Black hole’s gravity gives it away
Black holes, as their name suggests, give off no light of their own, making them notoriously difficult to detect. But often, black holes appear in binary systems with a normal star. In these cases, the gravitational pull of the black hole on its visible companion may be the only sign of its presence.
That’s exactly what researchers saw within NGC 1850, a cluster of stars some 160,000 light-years away in the Large Magellanic Cloud. They used the Multi Unit Spectroscopic Explorer on the Very Large Telescope in Chile; this instrument allows astronomers to look closely at the light of individual stars — even those packed within the distant, crowded cluster. By doing so, scientists discovered an invisible member: a black hole 11 times the mass of our Sun, causing noticeable motion in a visible star five times the mass of the Sun.
The research, published Nov. 11 in Monthly Notices of the Royal Astronomical Society, is the first time scientists have found such a small black hole in another galaxy using only its gravitational influence. The researchers say this type of gravitational, or dynamical, influence is the best way to spot many stellar-mass black holes, which have masses up to about 100 times that of the Sun. That’s because other methods for finding these objects require the black hole to be actively feeding or in the throes of smashing into its partner.
What’s more, NGC 1850 is relatively young — just
100 million years old. In such young clusters, it’s even less likely that black holes will be feeding or colliding, making dynamical interaction the only technique available to find them in such environments. So, targeting young clusters using this method is an opportunity to increase the known number of younger stellar-mass black holes. Astronomers can then compare them to older black holes found via other methods to study the life cycle of these extreme objects.