ESA/Hubble & NASA, M. Häberle
Supermassive black holes appear to reside at the center of every galaxy since galaxies formed early in the history of the universe. However, we currently cannot fully explain their existence because it is difficult to understand how they could have grown fast enough to reach the supermassive limit as quickly as they did.
Possible evidence was recently found using about 20 years of data from the Hubble Space Telescope. The data comes from a globular cluster, believed to be the remnant 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 entirely. That means they’re being held there by something massive, which scientists say is a rare intermediate-mass black hole weighing more than 8,000 times the mass of the Sun.
He moves fast
The fast-moving stars reside in Omega Centauri, the largest globular star cluster in the Milky Way. With an estimated 10 million stars, it’s a crowded environment, but observations are aided by its relative proximity, “only” 17,000 light-years away. These observations suggested that there might be a central black hole in the globular cluster, but the evidence was inconclusive.
The new work, carried out 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 in the cluster, allowing them to estimate their speed relative to the cluster’s center of gravity. Although this was done earlier, the latest data allowed for an update that reduced the uncertainty in the stellar velocities.
In the updated data, a number of stars near the center of the cluster were characterized by their extreme velocities: seven of them were moving fast enough that the cluster’s gravitational force was not enough to keep them there. All seven were expected to be lost from the cluster within 1000 years, although uncertainty remained large for two of them. Based on the size of the cluster, there shouldn’t be a single foreground star between HST and the Omega cluster, so they appear to be inside the cluster despite their speed.
The simplest explanation is that they are held in place by additional mass. It could potentially be several massive objects, but the close proximity of all these stars to the center of the cluster favors a single compact object. Which means black hole.
Based on the velocities, scientists estimate that the object has a mass at least 8,200 times that of the Sun. A couple of stars seem to be accelerating; if it holds up based on further observations, it would mean the black hole has more than 20,000 solar masses. This places it firmly in black hole territory, even though they are smaller than supermassive black holes, which are viewed as having the mass of about a million Suns or more. And it’s significantly larger than you’d expect from black holes created by the death of a star, which aren’t expected to be much larger than 100 times the mass of the Sun.
This places 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 for no other reason than that it may be the least controversial observation of an intermediate-mass black hole yet.
What does this tell us?
There are still considerable uncertainties in some details – but there are prospects for the situation to improve. Observations with the Webb Space Telescope could potentially pick up faint emissions from gas falling into a black hole. And it can track the seven stars identified here. Its spectrographs could also potentially pick up the red and blue shifts in light caused by the motion of the star. Its location at a significant distance from Hubble could also provide a more detailed three-dimensional picture of the central structure of Omega Centauri.
Finding this out could potentially tell us more about how black holes grow to supermassive scales. Previous potential observations of intermediate-mass black holes have also occurred in globular clusters, which may indicate that this is a general feature of large star clusters.
However, Omega Centauri differs from many other globular clusters, which often contain large populations of stars that all formed at roughly the same time, suggesting that the 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 sucked into the Milky Way.
If so, then its central black hole is an analogue of the supermassive black holes found in real dwarf galaxies – which begs the question of why it has only an intermediate mass. Did something about its interactions with the Milky Way interfere with the growth of the black hole?
And finally, none of this sheds light on how any black hole becomes much more massive than any star it might have formed from. Getting a better idea of ​​the history of this black hole could provide more insight into some of the questions currently troubling astronomers.
Nature, 2024. DOI: 10.1038/s41586-024-07511-z (About DOI).