University of Sheffield: ‘Black hole police’ discover dormant black hole outside our galaxy

A stellar-mass black hole has been discovered in a neighbouring galaxy by a team of international scientists, including researchers from the University of Sheffield.

The team found that the star that gave rise to the black hole vanished without any sign of an associated supernova explosion. The discovery was made thanks to six years of observations obtained with the European Southern Observatory’s (ESO’s) Very Large Telescope (VLT).

Stellar-mass black holes are formed when massive stars reach the end of their lives and collapse under their own gravity. In a binary, a system of two stars revolving around each other, this process leaves behind a black hole in orbit with a luminous companion star. The black hole is ‘dormant’ if it does not emit high levels of X-ray radiation, which is how such black holes are typically detected.

Dormant black holes are particularly hard to spot since they do not interact much with their surroundings. The team has been looking for black-hole-binary systems for more than two years and they are particularly excited about the discovery, known as VFTS243, which they consider to be the most convincing candidate reported to date.

Professor Paul Crowther, Head of the Department of Physics and Astronomy, and Professor of Astrophysics at the University of Sheffield, said: “It’s a very exciting discovery. Although a number of `dormant’ black hole candidates have been proposed, this is the first to be unambiguously detected outside our galaxy.”

As part of the international research team, Professor Crowther has been working alongside Tomer Shenar from the Institute of Physics and Astronomy, who started the study at KU Leuven in Belgium and is now a Marie-Curie Fellow at Amsterdam University in the Netherlands.

Dr Shenar explains: “We identified a `needle in a haystack’. The star that formed the black hole in VFTS 243 appears to have collapsed entirely, with no sign of a previous explosion. Evidence for this ‘direct-collapse’ scenario has been emerging recently, but our study arguably provides one of the most direct indications. This has enormous implications for the origin of black-hole mergers in the cosmos.”

To find VFTS 243, the collaboration searched nearly 1000 massive stars in the Tarantula Nebula region of the Large Magellanic Cloud, looking for the ones that could have black holes as companions. Identifying these companions as black holes is extremely difficult, as so many alternative possibilities exist.

The discovery also allows the team a unique view into the processes that accompany the formation of black holes. Astronomers believe that a stellar-mass black hole forms as the core of a dying massive star collapses, but it remains uncertain whether or not this is accompanied by a powerful supernova explosion. Co-author Pablo Marchant of KU Leuven adds: “It is incredible that we hardly know of any dormant black holes, given how common astronomers believe them to be.”

The black hole in VFTS 243 was found using six years of observations of the Tarantula Nebula by the Fibre Large Array Multi Element Spectrograph (FLAMES) instrument on ESO’s VLT [3]. Historically, binaries hosting stellar mass black holes have been identified through the presence of bright X-ray emission from the accretion disk surrounding the black hole, but deep Chandra X-ray observations reveal VFTS 243 to be X-ray faint [4].

The team actively encourages scrutiny, and hopes that their work, published today in Nature Astronomy, will enable the discovery of other stellar-mass black holes orbiting massive stars, thousands of which are predicted to exist in Milky Way and in the Magellanic Clouds.

“We encourage other scientists in the field to explore our analysis carefully, and to try to offer alternative models as an explanation,” concludes Professor Crowther.