A faint flash of infrared light in the Andromeda galaxy was emitted at the birth of a stellar-mass black hole – according to a team of astronomers in the US. Kishalay De at Columbia University and the Flatiron Institute, and colleagues, noticed that the flash was followed by the rapid dimming of a once-bright star. They say that the star collapsed, with almost all of its material falling into a newly forming black hole. Their analysis suggests that there may be many more such black holes in the universe than previously expected.

When a massive star runs out of fuel for nuclear fusion it can no longer avoid gravitational collapse. As it implodes, such a star is believed to emit an intense burst of neutrinos, whose energy can be absorbed by the star’s outer layers.

In some cases, this energy is enough to tear material away from the core, triggering spectacular explosions known as core-collapse supernovae. Sometimes, however, this energy transfer is insufficient to halt the collapse, which continues until a stellar-mass black hole is created. These stellar deaths are far less dramatic than supernovae, and are therefore very difficult to observe.

Observational evidence for these stellar-mass black holes include their gravitational influence on the motions of stars; and the gravitational waves emitted when they merge together. So far, however, their initial formation has proven far more difficult to observe.

Mysterious births

“While there is consensus that these objects must be formed as the end products of the lives of likely very massive stars, there has remained little convincing observational evidence of watching stars turn into black holes,” De explains. “As a result, we don’t even have constraints on questions as fundamental as which stars can turn into black holes.”

The main problem is the low key nature of the stellar implosions. While core-collapse supernovae shine brightly in the sky, “finding an individual star disappearing in a galaxy is remarkably difficult,” De says. “A typical galaxy has a 100 billion stars in it, and being able to spot one that disappears makes it very challenging.”

Fortunately, it is believed that these stars do not vanish without a trace. “Whenever a black hole does form from the near complete inward collapse of a massive star, its very outer envelope must be still ejected because it is too loosely bound to the star,” De explains. As it expands and cools, models predict that this ejected material should emit a flash of infrared radiation – vastly dimmer than a supernova, but still bright enough for infrared surveys to detect.

To search for these flashes, De’s team examined data from NASA’s NEOWISE infrared survey and several other telescopes. They identified a near-infrared flash that was observed in 2014 and closely matched their predictions for a collapsing star. That flash was emitted by a supergiant star in the Andromeda galaxy.

Nowhere to be seen

Between 2017 and 2022, the star dimmed rapidly before disappearing completely across all regions of the electromagnetic spectrum.  “This star used to be one of the most luminous stars in the Andromeda Galaxy, and now it was nowhere to be seen,” says De.

“Astronomers can spot supernovae billions of light years away – but even at this remarkable proximity, we didn’t see any evidence of an explosive supernova,” De says. “This suggests that the star underwent a near pure implosion, forming a black hole.”

The team also examined a previously-observed dimming in a galaxy 10 times more distant. While several competing theories had emerged to explain that disappearance, the pattern of dimming bore a striking resemblance to their newly-validated model, strongly suggesting that this event too signalled the birth of a stellar-mass black hole.

Because these events occurred so recently in ordinary galaxies like Andromeda, De’s team believe that similar implosions must be happening routinely across the universe – and they hope that their work will trigger a new wave of discoveries.

“The estimated mass of the star we observed is about 13 times the mass of the Sun, which is lower than what astronomers have assumed for the mass of stars that turn into black holes,” De says. “This fundamentally changes out understanding of the landscape of black hole formation – there could be many more black holes out there than we estimate.”

The research is described in Science.

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