Hubble and Webb Find Omega Centauri's First Stellar-Mass Black Hole, One of an Estimated 10,000 Missing From the Cluster
oMEGACat BH-2, a 4.46-solar-mass black hole orbited by a main-sequence star once every 94 years, is the first stellar-mass black hole detected in the globular cluster Omega Centauri.
Overview
Astronomers using more than two decades of imaging from NASA’s Hubble Space Telescope and recent data from the James Webb Space Telescope have identified the first stellar-mass black hole in Omega Centauri, a dense globular star cluster in the Milky Way. The object, designated oMEGACat BH-2, was announced on July 13 as the first of the cluster’s long-sought “missing” black holes, according to NASA Science and ESA/Hubble.
Models predict that Omega Centauri should contain about 10,000 stellar-mass black holes left behind by exploding stars, yet direct evidence for them had remained scarce, according to NASA Science. The new detection, published in The Astrophysical Journal Letters, is the first to pin one down.
What We Know
oMEGACat BH-2 has a mass of about 4.46 solar masses and lies roughly 18,000 light-years away inside Omega Centauri, and it is bound to a visible main-sequence companion star of 0.78 solar masses, according to Phys.org. The two objects orbit each other once every 94 years, making it the longest-period black hole binary known to date, NASA Science reported.
The research team, led by Matthew Whitaker of the University of Utah, did not observe the black hole directly. Instead the astronomers used astrometry, tracking the small motion of the companion star across Hubble and Webb images taken between 2002 and 2023, according to ESA/Hubble. “With Hubble and Webb data, we were able to see the motion of the visible main sequence star that is part of this binary, which is about 18,000 light-years away in the dense environment of Omega Centauri,” Whitaker said. “The precision of these measurements is incredible, down to a fraction of a pixel on Hubble and Webb’s detectors. It would not have been possible to find this black hole without these two space telescopes,” according to NASA Science.
The underlying paper, “A Long Period Stellar-Mass Black Hole Binary in ω Centauri,” puts the black hole mass at 4.46 solar masses with a formal uncertainty of about +1.2 and −1.0 solar masses, and describes an orbit with a semi-major axis of roughly 31 astronomical units and an eccentricity of 0.72, according to the arXiv preprint. The authors write that “This is the first astrometric discovery of a stellar-mass black hole in a globular cluster, and is the longest period black hole binary system yet discovered,” per the arXiv preprint.
Knowing the companion’s mass let the team weigh its unseen partner. “While we already knew that the star was 0.78 solar masses, we can now calculate the black hole’s mass, which is 4.46 solar masses and therefore too heavy to be a neutron star,” coauthor Anil Seth of the University of Utah said, according to NASA Science. Seth added that the mass “is much lower than would be expected in a metal-poor environment like Omega Centauri,” a result he called “surprising and exciting,” per NASA Science.
Why It Matters
Seth framed the find as a way to probe how black holes form and pair up in crowded stellar environments. “It’s important to understand black hole populations in globular clusters because there’s uncertainty about their physics and formation,” he said, adding that “understanding the process of forming black holes and then dynamically forming binaries is vital, because it affects our ability to interpret and understand gravitational wave events,” according to NASA Science. Dense clusters such as Omega Centauri are considered primary sites where black hole binaries can form and eventually merge to produce the gravitational waves detected from Earth.
What We Don’t Know
Why a metal-poor star produced a black hole of this mass is unresolved; Seth said the team now needs to “figure out how that happens,” per NASA Science. The detection also leaves the vast majority of the cluster’s predicted black hole population unaccounted for, with oMEGACat BH-2 standing as a single confirmed object against a modeled census of roughly 10,000, according to ESA/Hubble. The team intends to keep looking. “With Hubble and Webb, we can continue to look at Omega Centauri and expand our search for similar systems within other clusters,” Whitaker said, according to NASA Science.