Astronomers using NASA’s Hubble Space Telescope have discovered what they believe to be some of the greatest evidence yet for the presence of a rare type of “intermediate-sized” black hole lurking in the heart of the nearest globular star cluster to Earth, about 6,000 light-years distant.
What are the formation, distribution, and rarity of intermediate-mass black holes?
Almost all black holes appear to come in two sizes, similar to strong gravitational pits in the fabric of space: small and gargantuan. Our galaxy is thought to be strewn with 100 million tiny black holes (many times the mass of our Sun) formed by exploding stars. The cosmos is teeming with supermassive black holes, which are situated in the centers of galaxies and weigh millions or billions of times the mass of our Sun.
An intermediate-mass black hole, weighing between 100 and 100,000 solar masses, is a long-sought missing link.
How would they form, where would they congregate, and why do they appear to be so uncommon?
Using a variety of observational approaches, astronomers have detected more probable intermediate-sized black holes. Three of the finest possibilities — 3XMM J215022.4055108, discovered by Hubble in 2020, and HLX-1, discovered in 2009 — live in dense star clusters on the edges of neighboring galaxies. Each of these hypothetical black holes has tens of thousands of suns in mass and may have formerly resided in the centers of dwarf galaxies. NASA’s Chandra X-ray Observatory has also aided in the discovery of numerous probable intermediate black holes, including a large sample in 2018.
Much closer to home, a number of probable intermediate-sized black holes have been discovered in dense globular star clusters around our Milky Way galaxy. For example, Hubble researchers reported the possible presence of an intermediate-mass black hole in the globular cluster Omega Centauri in 2008. These and other intermediate-mass black hole discoveries remain inconclusive and do not rule out alternate hypotheses for a variety of reasons, including the need for further data.
Hubble’s unique capabilities have now been utilized to hone in on the core of the globular star cluster Messier 4 (M4), allowing for more precise black-hole hunting than prior efforts. “You can’t do this kind of science without Hubble,” said Eduardo Vitral, lead author of an article to be published in the Monthly Notices of the Royal Astronomical Society.
What Vitral’s team discovered and what was its significance?
Vitral’s team discovered a probable 800 solar-mass intermediate-sized black hole. Although the alleged object cannot be seen, its mass can be determined by observing the motion of stars caught in its gravitational field, similar to bees swarming around a hive. Measuring their movement needs time and precision. This is where Hubble achieves something that no other modern telescope can. Astronomers examined 12 years of Hubble M4 data and resolved pinpoint stars.
His team believes the black hole in M4 could be 800 times the mass of our Sun. Alternative possibilities for this object, such as a compact center cluster of unresolved stellar remains like neutron stars or smaller black holes revolving around one other, are ruled out by Hubble’s observations.
“We are confident that we have a very small region with a large concentration of mass.” “It’s about three times smaller than the densest dark mass we’ve found in other globular clusters,” Vitral added. “When we consider a collection of black holes, neutron stars, and white dwarfs segregated at the cluster’s center, the region is more compact than what we can reproduce with numerical simulations.” They are not capable of forming such a dense concentration of mass.”
Credits: NASA’s Goddard Space Flight Center; Lead Producer: Paul Morris
What is the nature of the central mass in the globular cluster and its impact on stellar motions?
A collection of closely packed objects would be dynamically unstable. If the object isn’t a single intermediate-sized black hole, the observed stellar motions would require an estimated 40 smaller black holes squeezed into an area only one-tenth of a light-year across. As a result, they would merge and/or be expelled in an interplanetary pinball game. “We measure the motions and positions of stars and apply physical models to try to reproduce these motions.”
“We end up with a measurement of a dark mass extension in the center of the cluster,” Vitral explained. “The stars move more randomly as they get closer to the central mass.” And, as the center mass increases, so do the stellar velocities.”
Because intermediate-mass black holes in globular clusters have been so difficult to find, Vitral warns, “While we cannot completely confirm that it is a central point of gravity, we can show that it is very small.” It’s too little for us to explain anything other than a solitary black hole. Alternatively, there could be a stellar mechanism that we are simply unaware of, at least in terms of present physics.”