Astronomers find missing link in massive black hole formation

Omega Centauri is a spectacular collection of 10 million stars, visible as a smudge in the night sky from Southern latitudes. Through a small telescope, it looks no different from other so-called globular clusters; a spherical stellar collection so dense towards the center that it becomes impossible to distinguish individual stars. But a new study, led by researchers from the University of Utah and the Max Planck Institute for Astronomy, confirms what astronomers had argued about for over a decade: Omega Centauri contains a central black hole. The black hole appears to be the missing link between its stellar and supermassive kin—stuck in an intermediate stage of evolution, it is considerably less massive than typical black holes in the centers of galaxies. Omega Centauri seems to be the core of a small, separate galaxy whose evolution was cut short when it was swallowed by the Milky Way.

“This is a once-in-a-career kind of finding. I’ve been excited about it for nine straight months. Every time I think about it, I have a hard time sleeping,” said Anil Seth, associate professor of astronomy at the U and co-principal investigator (PI) of the study. “I think that extraordinary claims require extraordinary evidence. This is really, truly extraordinary evidence.”

A clear detection of this black hole had eluded astronomers until now. The overall motions of the stars in the cluster showed that there was likely some unseen mass near its center, but it was unclear if this was an intermediate-mass black hole or just a collection of the stellar black holes. Maybe there was no central black hole at all. 

“Previous studies had prompted critical questions of ‘So where are the high-speed stars?’ We now have an answer to that, and the confirmation that Omega Centauri contains an intermediate-mass black hole. At about 18,000 light-years, this is the closest known example for a massive black hole,” said Nadine Neumayer, a group leader at the Max Planck Institute and PI of the study. For comparison, the supermassive black hole in the center of the Milky Way is about 27,000 light-years away.

The paper was published in the journal Nature on July 11, 2024. Watch the research come to life on August 8, 2024, at 7:00 p.m. when Anil Seth will present these once-in-a-lifetime findings at the Clarke Planetarium IMAX theater.

A range of black hole masses

In astronomy, black holes come in different mass ranges. Stellar black holes, between one and a few dozen solar masses, are well known, as are the supermassive black holes with masses of millions or even billions of suns. Our current picture of galaxy evolution suggests that the earliest galaxies should have had intermediate-sized central black holes that would have grown over time, gobbling up smaller galaxies done or merging with larger galaxies.

Such medium-sized black holes are notoriously hard to find. Although there are promising candidates, there has been no definite detection of such an intermediate-mass black hole—until now.

“There are black holes a little heavier than our sun that are like ants or spiders—they’re hard to spot, but kind of everywhere throughout the universe. Then you’ve got supermassive black holes that are like Godzilla in the centers of galaxies tearing things up, and we can see them easily,” said Matthew Whittaker, an undergraduate student at the U and co-author of the study. “Then these intermediate-mass black holes are kind of on the level of Bigfoot. Spotting them is like finding the first evidence for Bigfoot—people are going to freak out.”

Needle in an archival haystack

When Seth and Neumayer designed a research project to better understand the formation history of Omega Centauri in 2019, they realized they could settle the question of the cluster’s central black hole once and for all. If they found fast-moving stars around its center, they would have the proverbial smoking gun, as well as a way of measuring the black hole’s mass. 

The arduous search became the task of Maximilian Häberle, a doctoral student at the Max Planck Institute. Häberle led the work of creating an enormous catalogue for the motions of stars in Omega Centauri, measuring the velocities for 1.4 million stars by studying over 500 Hubble images of the cluster. Most of these images had been produced for the purpose of calibrating Hubble’s instruments rather than for scientific use. But with their ever-repeating views of Omega Centauri, they turned out to be the ideal data set for the team’s research efforts. 

“Looking for high-speed stars and documenting their motion was the proverbial search for a needle in a haystack,” Häberle said. In the end, Häberle not only had the most complete catalog of the motion of stars in Omega Centauri yet, he also found seven needles in his archival haystack—seven tell-tale, fast-moving stars in a small region in the center of Omega Centauri.

Uncovering a black hole

The seven stars move fast because of the presence of a concentrated nearby mass. For a single star, it would be impossible to tell whether it is fast because the central mass is large or because the star is very close to the central mass—or if the star is merely flying straight, with no mass in sight. But seven such stars, with different speeds and directions of motion, allowed the team to separate the different effects and determine that there is a central mass in Omega Centauri, with the mass of at least 8,200 suns. The images do not indicate any visible object at the inferred location of that central mass, as one would expect for a black hole.

The broader analysis also allowed the team to narrow down the location of Omega Centauri’s central region at 3 light-months in diameter (on images, 3 arc seconds). In addition, the analysis provided statistical reassurance: A single high-speed star in the image might not even belong to Omega Centauri. It could be a star outside the cluster that passes right behind or in front of Omega Centauri’s center by chance. The observations of seven such stars, on the other hand, cannot be pure coincidence, and leaves no room for explanations other than a black hole. 

An intermediate-mass black hole at last

Given their findings, the team now plans to examine the center of Omega Centauri in even more detail. The U’s Seth is leading a project has gained approval to use the James Webb Space Telescope for measuring the high-speed star’s movement towards or away from Earth, and there are future instruments (GRAVITY+ at ESO’s VLT, MICADO at the Extremely Large Telescope) that could pinpoint stellar positions even more accurately than Hubble. The long-term goal is to determine how the stars accelerate: how their orbits curve. Following those stars once around their whole orbit, as in the Nobel-prize-winning observations near the black hole in the center of the Milky Way, is a project for future generations of astronomers, though. The smaller black hole mass for Omega Centauri means ten times larger time scales than for the Milky Way: orbital periods of more than a hundred years.

Background information
The work described here has been published as M. Häberle et al., “Fast-moving stars around an intermediate-mass black hole in ω Centauri” in the journal Nature. The star catalog on which the work is based has been accepted for publication as M. Häberle et al., “oMEGACat II—Photometry and proper motions for 1.4 million stars in Omega Centauri and its rotation in the plane of the sky” in the Astrophysical Journal.

Other authors include the Max Plank Institute of Astronomy researchers Antoine Dumont, Callie Clontz (also University of Utah), Anja Feldmeier-Krause (also University of Vienna) and Maria Selina Nitschai in collaboration with Andrea Bellini (Space Telescope Science Institute), Mattia Libralato (ESA and INAF Padova), Holger Baumgardt (The University of Queensland), Mayte Alfaro Cuello (Universidad Central de Chile), Jay Anderson (Space Telescope Science Institute), Nikolay Kacharov (Leibniz Institute for Astrophysics Potsdam), Sebastian Kamann (Liverpool John Moores University), Antonino Milone (University of Padova), Renuka Pechetti (Liverpool John Moores University) and Glenn van de Ven (University of Vienna).