ESA/Hubble and NASA, M. Häberle
Supermassive black holes appear to reside at the center of every galaxy and have done so since galaxies formed early in the history of the Universe. At present, however, we cannot fully explain their existence, as it is difficult to understand how they could grow fast enough to reach the supermassive limit as quickly as they did.
A possible piece of evidence was recently found using nearly 20 years of data from the Hubble Space Telescope. The data comes from a globular cluster of stars thought to be the remnants of a dwarf galaxy, and shows that a group of stars near the cluster’s core are moving so fast that they should have been ejected from it entirely. This implies that something massive is holding them there, which the researchers argue is a rare intermediate-mass black hole weighing over 8,000 times the mass of the Sun.
Moving fast
Fast-moving stars reside in Omega Centauri, the largest globular cluster in the Milky Way. With about 10 million stars, it’s a crowded environment, but observations are aided by its relative proximity, at “only” 17,000 light-years away. These observations have hinted that there may be a central black hole within the globular cluster, but the evidence has not been conclusive.
The new work, done by a large international team, used over 500 images of Omega Centauri taken by the Hubble Space Telescope over 20 years. This allowed them to track the motion of the stars within the cluster, allowing an estimate of their velocity relative to the cluster’s center of mass. While this has been done before, the latest data allowed an update that reduced the uncertainty in the star’s velocity.
Within the update data, a number of stars near the cluster’s center stood out for their extreme velocities: seven of them were moving fast enough that the cluster’s gravitational pull wasn’t enough to keep them there. All seven should have been lost from the cluster within 1,000 years, although uncertainties remained large for two of them. Based on the size of the cluster, there shouldn’t be a single foreground star between Hubble and the Omega cluster, so these really do appear to be within the cluster despite their speed.
The simplest explanation for this is that there is an extra mass holding them in place. It could be several massive objects, but the proximity of all these stars to the center of the cluster favors a single compact object. Which means a black hole.
Based on the velocities, researchers estimate that the object has a mass of at least 8,200 times that of the Sun. Some stars appear to be accelerating; if this holds up based on further observations, it would indicate that the black hole is over 20,000 solar masses. This puts it firmly within black hole territory, albeit smaller than supermassive black holes, which are seen as those of about a million solar masses or more. And it’s much bigger than you’d expect from black holes formed by the death of a star, which aren’t expected to be much larger than 100 times the mass of the Sun.
This puts it in the category of intermediate-mass black holes, of which there are only a handful of possible observations, none of which are universally accepted. So this is a significant discovery if for no other reason than it may be the least controversial point of an intermediate-mass black hole.
What is this telling us?
At the moment, there is still considerable uncertainty in some of the details here – but the prospects for the situation to improve do exist. Observations with the Webb Space Telescope could potentially pick up faint emissions from gas falling into the black hole. And it can track the seven stars identified here. Its spectrographs can also pick up the red and blue shifts in light caused by the star’s motion. Its location at a significant distance from Hubble may also provide a more detailed three-dimensional view of the central structure of Omega Centauri.
Finding this out could tell us more about how black holes grow to supermassive scales. Previous possible sightings of intermediate-mass black holes have also come in globular clusters, which may suggest that they are a general feature of large star clusters.
But Omega Centauri differs from many other globular clusters, which often contain large populations of stars that all formed around the same time, suggesting clusters formed from a single giant cloud of material. Omega Centauri has stars with a wide range of ages, which is one reason people think it is the remnant of a dwarf galaxy that was absorbed into the Milky Way.
If so, then its central black hole is an analogue of the supermassive black holes found in true dwarf galaxies – which begs the question of why it is only intermediate in mass. Did something about its interactions with the Milky Way interfere with the growth of the black hole?
And, in the end, none of this sheds light on how any black hole becomes so much more massive than any star it could have formed from. Gaining a better understanding of the history of this black hole may provide more perspective on some questions currently troubling astronomers.
Nature, 2024. DOI: 10.1038/s41586-024-07511-z (About DOIs).