100 million tiny black holes, each several times as massive as our sun, are believed to be scattered throughout our galaxy. These black holes were created when stars exploded. Supermassive black holes, located in the galactic centers and millions or billions of times more massive than our sun, are abundant throughout the cosmos.

A medium-mass black hole, weighing about 100 to 100,000 times the mass of our sun, is a long-sought missing piece. Why do they seem so unusual, how would they arise and where would they hang out?

Through various observing techniques, astronomers have discovered additional potential medium-mass black holes. Two of the leading contenders, HLX-1, found in 2009, and 3XMM J215022.4-055108, which Hubble helped discover in 2020, are on the fringes of other galaxies. These potential black holes, containing the mass of tens of thousands of suns, may have once served as the centers of dwarf galaxies.

Astronomers using the NASA/ESA Hubble Space Telescope have provided their best evidence yet for the presence of a rare class of medium-sized black holes. They found a strong candidate lurking at the heart of the closest globular cluster to Earth, 6,000 light-years away.

Hubble targets the core of the globular star cluster Messier 4 (M4) to hunt for black holes with greater precision than previous searches.

Eduardo Vitral of the Space Telescope Science Institute in Baltimore, Maryland, and formerly of the Institut d’Astrophysique de Paris (IAP, Sorbonne University) in Paris, France, said: “You can’t do this kind of science without Hubble.”

A team of scientists has found a potential black hole with an average mass of about 800 solar masses. Although the supposed object is invisible, its mass can be determined by observing how stars move when they are pulled into the gravitational field, much like how bees are drawn to a hive. It takes a little time and great care to measure their movements accurately. This is where Hubble excels against all other modern telescopes. Hubble M4 observations over a 12-year period were examined by astronomers, resolving precise stars.

With scans of more than 6,000 stars constraining the cluster’s overall structure and mass, the European Space Agency’s (ESA) Gaia mission also contributed to this result. Hubble’s observations mostly rule out alternative explanations for this object, such as a dense central cluster of unresolved stellar remnants such as neutron stars or smaller black holes orbiting each other.

Vitral said, “Using the latest data from Gaia and Hubble, it was impossible to distinguish between a dark population of stellar remnants and a single larger point-like source. So one of the possible theories is that this dark mass, rather than many separate small dark objects could be a medium-sized black hole.”

“We are very confident that we have a very small region with a lot of concentrated mass. It is about three times smaller than the densest dark mass we had previously found in other globular star clusters. The region is more compact than we can reproduce with numerical simulations when we look at a collection of black holes, neutron stars and white dwarfs separated in the center of the cluster, they are not able to form such a compact mass concentration.”

Vitral said, “Because medium-mass black holes in globular clusters are so elusive, Vitral warns: “While we cannot fully confirm that it is a center of gravity, we can show that it is very small. It’s too small for us to explain other than that it’s a single black hole. Alternatively, there could be a stellar mechanism we know nothing about, at least within current physics.”

Magazine reference:

  1. Eduardo Vitral, Mattia Libralato et al. An elusive dark central mass in the globular star cluster M4. Monthly communications from the Royal Astronomical Society. DOI: 10.1093/mnras/city1068