They’ve Finally Found the ‘Missing Link’ of Black Holes

But now a new study led by Maximilian Heberle of the Max Planck Institute for Astronomy and published in the journal Nature has confirmed what astronomers have long suspected: Omega Centauri, which is located about 18,000 light-years from Earth, has a black hole at its center. And not just any black hole, but what appears to be the “missing link” of black holes. Stuck in an intermediate stage of its evolution, it is significantly smaller and less massive than its cousins ​​living in galactic centers. Scientists believe that Omega Centauri may have been the central core of a small galaxy close to our own, whose evolution was interrupted when it was swallowed up by the Milky Way.

“Missing link”

In astronomy, black holes come in a range of masses. On the one hand, there are “stellar” stars, with masses ranging from one to several tens of times that of the Sun, formed by the gravitational collapse of very massive stars. On the other hand, there are supermassive black holes at the centers of most galaxies, with masses of millions or even billions of suns. But our current picture of galactic evolution suggests that the first galaxies must have had intermediate-sized central black holes, which would then grow over time as those galaxies evolved, either by swallowing smaller galaxies (as our Milky Way did) or by merging with larger galaxies.

However, finding these intermediate-sized black holes has proven very difficult. For example, galaxies like ours have long since passed this intermediate phase and now contain much larger central black holes. And the galaxies that remain small, called “dwarf galaxies,” are generally difficult to observe. In fact, with current technology, observing their central regions presents a real challenge for astronomers. And while there are promising candidates, no one has yet been able to definitively detect an intermediate-mass black hole.

And this is where Omega Centauri becomes something special, because if it is true that it was once the core of a separate galaxy that then merged with the Milky Way and lost everything but its central group of stars, the remaining galactic core, and its central black hole, the hole would remain “frozen in time” since there would be no more mergers or any other way for the central black hole to continue growing. In other words, the black hole would remain the same size it was when Omega Centauri was absorbed into the Milky Way. It would thus be an intermediate-mass black hole, allowing the first observation of the missing link between the first low-mass black holes and the later supermassive black holes.

However, to test this hypothesis, it would first be necessary to definitively detect the central black hole in Omega Centauri, something that had eluded astronomers until now. Although there was evidence of large-scale patterns in the motion of stars in the cluster, that evidence left room for doubt: perhaps there was no central black hole. Now those doubts have been dispelled.

Hunting for a Black Hole

When Nadine Neumayer, a team leader at the Max Planck Institute for Astronomy, and Anil Seth of the University of Utah designed a research project in 2019 to better understand the formation history of Omega Centauri, they realized there was a way to settle the question of the cluster’s central black hole once and for all: They simply needed to find a way to identify the stars moving rapidly around it. That would reveal the black hole’s presence and, crucially, measure its mass.

The difficult search was the task of Maximilian Heberle, a PhD student at the Max Planck Institute for Astronomy, who led the daunting task of creating a massive catalog of the motions of stars in Omega Centauri, measuring the velocities of 1.4 million stars by studying more than 500 images of the cluster taken by the Hubble Space Telescope. Most of these images were taken to calibrate Hubble’s instruments rather than for scientific use, but the constant repetition of images of Omega Centauri made them an ideal data set for the team’s research efforts.

Needle in a haystack

“Finding high-speed stars and documenting their motion,” Heberle recalls, “was like looking for a needle in a haystack.” But not only did he end up with the most complete catalog of Omega Centauri star motions (published in a separate paper). He also found not one but seven “needles” in the haystack: seven distinctive stars moving rapidly in a small region of central Omega Centauri.

It became clear that the fast motion of these stars was due to the presence of a large mass nearby. In the case of a single star, it would have been impossible to know whether it was fast just because its central mass was large, or whether its speed was due to the star being very close to that central mass, or even if the star was simply flying in a straight line, without a central mass. But seven fast stars with different speeds and directions of motion allowed Heberle and his colleagues to separate the different effects and determine that there was indeed “something” at the center of Omega Centauri, something with a mass of at least 8,200 times that of the Sun. And as one would expect from a black hole, the Hubble images showed no visible objects at that point.

Further analysis not only allowed Heberle to accurately determine the velocities of his seven high-velocity stars, but also located a central region three light-months in diameter (three seconds of arc in the images) within Omega Centauri. The observation of seven of these stars clearly cannot be pure coincidence and leaves no room for doubt about the presence of a black hole.

According to Neumayer, “Previous studies have already raised important questions such as: ‘Where are the high-velocity stars then?’ Now we have an answer to that question and confirmation that Omega Centauri does indeed contain an intermediate-mass black hole. “At a distance of about 18,000 light years, it is the closest known example of a massive black hole.”

With the discovery behind them, Neumayer, Heberle and their colleagues now plan to study the center of Omega Centauri in more detail. They already have the necessary permission to use the James Webb Space Telescope, and instruments are already being built for future large telescopes like the ELT, the Extremely Large Telescope, which will be able to determine the positions of stars with even greater precision than Hubble. The long-term goal is to determine how the stars are accelerating and how their orbits are warped. Orbits that would take more than a century to complete due to Omega Centauri’s smaller black hole, so observing them in detail is something that future generations of astronomers will have to do.