Astronomers might have spotted a stellar-mass black hole in another galaxy — but not everyone agrees with what they found.

Recently, European Southern Observatory (ESO) scientists announced that they spotted a stellar-mass black hole outside the Milky Way — the first time an extragalactic black hole had been detected by measuring its influence on the motion of a nearby star. The black hole was also the first uncovered within a young globular cluster; NGC 1850 is a sprightly 100 million years old.

Black hole in another galaxy (artist's illustration)
This artist’s impression shows a black hole with 11 times the Sun's mass and the five-solar-mass star orbiting it. The two objects are located in NGC 1850, a cluster of thousands of stars roughly 160,000 light-years away in the Large Magellanic Cloud, a Milky Way neighbor. The black hole's strong gravitational force distorts the star’s shape and influences its orbit. 
ESO / M. Kornmesser

But just days later, independent experts Kareem El-Badry (Harvard University) and Kevin Burdge (MIT) released a very different interpretation on the science pre-print service arXiv. They argued that the system is somewhat less exotic but no less interesting, consisting of a Sun-like star and a more massive, less luminous — and wholly stellar — companion.

An Extragalactic Black Hole?

The discrepancy boils down to the interpretation of results from ESO’s Multi Unit Spectroscopic Explorer (MUSE), an instrument mounted on the Very Large Telescope in Chile. MUSE can provide information about thousands of stars in one shot, enabling researchers to tease out light from individual stars in crowded regions such as dense stellar clusters.

From this data, researchers can even infer whether a star is part of a binary star system — though, crucially, the instrument cannot distinguish individual stars within such systems.

When the ESO team pored over data on NGC 1850 that MUSE had collected over two years, they spotted one star acting abnormally. It was moving back and forth at speeds in excess of 300 km/s (700,000 mph), probably in orbit around an unseen companion.

Globular cluster NGC 1850
This image shows NGC 1850, a globular cluster containing thousands of stars. The cluster is roughly 160,000 light-years away in the Large Magellanic Cloud, a Milky Way neighbor. The reddish filaments surrounding the cluster, made of vast clouds of hydrogen, are thought to be the remnants of supernova explosions.
ESO / NASA / ESA / R. Gilmozzi / S. Casertano / J. Schmidt

The researchers next combined their MUSE results with archival data from the Hubble Space Telescope and ground-based photometric images from the Optical Gravitational Lensing Experiment to glean more precise information about this binary. This additional data led them to the conclusion that the mass of the luminous partner is five times that of the Sun, while the unseen companion has about 11 times the Sun’s mass. The only explanation was that the latter must be a black hole.

To add to the evidence, the researchers then began trawling through archival data from the Chandra X-ray Observatory for telltale flares that often appear around black holes as they feed. They spotted a faint signature. “The faint X-ray signal suggests that the black hole is rather dormant, i.e. does not accrete a lot of material at the moment,” says team member Sebastian Kamann (Liverpool John Moores University, UK). “This is in agreement with the appearance of the MUSE spectra.”

Reacting immediately after the ESO team’s announcement, Abbas Askar (Lund University, Sweden), who was not involved in the study, was excited by the breakthrough. “Finding a black hole through radial velocity measurements outside the Milky Way is no easy task,” he said. “The discovery . . . is truly awe-inspiring.”

Or a Stripped-Star System?

However, his enthusiasm was soon tempered when he read El-Badry and Burdge’s response to the finding. In it, the researchers argue that the luminous star is actually similar in mass to our Sun, rather than five times more massive.

Despite its mass, it’s not a Sun-like star. “The star's evolution has been affected significantly by its companion,” El-Badry posits. “In particular, the companion has stripped most of the mass from the star, leaving only a low-mass, bloated core.”

Such stripped stars appear more luminous than they should, masquerading as their more massive siblings. If this is indeed the case, and the star’s mass is much lower than thought, then the inferred mass of its companion also goes way down — so low, in fact, that there is no need to invoke the presence of a black hole.

Both arguments have their merits. In the ESO team’s favor, a largely different team of researchers used the same techniques and instruments to robustly detect three stellar-mass black holes lurking in the globular cluster NGC 3201 within the Milky Way.

But, bolstering El-Badry & Burdge’s case, invoking stripped stars has already cast doubt on similarly extraordinary findings. In 2019, an announcement in Nature claimed the binary system LB-1 contained a black hole of 70 solar masses, twice as massive as was thought possible. El-Badry and a colleague showed that it was more likely a stripped star system with no black hole at all.

“It's easy to get excited with the announcement of such discoveries,” adds Askar. “But one needs to be very careful about interpreting data, given the complexity of the processes involved in binary stellar evolution and cluster dynamics.”

Scientific Method in Action

Kamann says that he and the other ESO authors are already in talks with El-Bandry and Burdge to figure out how they can settle the debate. One way might be to tease apart the spectra of the star and its companion.

“They can't be spatially resolved because we don't have high enough angular resolution,” says El-Badry. But it’s possible the companion could be detected in the spectrum with careful analysis. “Because it's fainter and rapidly rotating (which blurs its spectral lines), it's not easy.”

"In my opinion, what is happening here is a fundamental part of science,” says Kamann, “namely that you present your results not only for others to acknowledge them but to scrutinize and criticize them."


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