Dr. Vincent Hénault-Brunet and Nolan Dickson MSc’22
Less than a decade ago, gravitational waves originating from two black holes merging in a distant galaxy were observed for the first time. Since then, the number of such detections has steadily increased, and much effort has been devoted to understanding the astrophysical origin of these cataclysmic events that send ripples through space-time.
Two recent studies, spearheaded by Saint Mary’s researchers, shed new light on this topic by probing the populations of black holes hiding in the globular star clusters surrounding our Milky Way galaxy. The work was led by SMU graduate students Nolan Dickson and Peter Smith under the supervision of Dr. Vincent Hénault-Brunet and done in collaboration with researchers in Spain and Australia.
Globular star clusters are promising cradles for black holes to form, remain close to one another, and sometimes gravitationally capture each other. This can produce tight binary systems in which the two black holes spiral down to merge, emitting gravitational waves in the process. These clusters are extremely compact and can contain up to a million stars that interact through gravity. Shortly after they formed 10 to 12 billion years ago, their short-lived massive stars died and left behind black holes. But until recently, it was thought that globular clusters could not hold on to their black holes. Recent theoretical work and simulations have suggested otherwise. While some black holes may experience strong enough natal kicks to eject themselves from their host cluster promptly, and others also escape after energetic “slingshot” interactions with other black holes, it doesn’t seem to deplete the black hole population of most clusters completely.
In their study published in Monthly Notices of the Royal Astronomical Society (MNRAS), Nolan Dickson MSc’22, PhD candidate and Durland Scholar, and collaborators put these ideas on firmer ground. By carefully comparing the observed spatial distribution and motions of stars in a sample of globular clusters with detailed dynamical models, they were able to infer the presence of up to a few hundred black holes near the centre of many clusters. These black holes typically amount to less than 1% of the cluster mass in these systems, which is small but still has important ramifications.
“Although we don’t see the black holes directly, they have a subtle but noticeable effect on the visible stars, effectively injecting energy into the rest of the cluster,” says Dickson. “Other research groups around the world have reached similar conclusions, but our study is the first one that simultaneously compares models to such a wide range of observations coming from different telescopes on the ground and in space. Our models are also very fast to compute, so we can easily explore a very large number of configurations with clusters containing varying amounts of black holes and make sure we identify the models that best match all of these rich datasets.”
Peter Smith MSc’24
In a second study recently published in The Astrophysical Journal (ApJ), Peter Smith MSc’24, now a PhD candidate at the Max Planck Institute for Astronomy in Heidelberg, Germany, took this method one step further by using exotic astrophysical accelerometers. In addition to analyzing the positions and motions of stars in globular clusters, he also considered the precise timing of “millisecond pulsars”.
Pulsars are like cosmic lighthouses, rapidly rotating compact stellar remnants that emit beams of radiation out of their magnetic poles. When the beams periodically align with Earth (once per rotation, every 0.001 seconds or so for a millisecond pulsar!), pulses of energy can be detected. This makes them very precise and stable clocks. As pulsars move inside a globular cluster and are accelerated by all the stars and black holes, the measured time intervals between pulses change slightly.
“This allows us to use them as powerful probes of the dark mass inside globular clusters, in particular black holes. It’s especially helpful when not much is known about the motions of the stars, which can be difficult to measure in distant clusters obscured by interstellar dust”, comments Smith. Using this method, he and collaborators provided much improved constraints on the black hole content of two clusters that had been the subject of constant debate in the scientific community. “And things will only get better in the future, as the next generation of radio telescopes is expected to dramatically increase the number of detected pulsars in globular clusters,” Smith adds.
These findings offer new insight and further evidence that globular star clusters can keep hold of a significant number of black holes. “It’s an exciting time to be working in this field. Following the detection of gravitational waves from black hole mergers, the study of globular clusters has experienced a renaissance”, says Dr. Vincent Hénault-Brunet. “But there is still a lot that we don’t understand. We are already working on how we can use our recent results to explore the physics of black hole formation and the birth conditions of globular clusters during the earliest stages of galaxy formation in the early universe.”
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By Dr. Vincent Hénault-Brunet