ESA/Hubble & 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. However, we currently cannot fully explain their existence, as it is difficult to understand how they could grow fast enough to reach the supermassive threshold as quickly as they did.
A possible piece of evidence was recently found using about 20 years of data from the Hubble Space Telescope. The data come from a globular cluster thought to be the remains of a dwarf galaxy, and show that a group of stars near the cluster’s core are moving so fast that they should have been flung completely out. That implies that something big is holding them there, which the researchers believe is a rare intermediate-mass black hole, with a mass more than 8,000 times that of the sun.
Move fast
The fast-moving stars are in Omega Centauri, the largest globular cluster in the Milky Way. With an estimated 10 million stars, it’s a busy place, but observations are aided by its relative proximity, at “only” 17,000 light-years away. Those observations have suggested that the globular cluster might harbor a central black hole, but the evidence has been inconclusive.
The new work, carried out by a large international team, used more than 500 images of Omega Centauri taken by the Hubble Space Telescope over the course of 20 years. This allowed them to track the motion of stars within the cluster, allowing an estimate of their velocity relative to the cluster’s center of mass. Although this has been done before, the most recent data allowed an update that reduced the uncertainty in the stars’ velocity.
In the updated data, a number of stars near the cluster center stood out for their extreme velocities: seven of them were moving so fast that the cluster’s gravity wasn’t enough to hold them there. All seven should be lost from the cluster within 1,000 years, although uncertainties remain large for two of them. Based on the size of the cluster, there shouldn’t be even a single foreground star between Hubble and the Omega Cluster, so these appear to be truly within the cluster, despite their speed.
The simplest explanation for this is that there is some extra mass holding them in place. Potentially, these could be multiple massive objects, but the proximity of all these stars to the center of the cluster favors a single, compact object. And that means a black hole.
Based on the velocities, the researchers estimate that the object has a mass of at least 8,200 times that of the sun. A few stars appear to be accelerating; if that’s correct based on further observations, that would suggest the black hole has more than 20,000 solar masses. That puts it firmly within black hole territory, though smaller than supermassive black holes, which are considered to be those with about a million solar masses or more. And it’s considerably larger than you’d expect from black holes formed by the death of a star, which aren’t expected to be much larger than 100 solar masses.
This puts it in the category of intermediate-mass black holes, of which there are only a handful of potential observations, none of which are universally accepted. So this is a significant finding, if only because it is perhaps the least controversial observation of an intermediate-mass black hole to date.
What does this tell us?
For now, there are still significant uncertainties in some of the details here, but there are prospects for improvement. Observations with the Webb Space Telescope could potentially pick up the faint emissions from gas falling into the black hole. And it could track the seven stars identified here. The spectrographs could also potentially pick up the red and blue shifts in light caused by the motion of the star. The location at a considerable distance from Hubble could also provide a more detailed three-dimensional view of Omega Centauri’s central structure.
Figuring this out could help us learn more about how black holes grow to supermassive scales. Previous potential observations of medium-mass black holes have also been made in globular clusters, suggesting that they are a common feature of large collections of stars.
But Omega Centauri is different from many other globular clusters, which often contain large populations of stars that all formed at about the same time, suggesting that the clusters formed from a single giant cloud of materials. Omega Centauri has stars with a wide range of ages, which is one reason people think it’s the remnants of a dwarf galaxy that was sucked into the Milky Way.
If so, then the central black hole is an analog of the supermassive black holes found in real dwarf galaxies, which raises the question of why it has only an average mass. Did something in the interactions with the Milky Way disrupt the growth of the black hole?
And ultimately, none of this sheds any light on how a black hole becomes so much more massive than the star from which it may have formed. A better understanding of the history of this black hole could shed more light on a number of questions that astronomers are currently grappling with.
Nature, 2024. DOI: 10.1038/s41586-024-07511-z (About DOIs).