Hubble begins its hunt for hard-to-identify medium-sized black holes

Throughout the history of astronomy, humanity has found many 'small' black holes, with a mass 100 times less than that of the sun. Along with that are also countless giant black holes, with masses millions or even billions of times that of the sun. But the strange thing is that we hardly find any black holes in the mid-mass range. In other words, the confirmation that mid-sized black holes exist or not, and what properties they possess is still very vague for global astrophysicists.

As part of an effort to solve this conundrum, astronomers are planning to use the Hubble Space Telescope to hunt down mysterious medium-sized black holes floating in the universe. In fact, Hubble has repeatedly found some clues about mid-sized black holes in the past, especially within a few thousand light-years of Earth, and this is also the reason it was chosen in this difficult task.

The reason why the discovery of mid-sized black holes is so challenging is because their impact on surrounding stars is much more modest than the massive black holes that astronomers usually find easily. Hubble has observed targets like Messier 4, a globular cluster thought to contain a black hole about 800 times the mass of the sun. In theory, a black hole cannot be directly observed, but its presence can be inferred by analyzing its own effects on nearby stars.

Besides Hubble, the scientists also plan to use data from Gaia, a project to create 3D maps of stars in the Milky Way, to help provide information about the shape of globular clusters. However, even with the help of these two powerful telescope systems, researchers still cannot say with certainty whether they are observing a black hole or a series of less 'condensed' objects such as neutron stars or white dwarfs.

In theory, if there were a group of objects lying close together, they would have to be crammed together in an unstable formation. A more plausible explanation is the existence of a single black hole of average mass. So, one of the possible theories is that instead of many small dark objects lying apart, this dark mass could be a medium-sized black hole.

' A mid-sized black hole could be a relatively small dark region with a lot of concentrated mass. It is about three times smaller than the densest dark masses we have found previously in other globular clusters. This region is more compact than what we can reconstruct with numerical simulations when taking into account a collection of black holes, neutron stars and white dwarfs separated at the center of the cluster. They cannot form such a compact concentrated mass ,' said Professor Eduardo Vitral, who led the study.

Overall, researchers can't be completely certain whether they've identified a particular dark region as a mid-sized black hole, but the level of accuracy can be enhanced through data analysis. And that means even more interesting research to come. Science rarely discovers something new in a single instant, but often has to go through the process of analysis and proof of a conclusion to a reasonable, universally accepted level.

Micah Soto

Update 24 July 2023