A Dark Central Mass is Lurking at the Hub of a Glittering Stellar Island
Gravitational traps in space, black holes, come in different sizes. Or more correctly, different masses, because they are all infinitely small. The first black hole ever discovered, in 1971, weighed in at 21 times our Sun’s mass. It was formed by the explosion and collapse of a star. Examples of a completely different class of black hole were identified in the 1960s-1970s. They weighed in at millions to billions of times our Sun’s mass. Like all supermassive black holes, those monsters dwell in the center of major galaxies.
So, black holes can be super-big or super-small. The missing link is an intermediate-mass black hole, weighing roughly 100 to 1,000 times our Sun’s mass. A handful have been found in other galaxies. Perhaps they are on the road to growing into supermassive black holes.
The cores of globular star clusters are hunting grounds for intermediate-mass black holes. They are smaller than galaxies and should have correspondingly smaller black holes. Over 150 of these snow-globe-shaped collections of hundreds of thousands of stars orbit our Milky Way galaxy, like artificial satellites whirling around Earth. Searches for intermediate-mass back holes in these clusters have been elusive. The suspected central black hole can’t be directly observed, of course. Astronomers gather circumstantial evidence by watching stars swarming around the black hole, like bees around a hive. Based on their speeds, the invisible central mass can be calculated using straightforward Newtonian laws of physics.
Tracking the stars is meticulous work that’s cut out for the Hubble Space Telescope’s sharp resolution and longevity. Astronomers looking through over a decade of Hubble observations of the nearby globular star cluster Messier 4 calculated there is a very dense central object of about 800 solar masses. It is so compact, the observations tend to rule out alternative theories as to what’s happening in the heart of the cluster.
This is a simulation of the motions of stars around a suspected black hole in the core of the globular cluster M4. It is based on the “HST Legacy Survey of Galactic Globular Clusters: Shedding UV Light on Their Populations and Formation,” which includes stellar clusters’ internal kinematics. After the zoom into M4, the center of the cluster, where the suspected black hole resides, is highlighted by a red “X.” The red circle has a radius of a little less than 1 light-year across. It is the sphere of influence of the putative intermediate-mass black hole. Estimated to be 800 solar masses, the black hole has an event horizon that is a little more than half the diameter of our moon. The sphere of influence of the black hole is the region where its gravitational potential dominates over the gravitational potential of the starfield. It is a region where stellar motions are significantly affected by the black hole’s gravitational pull. Only Hubble’s sharp resolution can plot this motion over more than a decade of observing. The fastest moving stars in this video are not in the cluster, but are much closer to us, in the foreground of Milky Way stars. Credit: NASA, ESA, Mattia Libralato (AURA/STScI for ESA)
Hubble Space Telescope Hunts for Intermediate-Sized Black Hole Close to Home
Astronomers using NASA’s Hubble Space Telescope have come up with what they say is some of their best evidence yet for the presence of a rare class of “intermediate-sized” black hole that may be lurking in the heart of the closest globular star cluster to Earth, located 6,000 light-years away.
Like intense gravitational potholes in the fabric of space, virtually all black holes seem to come in two sizes: small and humongous. It’s estimated that our galaxy is littered with 100 million small black holes (several times the mass of our Sun) created from exploded stars. The universe at large is flooded with supermassive black holes, weighing millions or billions of times our Sun’s mass and found in the centers of galaxies.
A long-sought missing link is an intermediate-mass black hole, weighing in somewhere between 100 and 100,000 solar masses. How would they form, where would they hang out, and why do they seem to be so rare?
Astronomers have identified other possible intermediate-mass black holes through a variety of observational techniques. Two of the best candidates — 3XMM J215022.4−055108, which Hubble helped discover in 2020, and HLX-1, identified in 2009, reside in dense star clusters in the outskirts of other galaxies. Each of these possible black holes has the mass of tens of thousands of suns, and may have once been at the centers of dwarf galaxies. NASA’s Chandra X-ray observatory has also helped make many possible intermediate black hole discoveries, including a large sample in 2018.
Looking much closer to home, there have been a number of suspected intermediate-mass black holes detected in dense globular star clusters orbiting our Milky Way galaxy. For example, in 2008, Hubble astronomers announced the suspected presence of an intermediate-mass black hole in the globular cluster Omega Centauri. For a number of reasons, including the need for more data, these and other intermediate-mass black hole findings still remain inconclusive and do not rule out alternative theories.
Hubble’s unique capabilities have now been used to zero in on the core of the globular star cluster Messier 4 (M4) to go black-hole hunting with higher precision than in previous searches. “You can’t do this kind of science without Hubble,” said Eduardo Vitral of the Space Telescope Science Institute in Baltimore, Maryland, lead author on a paper published today (May 23, 2023) in the Monthly Notices of the Royal Astronomical Society.
Vitral’s team has detected a possible intermediate-mass black hole of roughly 800 solar masses. The suspected object can’t be seen, but its mass is calculated by studying the motion of stars caught in its gravitational field, like bees swarming around a hive. Measuring their motion takes time, and a lot of precision. This is where Hubble accomplishes what no other present-day telescope can do. Astronomers looked at 12 years’ worth of M4 observations from Hubble and resolved pinpoint stars.
His team estimates that the black hole in M4 could be as much as 800 times our Sun’s mass. Hubble’s data tend to rule out alternative theories for this object, such as a compact central cluster of unresolved stellar remnants like neutron stars, or smaller black holes swirling around each other.
“We have good confidence that we have a very tiny region with a lot of concentrated mass. It’s about three times smaller than the densest dark mass that we had found before in other globular clusters,” said Vitral. “The region is more compact than what we can reproduce with numerical simulations when we take into account a collection of black holes, neutron stars, and white dwarfs segregated at the cluster’s center. They are not able to form such a compact concentration of mass.”
A grouping of close-knit objects would be dynamically unstable. If the object isn’t a single intermediate-mass black hole, it would require an estimated 40 smaller black holes crammed into a space only one-tenth of a light-year across to produce the observed stellar motions. The consequences are that they would merge and/or be ejected in a game of interstellar pinball.
“We measure the motions of stars and their positions, and we apply physical models that try to reproduce these motions. We end up with a measurement of a dark mass extension in the cluster’s center,” said Vitral. “The closer to the central mass, more randomly the stars are moving. And, the greater the central mass, the faster these stellar velocities.”
Because intermediate-mass black holes in globular clusters have been so elusive, Vitral cautions, “While we cannot completely affirm that it is a central point of gravity, we can show that it is very small. It’s too tiny for us to be able to explain other than it being a single black hole. Alternatively, there might be a stellar mechanism we simply don’t know about, at least within current physics.”
Reference: “An elusive dark central mass in the globular cluster M4” by Eduardo Vitral, Mattia Libralato, Kyle Kremer, Gary A Mamon, Andrea Bellini, Luigi R Bedin and Jay Anderson, 23 May 2023, Monthly Notices of the Royal Astronomical Society.
The Hubble Space Telescope is a project of international cooperation between NASA and ESA. NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble and Webb science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C.