An article (link to the file in PDF format) published in the journal “Nature Astronomy” reports the discovery of a probable dormant black hole in the Large Magellanic Cloud, one of the dwarf galaxies satellite of the Milky Way. A team of researchers spotted it in the VFTS 243 system after a thorough examination of a binary system located in the Tarantula Nebula in which a candidate was identified to be tested in the search for black holes. Six years of observations conducted with ESO’s VLT allowed to rule out other possible explanations for the nature of the objects studied. An interesting conclusion is that the black hole discovered is the result of a collapse of the parent star that occurred without a supernova.
Stellar-mass black holes are one of the possible remnants of a massive star after the end of its normal life. Massive stars usually explode into supernovae leaving a core in which the force of gravity is no longer opposed by nuclear reactions. If the mass is sufficient, that core can contract to the point of having such a force of gravity that even light can’t escape it. However, in recent years some cases were found where a massive star at the end of its life collapsed directly into a black hole with no intermediate stage. The observations of the VFTS 243 system show a binary system with a circular orbit of the two objects with a motion that suggests that the black hole was born as a result of a direct collapse.
Locating VFTS 243’s black hole was a long and complex job because it’s a dormant black hole showing no signs of activity like others that are surrounded by materials that get heated and emit detectable electromagnetic radiation or strip gas from a companion star. The authors of this study have long been studying candidates that could be objects of this type but usually end up finding other explanations for the observations obtained. That includes what was initially believed to be a black hole in the HR 6819 system. For this reason, they have earned the nickname of black holes police.
In the case of VFTS 243, the researchers used data collected by the FLAMES (Fiber Large Array Multi Element Spectrograph) instrument mounted on ESO’s VLT (Very Large Telescope) over six years. Its discovery came by examining nearly a thousand massive stars in the Tarantula Nebula in the Large Magellanic Cloud in the hopes of finding one that had a black hole as a companion. More data was found in the Tarantula Massive Binary Monitoring program archive, which also used the FLAMES instrument and also another VLT spectrograph, GIRAFFE, to study blue giants in the Tarantula Nebula.
The image (ESO/L. Calçada) shows an artist’s representation of the VFTS 243 system. A giant blue star that has a mass that is about 25 times the Sun’s has as a companion a black hole, whose mass is estimated at about 9 times the Sun’s. The image of the black hole is surrounded by a lens effect around it which is purely illustrative. The sizes of the two objects are not to scale: actually, the star is about 200,000 times larger than the black hole.
The clues point to the black hole nature of the object found in the VFTS 243 system but now the black hole police will see their work under scrutiny by other astronomers. It’s possible that other observations conducted with other instruments bring useful data to provide a final answer.
It would be important to be certain that the object in the VFTS 243 system is a black hole also because it would confirm that the criteria are correct in the search for dormant black holes. Discovering black holes generated by a direct collapse of a star would be even better because it could offer new clues to a process that’s still poorly understood of very massive star death.
