Hubble Space Telescope detects strong candidate for intermediate-sized black hole at the heart of nearby star cluster

A group of astronomers utilizing the NASA/ESA Hubble Space Telescope has made a significant breakthrough in their quest to detect a rare class of intermediate-sized black holes. They have discovered a compelling candidate located at the core of the nearest globular star cluster to Earth, situated 6,000 light-years away.

Black holes typically fall into two size categories: small and enormous, akin to intense gravitational sinkholes in the fabric of space. Within our galaxy, it is estimated that there exist 100 million small black holes, formed from the remnants of exploded stars and several times the mass of our sun. On a larger scale, the universe is teeming with supermassive black holes, weighing millions or billions of times the mass of our sun and typically found at the centers of galaxies.

The elusive intermediate-mass black holes, weighing approximately 100 to 100,000 times the mass of our sun, have long been sought after. Questions surrounding their formation, whereabouts, and scarcity have puzzled scientists.

Astronomers have previously identified potential intermediate-mass black holes using various observational methods. Two of the most promising candidates—3XMM J215022.4-055108, which was discovered with the assistance of Hubble in 2020, and HLX-1, identified in 2009—reside on the outskirts of other galaxies. These black holes each possess the mass of tens of thousands of suns and might have originally occupied the cores of dwarf galaxies.

Turning their attention closer to home, scientists have detected several suspected intermediate-mass black holes in dense globular star clusters orbiting our Milky Way galaxy. One notable example is the globular cluster Omega Centauri, where astronomers announced in 2008 the potential presence of an intermediate-mass black hole. However, due to various factors, such as the need for additional data, these findings and others regarding intermediate-mass black holes remain inconclusive and do not rule out alternative theories.

Leveraging Hubble’s distinctive capabilities, researchers have now concentrated their efforts on the core of the globular star cluster Messier 4 (M4) to conduct more precise black hole investigations compared to previous searches. “This kind of scientific exploration would be impossible without Hubble,” stated Eduardo Vitral, the lead author of a paper published in the Monthly Notices of the Royal Astronomical Society. Vitral, currently affiliated with the Space Telescope Science Institute in Baltimore, Maryland, previously worked at the Institut d’Astrophysique de Paris (IAP, Sorbonne University) in Paris, France.

Vitral’s team has made a significant discovery, potentially detecting an intermediate-mass black hole weighing around 800 times the mass of the Sun. Although the suspected object remains invisible, its mass has been determined by studying the movement of stars influenced by its gravitational field, akin to bees swarming around a hive. This intricate motion analysis requires both time and precision, and the Hubble telescope excels in providing these capabilities. By examining 12 years’ worth of observations of M4 from Hubble, astronomers were able to precisely identify individual stars.

The Gaia spacecraft, operated by the European Space Agency (ESA), also played a crucial role in this breakthrough. Gaia conducted scans of over 6,000 stars within the cluster, aiding in the determination of its overall shape and mass. The data gathered by Hubble effectively dismisses alternative theories for this object, such as a compact central cluster consisting of unresolved remnants like neutron stars or smaller black holes orbiting one another.

Vitral explains, “Using the latest Gaia and Hubble data, it was not possible to distinguish between a dark population of stellar remnants and a single larger point-like source. So one of the possible theories is that rather than being lots of separate small dark objects, this dark mass could be one medium-sized black hole.”

The team is confident that they have identified a highly concentrated mass within a minuscule region. In fact, it is three times denser than the densest dark mass previously observed in other globular clusters. Numerical simulations that account for a collection of black holes, neutron stars, and white dwarfs segregated at the cluster’s center fail to reproduce such a compact mass concentration, rendering this scenario unlikely.

A collection of closely packed objects would be dynamically unstable. If the observed object is not a single intermediate-mass black hole, it would necessitate approximately 40 smaller black holes squeezed into a region only one-tenth of a light-year across in order to explain the observed stellar motions. In such a scenario, these objects would either merge or be flung away like pinballs in interstellar space.

Vitral elaborates on the methodology, stating, “We measure the motions of stars and their positions, and we apply physical models that try to reproduce these motions. We end up with a measurement of a dark mass extension in the cluster’s center. The closer stars are to the central mass, the more random their movements become. Moreover, the greater the central mass, the faster the stellar velocities.”

While acknowledging that confirming the central point of gravity is challenging, Vitral emphasizes the object’s exceptionally small size. Its dimensions are too minute to be explained by anything other than a single black hole. Alternatively, there may exist an unknown stellar mechanism that currently eludes our understanding within the realm of existing physics.

Timo Prusti, a Gaia mission scientist, highlights the incremental nature of scientific discovery, stating, “Science is rarely about discovering something new in a single moment. It’s about becoming more certain of a conclusion step by step, and this could be one step towards being sure that intermediate-mass black holes exist.” Prusti notes the significance of Gaia Data Release 3, which provided crucial information about the proper motion of stars in the Milky Way. Future data releases from Gaia, as well as follow-up studies using the Hubble and James Webb Space Telescopes, hold the potential to shed further light on this intriguing subject.

Source: ESA/Hubble Information Centre

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