Intermediate-mass black hole

Intermediate-mass black holes are defined by their mass, typically between 10² and 10⁵ solar masses¹. This places them between stellar-mass black holes, which form from the collapse of massive stars (about 3–100 M_☉), and supermassive black holes, which reside at the centers of most large galaxies and have masses from millions to billions of solar masses². The existence of IMBHs is important because they may be the 'seeds' from which supermassive black holes grow, and their study can reveal how black holes form and evolve over cosmic time³.

Several mechanisms have been proposed for the formation of IMBHs. One leading idea is that they form through the runaway coalescence of massive stars in dense star clusters, such as globular clusters⁴. Another scenario involves the direct collapse of gas clouds in the early universe, which could produce seeds of ~10³–10⁵ M_☉⁵. Additionally, IMBHs might form through the merger of smaller black holes in dense environments⁶. These formation channels have distinct observational signatures, such as the presence of an IMBH in a star cluster or the gravitational waves emitted during mergers.

Observational evidence for IMBHs has been accumulating. In 2009, a team led by Sean Farrell reported the discovery of HLX-1, an ultraluminous X-ray source in the galaxy ESO 243-49, with a mass of about 20,000 M_☉⁷. More recently, in 2024, astronomers using the Hubble Space Telescope and the Gemini Observatory confirmed a strong candidate IMBH in the globular cluster Omega Centauri, with a mass of about 8,200 M_☉⁸. Gravitational wave detections have also provided evidence: the LIGO-Virgo-KAGRA collaboration observed a merger of two black holes with masses of about 85 and 66 M_☉, producing a final black hole of about 142 M_☉, which falls in the IMBH range⁹.

Detecting IMBHs is challenging because they are smaller and less luminous than supermassive black holes, and they are often hidden within dense clusters or dwarf galaxies. X-ray observations of ultraluminous X-ray sources (ULXs) have been used to identify candidates, but many ULXs are now known to be neutron stars or stellar-mass black holes in super-Eddington accretion states¹⁰. Dynamical measurements, such as tracking the motion of stars near a suspected IMBH, are more reliable but require high-resolution observations. The Hubble Space Telescope and adaptive optics on ground-based telescopes have been used for such studies⁸.

IMBHs may play a key role in the evolution of dwarf galaxies and the growth of supermassive black holes. Many dwarf galaxies are thought to host IMBHs at their centers, and these could be the seeds for the supermassive black holes seen in larger galaxies today³. The presence of an IMBH can also affect the dynamics of its host galaxy, influencing star formation and the distribution of stars and gas¹¹. Studying IMBHs in dwarf galaxies can therefore provide insights into the co-evolution of black holes and galaxies.

Beyond the mainstream narrative, several lesser-known aspects of IMBHs are noteworthy. For instance, the first IMBH candidate, HLX-1, was discovered serendipitously in archival X-ray data⁷. Another intriguing case is the IMBH candidate in the galaxy NGC 404, which is only about 10 million light-years away¹². Additionally, some IMBHs may be 'wandering' — not located at the center of their host galaxy but in the halo or in globular clusters, as a result of galaxy mergers or ejection from the center¹³. The gravitational wave event GW190521, which produced a ~142 M_☉ black hole, is notable because the progenitor black holes were in the pair-instability mass gap, suggesting they formed via hierarchical mergers⁹. Finally, the concept of 'intermediate-mass' is not just about mass; it also implies a possible evolutionary link between stellar-mass and supermassive black holes, a connection that remains poorly understood.

Solar mass (M☉)

A unit of mass equal to the mass of the Sun, approximately 1.989 × 10^30 kg.

Ultraluminous X-ray source (ULX)

A point-like, off-nuclear X-ray source with luminosity exceeding 10^39 erg/s, often associated with accreting black holes or neutron stars.

Pair-instability mass gap

A range of stellar masses (roughly 130–250 M☉) in which pair-instability supernovae are expected to disrupt the star, leaving no black hole remnant.

Hierarchical merger

A process in which black holes merge repeatedly, producing more massive black holes, potentially leading to IMBHs.