Among black hole hunters, a group has dedicated itself to looking for specific types: singularities of intermediate size between the supermassives that roar within the galaxies (such as Sagittarius A *, in the center of the Milky Way) and those born from the collapse of massive stars (sometimes with the mass of the Sun). It was in search of this missing link that two astronomers from the Paris Institute of Astrophysics (IAP) discovered a group of small black holes in the center of a galaxy as old as the Universe.
“Ours is the first study to provide the mass and extent of what appears to be a collection of black holes in the center of a collapsed globular cluster,” said astronomer Eduardo Vitral.
Using the Hubble space telescope, he and his IAP colleague, also astronomer Gary Mamon, searched inside the globular cluster NGC 6397, 7,800 light-years from Earth, for the hypothetical intermediate-sized black hole. This galaxy has one of the most dense nuclei known and, when examining it, the researchers arrived at a result that did not correspond to what they imagined to find.
“We found very strong evidence of an invisible mass in the dense core of the globular cluster, but we were surprised to find that this extra mass is not punctual, that is, it is not in a single place in space (which would be expected for a massive black hole solitary), but spread over an area within the cluster, “said Stained Glass in a statement.
Astronomers have detected this “invisible mass” using the speeds at which stars travel within the cluster, thus determining the distribution of their total mass (including that of visible stars, those that are not so much and the so-called dark stars): the more mass there is in a region, the faster the stars will transit through it.
“Our analysis indicated that the orbits of the stars are almost random throughout the globular cluster, rather than maintaining systematic circular or elongated movements,” explained Mamon. What would make the stars behave in an astronomically bizarre manner would be remnants of massive stars (white dwarfs, neutron stars and black holes).
The NGC 6397 cluster is believed to have more than 20 black holes inside.
Source: ESA / Hubble / N. Bartmann Source: ESA / Hubble / N. Bartmann
Due to gravitational interactions, these remains ended up in the center of NGC 6397, while the less massive stars migrated to the periphery of the cluster. Thus, the hypothesis launched by IAP astronomers gains weight due to two recent studies, which, equally, argue that stellar remnants as black holes of stellar mass could migrate to the center of globular clusters.
The search for singularities of intermediate size also advances by another aspect: that the supermassive giants are, in fact, of size M, and that we have yet to discover singularities of size G – immeasurably large and stupendously monstrous, with mass 1 trillion. times greater than that of the Sun or 100 quintillions (100,000,000,000,000,000,000) of solar masses.
They would be larger than the largest known black hole, J2157, one of the first to have appeared in the Universe, feeding a burning quasar in the center of a galaxy 12 billion light years away. It is a black hole from the beginning of the Universe, swallowing the equivalent of a Sun every 2 days and growing like no other known black hole does.
The study by IAP researchers may help to understand how black holes grow. If they merge into larger singularities, the phenomenon can be captured by the Laser Interferometer Gravitational-Wave Observatory (LIGO).