An unexpected signal in an empty patch of sky
A team of astronomers hunting for ancient objects that formed soon after the Big Bang has found something it cannot explain. According to Gizmodo, which reported on a new paper in The Astrophysical Journal, the researchers used the James Webb Space Telescope to detect a single emission line coming from an unidentified source.
The glow came from what looked like an empty region in the southern constellation of Fornax. Earlier observations by both Webb and the Hubble Space Telescope had shown no visible object at that location, yet the light appeared twice over about three months.
How the team found it
Astrophysicist Mason Huberty of the University of Minnesota was searching for objects that had not yet been detected. Using Webb's Near InfraRed Spectrograph, Huberty and colleagues blocked light from known objects, including galaxies Hubble had already identified. They then searched the remaining data for emission lines.
Emission lines are bright spikes of light released when atoms or molecules give off energy. Astronomers use them to learn what distant objects are made of, how far away they are, and sometimes how they are moving.
Of seven detections, six could be matched to distant galaxies. The seventh appeared as a single bright line in two observing windows carried out 88 days apart, in August and November 2023. The team compared it with possible sources in earlier Webb images but could not pinpoint its origin.
Why the source is hard to pin down
The line had no other emission lines alongside it. According to the report, it would have been bright enough on its own to show up in earlier observations, which makes the lack of any visible object more puzzling.
Identifying the source depends first on knowing its distance, and the wavelength alone does not settle that. As the universe expands, light stretches toward longer, redder wavelengths. An infrared photon might therefore have started as high-energy light from a very distant object. It could also have been emitted at lower energy by something closer. Astronomers need more clues to tell the two cases apart.
The possible explanations
The researchers ruled out known sources but offered several candidates, each with drawbacks. One is a galaxy, perhaps a primitive one that formed a few hundred million years after the Big Bang. Such an object, however, should have appeared in earlier observations.
Another is an active galactic nucleus, the bright center of a galaxy powered by a black hole, in a dying phase. In that case the team would expect to have seen other emission lines from the source.
A third possibility is a short-lived event, such as a star being shredded by a black hole. That could explain why nothing appeared in images taken about a year earlier, but it too comes with its own problems, according to the report.
"Regardless of its true nature…this object is highly unusual and probes extreme physical conditions," the scientists wrote in the study.
