Large Magellanic Cloud
The Large Magellanic Cloud captured by ESO’s VISTA (Visible and Infrared Survey Telescope for Astronomy) telescope || Credits: ESO / VMC Survey (Enlarge).

A black hole, in and of itself, is difficult to detect. By not emitting visible light it is difficult to see them. If we analyze it in other bands of the electromagnetic spectrum, we can see some of the effects it produces. But seeing it, seeing it as such, is complicated. In fact, you will remember that famous first direct image of a black hole and all the technological deployment that had to be carried out to obtain it. And as in the circus, there is a “more difficult still”: what if the black hole was hidden in a cluster of stars? Today I bring you an investigation focused on the NGC 1850 object related to black holes.

The research has been carried out with the MUSE (Multi Unit Spectroscopic Explorer) instrument of the VLT (Very Large Telescope) at the ESO (European Southern Observatory) facilities. They have analyzed NGC 1850, a cluster of thousands of stars located about 160,000 light years away in the Large Magellanic Cloud (LMC), a satellite galaxy of our Milky Way. As I told you above, scientists have not been able to see the black hole directly. What they have been able to appreciate are the movements in a star close to it, one of the thousands that make up the cluster.

The principal investigator of this study, Sara Saracino of the Astrophysical Research Institute of the John Moores University of Liverpool (UK), compares this research with the most famous character created by Sir Arthun Conan Doyle. «We observe each and every one of the stars in that cluster and, like Sherlock Holmes when he followed the missteps of a criminal gang with his magnifying glass, we try to find some evidence of the presence of black holes, although without seeing them directly ».

NGC 1850 houses the “first criminal”
The research (Saracino, 2021) has been published in the scientific journal Monthly Notices of the Royal Astronomical Society. “The result we show would represent only one of the wanted criminals, but when you have found one, you are well on your way to discovering many others in different clusters,” explains Saracino. In fact, this first “criminal” is a black hole about 11 times more massive than our Sun. What scientists have been able to see of this “crime”, continuing with the analogy, is the smoke from the gun after being fired , which has manifested itself in this case in the form of gravitational influence in a star of about 5 solar masses that orbits the black hole.

How are these black holes detected?
It’s not the first time stellar-mass black holes like this have been detected in other galaxies. They have been seen capturing their emissions in the band of X-rays produced by engulfing the matter around them. They have also been appreciated from the gravitational waves produced when two of these stellar-mass black holes collide with each other. However, based on observations, discovering a black hole due to its consequences based on X-rays or gravitational waves is not typical. As stated by Stefan Dreizler, a scientist at the University of Göttingen (Germany) who is also involved in the research, “the presence of the vast majority can only be dynamically revealed.”

This dynamic method that Saracino and his team have used could allow the astronomical community to detect many more black holes. “When a black hole forms a system with a star, it will affect the motion of the star in a subtle but detectable way, so with sophisticated instruments, we will be able to find them,” explains Dreizler.

NGC 1850, a 100-million-year-old young cluster
On the other hand, the NGC 1850 cluster is very young. It is only 100 million years old, something that in cosmic time is a whisper. To give you an idea, there are dinosaurs that when they inhabited our planet, this cluster had not yet been born. This youth makes it the first time that a black hole has been detected in a young star cluster. In this way, using this method, new black holes could be found in young clusters and, therefore, it would be also young black holes, which could shed light on what the first steps of a black hole are when compared to black holes. older.

Related scientific articles
Saracino, S. et al (2021). Monthly Notices of the Royal Astronomical Society.