No Intermediate-Mass Black Hole for Omega Centauri?

 

From - Sky & Telescope

By - Arielle Frommer

Editted by - Amal Udawatta

The iconic globular cluster Omega Centauri globular cluster. home to some 10 million stars.
ESO / INAF-VST / OmegaCAM; Acknowledgement: A. Grado, L. Limatola / INAF-Capodimonte Observatory

Last month, astronomers discovered a giant black hole in Omega Centauri. But it might contain a swarm of stellar-mass black holes instead.

A new study has revealed that the Milky Way’s largest globular cluster, where astronomers recently reported the discovery of a central intermediate-mass black hole, may in fact contain thousands of stellar-mass black holes instead.

Around 17,000 light-years away in the constellation Centaurus, Omega Centauri is unique among globular clusters. With its large mass and complex stellar dynamics, astronomers think it originated as an dwarf galaxy that was swallowed by the Milky Way long ago. The discovery of an intermediate-mass black hole (IMBH) at its center was an exciting result, published in Nature, that promised to shed light on black hole evolution in such small galaxies.

However, a recent paper submitted to Astronomy & Astrophysics may temper that excitement. While the team’s findings didn’t rule out the mid-size black hole scenario, they found that Omega Centauri’s mass is better explained by a plethora of stellar-mass black holes swooping in and out of the cluster’s center.

Hidden Remnants

This Hubble Space Telescope image shows a portion of Omega Centauri, with stars resolved as individual points of light.
NASA / ESA / Hubble SM4 ERO Team

As the ancient stars of Omega Centauri swirl around its center, their trajectories are affected as the cluster’s central mass tugs on their orbits. Astronomers consider the motion of the cluster’s stellar population, or its stellar kinematics, to make their best guess on the make-up of the cluster’s central mass.

The study, led by Andrés Bañares Hernández (University of La Laguna, Spain), included observations from millions of stars and also accounted for the motion of pulsars — highly magnetized neutron stars that shoot rays of light towards Earth as they whip around on millisecond timescales. They rotate like ultra-precise lighthouses, which enables astronomers to track their movements through the cluster. Those motions affect the timing of when the pulses reach Earth; astronomers measured the pulsars’ accelerations  to estimate the cluster’s central mass.

By observing the movement of both stars and pulsars, the team determined that the center most likely contains a horde of 10,000 to 20,000 stellar-mass black holes — around 200,000 to 300,000 solar masses’ worth — rather than an intermediate-mass black hole.

The possibility of an IMBH in Omega Centauri still exists: “Our analysis does not rule out an IMBH, but rather sets a limit on its mass,” says team member Francesca Calore (French National Center for Scientific Research). The study predicts an upper limit of 6,000 solar masses for an IMBH, less than what Häberle’s team estimated its mass to be: between 8,200 and 50,000 solar masses.

One reason for this discrepancy is the different escape velocities that the two papers measured for the cluster. Häberle’s group determined that the seven stars’ velocities were higher than the expected escape velocity, so Omega Centauri needed an IMBH to keep those stars bound in orbit. However, Hernández’s study calculated a higher escape velocity for the cluster due to the presence of stellar remnants — like stellar-mass black holes. The mass of these additional, unseen stellar corpses allow the stars to move at high speeds without needing an IMBH to keep them bound.

However, even if an IMBH were present, the two studies estimate different masses for it. The team is therefore interested in including the velocity data from Häberle’s findings in later studies to see if they can reconcile the discrepancy.

Ancient Galaxy or Not?

The stakes for finding an IMBH in Omega Centauri are nothing less than the nature and origin of this cluster. If Omega Centauri is in fact the remnant core of a dwarf galaxy that started orbiting the Milky Way around 9 billion years ago, an IMBH is expected and would provide a unique window into black hole formation.

Entering orbit would have stripped stars from the dwarf’s outskirts, whittling the galaxy down to something resembling a globular cluster. Entry would also have “stopped the growth of its central black hole and kept it in an evolutionary state rather close to its initial mass,” says Nadine Neumayer (also at Max Planck), coauthor on the earlier Nature study. With the black hole’s growth arrested, it would never have grown into the supermassive type typical in most large galaxies. Finding an IMBH is akin to “finding a fossil that tells us about the early evolution of supermassive black holes,” Neumayer adds.

But others disagree, arguing that an IMBH isn’t expected. Gerry Gilmore (University of Cambridge, England), who was also not involved in either study, explained that, given that globulars are full of stars, stellar remnants such as neutron stars and stellar-mass black holes ought to be common. What’s more, as heavier objects, they should “sink” into the center of the cluster. So stellar-mass black holes in Omega Centauri’s core are entirely expected.

At present, the source of Omega Centauri’s central mass remains a mystery. Simon Portegies Zwart (Leiden Observatory, The Netherlands), who was not involved in either study, found the potential discovery of an IMBH “super exciting,” but he also acknowledges that the evidence isn’t quite there yet.

“Extraordinary discoveries require extraordinary proof,” he says. “Regretfully, I do not yet see sufficient proof.” For now, it seems possible that Omega Centauri hosts a swarm of ordinary stellar-mass black holes after all.