Progress in “gravitational monster research”: Astronomers have discovered a black hole in a star cluster outside the Milky Way using an indirect detection method. The comparatively small and still young specimen revealed itself on the basis of its gravitational influence on the movement of a nearby star. The method could now lead to the discovery of further black holes that have so far remained undetected. This in turn could shed light on the formation and development of the mysterious celestial bodies, say the researchers.
Their gigantic masses curve space so much that not even light can escape: Because of their spectacular features, black holes have long been the focus of astrophysics. What makes them so exciting, however, also creates major challenges for research: Due to their “dark” character, they are difficult to detect. However, they make themselves felt through certain effects on space and matter. This made it possible to draw conclusions about the masses of black holes. They are assigned to three categories: small and medium-sized black holes and the absolute gravitational giants – supermassive black holes with millions to billions of solar masses.
Search for “perpetrators”
The small representatives, which sometimes only have ten solar masses or less, are particularly difficult to discover. Above all, these “stellar” black holes have so far only rarely been found outside of our Milky Way. They mostly betrayed themselves by the X-ray light that they emitted when swallowing matter, or by gravitational waves that arise when black holes collide with one another or with neutron stars. Most black holes with stellar mass, however, do not show up in this way and therefore remain hidden. That is why the international team of researchers has now explored the possibility of detecting extragalactic black holes using a dynamic detection method based on movement patterns. “When black holes form a system with a star, they influence its course in a subtle but verifiable way so that we can find them with highly developed instruments,” explains co-author Stefan Dreizler from the University of Göttingen, explaining the basic principle of the approach.
As part of their investigations, the scientists targeted the cluster of thousands of stars, NGC 1850. It is about 160,000 light years away from us in the Large Magellanic Cloud – a galaxy neighboring the Milky Way. The Multi Unit Spectroscopic Explorer (MUSE) was used on the Very Large Telescope of the European Southern Observatory ESO in Chile. The researchers used the instrument to analyze the spectra of thousands of stars in the cluster at the same time. “Similar to Sherlock Holmes, who tracks down a criminal organization based on its deeds, we look at every single star in this cluster with a magnifying glass in hand and try to find clues to the existence of black holes without seeing them directly,” First author Sara Saracino from Liverpool John Moores University explains the approach.
Sinister dance partner
The manhunt finally led to the “machinations” of a black hole in a star with about five solar masses: It showed unusually recurring changes in its radiation pattern. As the researchers explain, this is due to the fact that the star periodically moves towards and away from us. However, the cause of this effect was not visible. It is therefore a hidden black hole near this star that, as it were, dances with it – it shapes its movements through its gravitational force.
From the analysis of the data obtained it finally emerged that it was a black hole with eleven solar masses. So the researchers have identified a small – and young – specimen. According to estimates, the star cluster itself is only around 100 million years old – a blink of an eye in astronomical terms. As they explain, evidence of young black holes can shed light on the evolutionary history of the dark celestial bodies: A comparison with larger, more developed representatives in older star clusters could provide clues as to how these objects grow, for example by devouring stars or with others Merge black holes. This could also provide information on their role as sources of gravitational waves.
The astronomers now see great potential in the dynamic detection process for the further search for black holes: In this way, more young black holes outside or inside the Milky Way could be discovered. “So far we have only identified one of the wanted ‘perpetrators’. But once you’ve found one, you’re well on your way to discovering many others in different heaps, ”says Saracino. Her colleague Mark Gieles from the University of Barcelona concludes: “Every single discovery we make will be important for our future understanding of star clusters and the black holes in them”.
Source: ESO