A small black hole that’s just hanging around, quietly minding its own business, has been detected outside the Milky Way galaxy.
This, the team says, marks the first time that we’ve been able to conclusively detect and identify an inactive black hole that isn’t within the confines of our own galaxy.
While a similar discovery was announced last year, evidence of its true identity left some wiggle room for doubt. This newer finding, according to the authors, is a solid bet.
Admittedly, the object isn’t all that far from the Milky Way, nestled within a satellite galaxy called the Large Magellanic Cloud. But the discovery is one that could help us find more such black holes in the future, and has implications for our understanding of black hole formation to start with.
The team behind the report is clearly thrilled, not least because of their skeptical reputation for casting doubt on previous dormant black hole findings.
“For the first time, our team got together to report on a black hole discovery, instead of rejecting one,” said astronomer Tomer Shenar of Amsterdam University in the Netherlands.
“We identified a ‘needle in a haystack’.”
Black holes are tricky little beasties. Their extreme density produces an extreme gravitational field that means not even light whizzing through a vacuum – the fastest anything can go in the Universe – is able to achieve escape velocity.
This means they are shrouded in darkness, emitting no light that we can detect. The exception is when they’re actively “feeding”, or accreting material. This is a messy process that produces a tell-tale signature of X-ray radiation from the vicinity immediately around the black hole.
Inactive, or quiescent, black holes are more or less invisible. But that pesky gravity can give it away… if you know how to look. If a stellar-mass black hole is in a binary system with another star, the orbital motion of the companion circling what appears to be empty space could indicate the black hole’s presence.
Not all dark masses are black holes, though. Other astronomers have been fooled before, the most famous example being a black hole touted as the closest yet to Earth ever found. A small, dim companion whose light cannot be discerned could be the culprit, meaning no option can be left unconsidered.
Shenar and members of his team, including astronomers Kareem El-Badry of the Harvard & Smithsonian Center for Astrophysics and Julia Bodensteiner of the European Southern Observatory, have been among those painstakingly debunking such discoveries. But that doesn’t mean they think such black holes aren’t out there; just that the evidence needs to be watertight.
“For more than two years now, we have been looking for such black-hole-binary systems,” Bodensteiner explained.
The focus of their search was the Tarantula Nebula in the Large Magellanic Cloud, a stellar nursery where young, extremely massive stars can be found. The researchers studied, in detail, around 1,000 of these young massive stars, looking for the telltale wobble of a binary orbit.
When any two objects orbit one another, they do so around a mutual center of gravity called their barycenter. For a system like Earth and the Sun, the barycenter would be close enough to the center of the Sun that it would be hard to see the star move around it from any great distance. Were Earth more massive, the Sun’s circling of the barycenter would be far easier to detect.
We can detect this wobbling movement, or radial velocity, in the spectrum of light from the object as it stretches into longer (redder) wavelengths moving away from us, and compresses into shorter (bluer) wavelengths moving towards us.
The team searched their sample for these spectral shifts and got a hit – with a massive blue-white O-type star 25 times the mass of the Sun, around 160,000 light-years away. When they calculated the mass of the object that could cause the observed wobble, they found that the companion was around 9 times the mass of the Sun. At that mass, the black hole’s event horizon would be around just 27 kilometers (17 miles) across.
And yet, it was invisible. The upper mass limit for neutron stars is around 2.3 times that of the Sun, so that rules out those. Other stars in their sample that wobbled were ruled out using the team’s techniques to detect light from faint companion stars, and modeling the light expected from a faint companion at the observed mass.
None of the alternative explanations fit the observation data.
“When Tomer asked me to double check his findings, I had my doubts,” El-Badry said. “But I could not find a plausible explanation for the data that did not involve a black hole.”
The binary, named VFTS 243, could hold important clues about how black holes form. Scientists believe that there are multiple scenarios. The first is a colossal supernova in which an unstable star erupts, exploding its outer material into space while the core collapses down into a black hole, born of fire and fury.
The second is a direct collapse in which a dying star, no longer supported by the outward pressure supplied by atomic fusion just… collapses in on itself, not with a bang but a quiet ker-floomp.
“The star that formed the black hole in VFTS 243 appears to have collapsed entirely, with no sign of a previous explosion,” Shenar said. “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.”
Although, of course, having scrutinized so many other black hole discoveries, the team now invites other astronomers to scrutinize theirs. Fair’s fair.
The research has been published in Nature Astronomy.