Did Astronomers Spot a Black Hole in Another Galaxy? Maybe Not

Recently, European Southern Observatory (ESO) scientists announced that they noticed a stellar-mass black hole outdoors the Milky Way — the primary time an extragalactic black hole had been detected by measuring its affect on the movement of a close by star. The black hole was additionally the primary uncovered inside a younger globular cluster; NGC 1850 is a sprightly 100 million years outdated.

Black hole in another galaxy (artist's illustration)
This artist’s impression exhibits a black hole with 11 instances the Sun’s mass and the five-solar-mass star orbiting it. The two objects are situated in NGC 1850, a cluster of 1000’s of stars roughly 160,000 light-years away in the Large Magellanic Cloud, a Milky Way neighbor. The black hole’s sturdy gravitational power distorts the star’s form and influences its orbit. 
ESO / M. Kornmesser

But simply days later, unbiased specialists Kareem El-Badry (Harvard University) and Kevin Burdge (MIT) launched a very completely different interpretation on the science pre-print service arXiv. They argued that the system is considerably much less unique however no much less attention-grabbing, consisting of a Sun-like star and a extra huge, much less luminous — and wholly stellar — companion.

The discrepancy boils all the way down to the interpretation of outcomes from ESO’s Multi Unit Spectroscopic Explorer (MUSE), an instrument mounted on the Very Large Telescope in Chile. MUSE can present details about 1000’s of stars in one shot, enabling researchers to tease out gentle from particular person stars in crowded areas similar to dense stellar clusters.

From this information, researchers may even infer whether or not a star is a part of a binary star system — although, crucially, the instrument can’t distinguish particular person stars inside such programs.

When the ESO staff pored over information on NGC 1850 that MUSE had collected over two years, they noticed one star appearing abnormally. It was shifting forwards and backwards at speeds in extra of 300 km/s (700,000 mph), in all probability in orbit round an unseen companion.

Globular cluster NGC 1850
This picture exhibits NGC 1850, a globular cluster containing 1000’s 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, fabricated from huge clouds of hydrogen, are regarded as the remnants of supernova explosions.
ESO / NASA / ESA / R. Gilmozzi / S. Casertano / J. Schmidt

The researchers subsequent mixed their MUSE outcomes with archival information from the Hubble Space Telescope and ground-based photometric photographs from the Optical Gravitational Lensing Experiment to glean extra exact details about this binary. This extra information led them to the conclusion that the mass of the luminous companion is 5 instances that of the Sun, whereas the unseen companion has about 11 instances the Sun’s mass. The solely rationalization was that the latter should be a black hole.

To add to the proof, the researchers then started trawling by archival information from the Chandra X-ray Observatory for telltale flares that usually seem round black holes as they feed. They noticed 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 staff member Sebastian Kamann (Liverpool John Moores University, UK). “This is in agreement with the appearance of the MUSE spectra.”

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

Or a Stripped-Star System?

However, his enthusiasm was quickly tempered when he learn El-Badry and Burdge’s response to the discovering. In it, the researchers argue that the luminous star is definitely related in mass to our Sun, moderately than 5 instances extra huge.

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 seem extra luminous than they need to, masquerading as their extra huge siblings. If that is certainly the case, and the star’s mass is far decrease than thought, then the inferred mass of its companion additionally goes way down — so low, in reality, that there isn’t any must invoke the presence of a black hole.

Both arguments have their deserves. In the ESO staff’s favor, a largely completely different staff of researchers used the identical methods and devices to robustly detect three stellar-mass black holes lurking in the globular cluster NGC 3201 inside the Milky Way.

But, bolstering El-Badry & Burdge’s case, invoking stripped stars has already solid doubt on equally extraordinary findings. In 2019, an announcement in Nature claimed the binary system LB-1 contained a black hole of 70 solar lots, twice as huge as was thought doable. El-Badry and a colleague confirmed that it was extra seemingly a stripped star system with no black hole in any respect.

“It’s easy to get excited with the announcement of such discoveries,” provides 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 opposite ESO authors are already in talks with El-Bandry and Burdge to determine how they’ll settle the controversy. One way is perhaps to tease aside 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 doable the companion may very well be detected in the spectrum with cautious evaluation. “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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