Black holes with masses between the stellar and supermassive regime are among the most elusive objects in the Universe. These intermediate-mass black holes are believed to reside in many dwarf galaxies. Using new, high-resolution supercomputer simulations, MPA scientists discovered that nuclear star clusters — compact, massive clusters of stars at the centres of galaxies — may be key to enabling these black holes to grow, thus shedding light on the origins of supermassive black holes.
All massive galaxies, including our own Milky Way, contain supermassive black holes at their centres, with masses ranging from millions to billions of solar masses. However, the formation and growth of these giants remains a mystery. Low-mass galaxies may hold the answer: some of them contain these elusive intermediate-mass black holes (IMBHs), which have masses ranging from hundreds to hundreds of thousands of times that of the Sun. These are more massive than stellar black holes, but have never reached the supermassive stage. IMBHs exert influence only in a tiny region around them. This makes it difficult for them to capture gas and stars, affect their host galaxies, or even be detected in the first place.
Many low-mass galaxies host nuclear star clusters: extremely dense systems of stars spanning only a few light years, yet containing a few percent of the entire galaxy’s stellar mass. Nuclear star clusters form an extremely compact and deep potential well at the centre of the galaxy. Recent observations suggest a strong correlation between nuclear star clusters and the existence of IMBHs at their centres. The nuclear star clusters in low-mass galaxies tend to be more massive than their IMBHs, and may play a crucial role in the evolution of galactic centres. The MPA team set out to study how such an environment affects IMBH growth.
Simulating black hole growth is complex as it requires tracking how interstellar gas flows from galactic scales down to the tiny sphere of influence of the black hole. The team of researchers used high-resolution simulations that resolve the black hole's sphere of influence and capture many relevant physical processes in the interstellar gas. The simulations also follow millions of individual stars, including the radiation they emit and the supernovae of the most massive ones. By heating and stirring the gas, these processes strongly influence whether black holes can feed and grow.
The team tested low-mass galaxies with IMBHs of different initial masses. They found that light IMBHs (those below 10,000 solar masses) are barely able to capture gas and grow unless a nuclear star cluster is present. If the IMBH is embedded in a cluster, its additional gravitational potential enables rapid gas accretion and swift black hole growth. More massive IMBHs accrete efficiently even without a nuclear star cluster, but the additional growth is small compared to their initial mass. This demonstrates that nuclear star clusters are particularly significant for the smallest black holes, where the cluster's mass far exceeds that of the black hole itselfas is typical in low-mass galaxies.
Even with a nuclear star cluster, growth can be easily disrupted by stellar feedback. Some of the gas captured by the nuclear star cluster forms stars, including massive stars. When these massive stars end their lives as supernovae, they can expel gas from the galaxy’s centre, temporarily starving the black hole. Consequently, IMBHs undergo cycles of activity and quiet phases. This means that many are likely to be missed in current surveys, which typically detect only actively feeding black holes through the radiation produced by the accretion process.
The study shows that nuclear star clusters are essential for the growth of intermediate-mass black holes in low-mass galaxies, which would otherwise remain stagnant. This is particularly exciting because many theories suggest that the first black hole seeds in the early Universe formed through stellar collisions inside such clusters. Therefore, the new results point not only to nuclear star clusters as the birthplace of intermediate-mass black holes, but also as the sites where they grow most efficiently.
All massive galaxies, including our own Milky Way, contain supermassive black holes at their centres, with masses ranging from millions to billions of solar masses. However, the formation and growth of these giants remains a mystery. Low-mass galaxies may hold the answer: some of them contain these elusive intermediate-mass black holes (IMBHs), which have masses ranging from hundreds to hundreds of thousands of times that of the Sun. These are more massive than stellar black holes, but have never reached the supermassive stage. IMBHs exert influence only in a tiny region around them. This makes it difficult for them to capture gas and stars, affect their host galaxies, or even be detected in the first place.
Many low-mass galaxies host nuclear star clusters: extremely dense systems of stars spanning only a few light years, yet containing a few percent of the entire galaxy’s stellar mass. Nuclear star clusters form an extremely compact and deep potential well at the centre of the galaxy. Recent observations suggest a strong correlation between nuclear star clusters and the existence of IMBHs at their centres. The nuclear star clusters in low-mass galaxies tend to be more massive than their IMBHs, and may play a crucial role in the evolution of galactic centres. The MPA team set out to study how such an environment affects IMBH growth.
Simulating black hole growth is complex as it requires tracking how interstellar gas flows from galactic scales down to the tiny sphere of influence of the black hole. The team of researchers used high-resolution simulations that resolve the black hole's sphere of influence and capture many relevant physical processes in the interstellar gas. The simulations also follow millions of individual stars, including the radiation they emit and the supernovae of the most massive ones. By heating and stirring the gas, these processes strongly influence whether black holes can feed and grow.
The team tested low-mass galaxies with IMBHs of different initial masses. They found that light IMBHs (those below 10,000 solar masses) are barely able to capture gas and grow unless a nuclear star cluster is present. If the IMBH is embedded in a cluster, its additional gravitational potential enables rapid gas accretion and swift black hole growth. More massive IMBHs accrete efficiently even without a nuclear star cluster, but the additional growth is small compared to their initial mass. This demonstrates that nuclear star clusters are particularly significant for the smallest black holes, where the cluster's mass far exceeds that of the black hole itselfas is typical in low-mass galaxies.
Even with a nuclear star cluster, growth can be easily disrupted by stellar feedback. Some of the gas captured by the nuclear star cluster forms stars, including massive stars. When these massive stars end their lives as supernovae, they can expel gas from the galaxy’s centre, temporarily starving the black hole. Consequently, IMBHs undergo cycles of activity and quiet phases. This means that many are likely to be missed in current surveys, which typically detect only actively feeding black holes through the radiation produced by the accretion process.
The study shows that nuclear star clusters are essential for the growth of intermediate-mass black holes in low-mass galaxies, which would otherwise remain stagnant. This is particularly exciting because many theories suggest that the first black hole seeds in the early Universe formed through stellar collisions inside such clusters. Therefore, the new results point not only to nuclear star clusters as the birthplace of intermediate-mass black holes, but also as the sites where they grow most efficiently.
Interstellar gas (top left) is stirred by radiation and supernova explosions from massive stars, creating hot, low-density regions (top middle). Occasionally, the IMBH captures gas (bottom left), fuelling growth and star formation in the galactic centre. In the IMBH’s immediate surroundings (bottom middle and right), supernovae clear gas from the galaxy’s core, temporarily stopping black hole feeding. Once most massive stars have exploded, new gas can be captured by the black hole. Stars with masses greater than 8 solar masses (and their potential massive remnants) are depicted as red, orange, and yellow star symbols, with colour indicating increasing mass.
Author:
Christian Partmann
partmann@mpa-Garching.mgp.de
Thorsten Naab
Scientific Staff
tnaab@mpa-garching.mpg.de
Original publication
Christian Partmann, Thorsten Naab, Natalia Lahén, Antti Rantala, Michaela Hirschmann, Jessica M Hislop, Jonathan Petersson, Peter H Johansson
The importance of nuclear star clusters for massive black hole growth and nuclear star formation in simulated low-mass galaxies
Monthly Notices of the Royal Astronomical Society, Volume 537, Issue 2, February 2025, Pages 956–977
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