In the traditional impression, black holes are often "entrenched" in the centers of galaxies. However,Researchers from the Shanghai Observatory of the Chinese Academy of Sciences discovered a "restless" black hole in a dwarf galaxy about 230 million light-years away from the Earth. It did not stay in the core of the galaxy, but deviated from the center by nearly 1 kiloparsec (about 3,000 light-years) and ejected radio jets. This "out-of-core, in-situ accretion, jet-bearing" black hole in a nearby dwarf galaxy is currently one of the cases with the lowest red shift and the most solid evidence.
This discovery further strengthens the understanding that "black hole growth is not limited to the center of galaxies" and provides a new perspective on understanding the rapid growth of supermassive black holes in the early universe.The relevant results were published online in Science Bulletin on September 5, 2025.
In our accustomed picture of the universe, black holes are often regarded as the "hearts" of galaxies. However, more and more observations show that some black holes do not stay in the center of galaxies, but deviate from the core and "wander" in the galactic disk or outer edges. Such objects are called "wandering black holes", wandering around in the universe like lost travelers.
Why look for them in dwarf galaxies? Dwarf galaxies have small masses and relatively simple evolutionary histories. They are like "cosmic fossils" that preserve clues to the growth of early black holes. The theory predicts that the gravitational wave recoil or multi-body interaction after the merger of galaxies can easily cause black holes to be kicked out of the center of dwarf galaxies with shallow gravitational potential wells, and become black holes wandering around the periphery of the galaxy. Some simulations even suggest that a significant proportion of dwarf galaxy black holes are offset from the center by as much as a thousand parsecs, but direct, unequivocal observational evidence has long been lacking.
An international team led by researcher An Tao of the Shanghai Observatory of the Chinese Academy of Sciences set their sights on a dwarf galaxy named MaNGA 12772-12704, which is only about 230 million light-years away from the Earth (redshift z≈0.017). Based on the integrated field of view spectral data of the Nearby Galaxy Spectral Survey (MaNGA), researchers found that this galaxy exhibits weak active galactic nuclei (AGN) features. The overall shape of the galaxy is regular, and there is no obvious sign of merger or double AGN. But the key point is that the radio radiation associated with it is not at the geometric center of the galaxy, but is offset by nearly a thousand parsecs from the center.
In order to confirm its nature, the team used the Very Long Baseline Array radio telescope (VLBA) to carry out deep imaging in the 1.6 GHz and 4.9 GHz bands. The results show that the angular distance between the source and the center of the galaxy is 2.68 arc seconds (corresponding to 0.94 kiloparsecs), the radio core brightness temperature exceeds one billion Kelvin, and a radio radiation structure extending about 2.2 parsecs (7.2 light years) along the southeast direction was observed on the 1.6 GHz image. These are typical AGN characteristics. Not only that, the team also systematically combed through the archive data from 1993 to 2023, and found that the source showed non-monotonic "strong and weak" changes on a scale of decades, consistent with the behavior of "long-term, in-situ accretion"; this is significantly different from the common law of monotonic decay of supernova remnants within a time scale of several years, thus effectively eliminating "pretenders". Combined with the mass of the star in its host galaxy, the empirical estimate of the black hole's mass is about 300,000 times the mass of the sun, which falls into the category of intermediate-mass black holes (IMBH). Combining multiple observational characteristics, it can be confirmed that this is a free black hole with active accretion and jets, and it is currently one of the closest (lowest redshift) dwarf galaxies. Assistant researcher Liu Yuanqi vividly said: "It's like a cosmic lighthouse lit by a 'wandering black hole'. Although it has left the center of the galaxy, it is still ejecting energy outwards."
This finding is particularly striking when viewed from a broader statistical perspective. Among more than 3,000 MaNGA dwarf galaxies, the researchers screened out 628 AGN candidates. About 62% of the sources had AGNs deviating from the optical center of the galaxy, indicating that denuclearization may not be uncommon. But "candidate" is not the same as "confirmation." An Tao's team conducted a multi-level screening of more than 600 candidates, and finally selected 11 targets that were most promising for detecting radio radiation, and used the highest sensitivity and resolution for tracking observations. Among the observations of these 11 target sources, only MaNGA 12772-12704 has the triple evidence of "dense high-brightness temperature, parsec jet, and 30-year time domain light variation" at the same time, becoming the only confirmed case so far.
Paper highlights
This research turns "wandering black holes" from theoretical conjecture to direct observation. With the arrival of the next generation of telescopes, "missing black holes" may no longer be rare - in the future, extremely large-aperture optical telescopes will be able to accurately measure the optical center and structure of dwarf galaxies, acquire high-resolution spectra at depth, resolve weak or masked AGN features, discover more candidates for off-core or free black holes, and expand the sample size.
With the completion of the FAST Core Array and the Square Kilometer Array Radio Telescope (SKA), astronomers will have the opportunity to conduct systematic sky surveys with higher sensitivity and resolution, detect weaker radio signals, and even directly resolve micro jets at the subparsec level, bringing breakthroughs in the confirmation and statistical research of eccentric black holes. Perhaps, in the future, we will realize that wandering black holes in the universe are not uncommon. They are like "invisible travelers" at the edges of galaxies, silently and profoundly affecting the life course of galaxies.