (AGN) signatures. Although its overall morphology is regular and shows no obvious signs of merging or a double AGN, one key feature stood out: The associated radio emission is offset from the geometric center of the galaxy by nearly one kiloparsec (kpc).

Follow-up observations with the Very Long Baseline Array (VLBA) at 1.6 and 4.9 GHz revealed that the source lies 2.68 arcseconds away from the galactic center (corresponding to 0.94 kpc), with a radio core brightness temperature exceeding 1 billion kelvins. At 1.6 GHz, they detected a jet structure extending about 2.2 parsec (7.2 light-years) southeastward. These are typical AGN features.

Furthermore, by analyzing archival data spanning 1993–2023, the researchers discovered that the source exhibits irregular, long-term variability, becoming brighter and dimmer over decades. This behavior is consistent with sustained, in-situ accretion onto a black hole, and clearly distinct from the monotonically decreasing typical of supernova remnants, effectively ruling out such "impostors."

Based on the stellar mass of the host galaxy, the black hole's mass is estimated to be about 300,000 times that of the sun, placing it in the intermediate-mass black hole (IMBH) category.

Together, they confirmed that this is indeed an actively accreting, off-nuclear black hole with jets—currently the nearest and most robustly confirmed case of its kind. "This is like a cosmic lighthouse lit by a wandering black hole, although it has strayed from the galactic center, it still shines outward with powerful energy," said Dr. Liu Yuanqi, a co-author of the study.

Rarity: Why is this one so important?

From a broader statistical perspective, this discovery is particularly striking. Out of more than 3,000 MaNGA dwarf galaxies, 628 showed possible AGN activity, and about 62% appeared offset from their optical centers. Only MaNGA 12772-12704 satisfied the "triple solid evidence" criteria: a compact high-brightness core, parsec-scale jets, and decades-long variability. This suggests that off-nuclear phenomena may not be uncommon. However, a "candidate" does not equal a confirmed detection.

"In dwarf galaxies especially, it is extremely difficult to obtain clear observational evidence for wandering AGN," said Dr. Mar Mezcua, a co-author of the study from the Institute of Space Sciences of Spain.

Scientific breakthrough: A new path for black hole growth

The prevailing view has long been that supermassive black holes primarily grow at galactic centers, rapidly feeding on central gas reservoirs. This study, however, demonstrates that an intermediate-mass black hole located outside the galactic nucleus can also sustain accretion and produce jets. These findings provide direct observational support for the idea that distributed feeding and multi-site growth as a potential pathway for the rapid formation of supermassive black holes in the early universe.

"This discovery prompts us to rethink black hole–galaxy co-evolution. Black holes are not only central 'engines,' they may also quietly reshape their host galaxies from the outskirts," said Dr. An.

Even when located in a galaxy's "suburbs," wandering black holes can inject energy into the surroundings through powerful outflows, influencing galactic dynamics and star formation.

Outlook: Unveiling the cosmic population of 'invisible' black holes

This study transforms wandering black holes from theoretical speculation into direct observational reality. With the advent of next-generation telescopes, such "lost black holes" may no longer be rare. In the near future, extremely large optical telescopes will measure galactic centers and structures with higher precision.

Meanwhile, deep, high-resolution radio surveys using facilities such as the Five-hundred-meter Aperture Spherical Telescope (FAST) core array and Square Kilometer Array will detect even fainter radio signals, potentially resolving sub-parsec-scale micro-jets. These advances will lead to breakthroughs in confirming and statistically studying off-nuclear black holes.

Perhaps one day, we will recognize that wandering black holes are not rare at all, but rather silent travelers at the edges of galaxies, quietly shaping their hosts' cosmic evolution.