Astronomers Pointed Webb at an Empty Spot in Space. Something ‘Highly Unusual’ Flashed Back

A team of astronomers scouring the distant universe for ancient objects that formed shortly after the Big Bang stumbled upon an unexpected discovery: a single emission line coming from an unidentified source.

Using the Webb space telescope, scientists detected the unusual glow in a seemingly empty patch of the sky in the southern constellation of Fornax. The light appeared twice over the course of three months, despite earlier observations by both Webb and the Hubble space telescope showing no visible object at that location. Its origin remains a mystery, and the possible candidates aren’t your garden-variety space objects. The findings are detailed in a new paper published in The Astrophysical Journal.

Staring into the void

Astrophysicist Mason Huberty of the University of Minnesota was searching for yet-to-be detected objects. Using Webb’s Near InfraRed Spectrograph, Huberty and his team blocked light from known objects such as galaxies that had been previously identified by Hubble. The researchers then searched the data for emission lines.

Six out of seven detections could be matched to distant galaxies. The seventh one, however, left the team puzzled. It appeared as a single bright line during two observations windows, which were carried out 88 days apart in August and November 2023.

The team compared the emission line to possible sources in earlier Webb images but could not pinpoint its origin. Both Hubble and Webb had stared at that corner of the sky before, but nothing could be seen there.

Emission lines are bright spikes of light emitted by atoms or molecules when they release energy. Astronomers use them to work out what distant objects are made of, how far away they are, and sometimes how they are moving.

This particular one had no other emission lines accompanying it. On its own, however, it would have been bright enough to show up in earlier observations.

Origin: unknown

The astronomers behind the discovery ruled out possible known sources for the emission line, but they haven’t been able to identify where it came from. To do that, they first need to know how far away it is.

As the universe expands, it stretches the light traveling through space, pushing it toward longer wavelengths and making it appear redder. An infrared photon could have started out as high-energy light from a distant object, stretched to longer wavelengths as the universe expanded. Or it could have been emitted at a lower energy by a closer object. To determine where it came from and what produced it, astronomers need more clues than its wavelength alone.

The team notes that the emission lines could have come from a galaxy—possibly a primitive one that formed just a few hundred million years after the Big Bang—though such an object should have shown up in earlier observations. It may also be an active galactic nucleus, the bright center of a galaxy powered by a black hole, that’s in its dying phase. If that were the case, however, the astronomers should’ve caught other emission lines from the source. Alternatively, the light may have come from a short-lived event, like a star being shredded by a black hole, which would explain why nothing appeared in images taken about a year earlier. But all of these explanations come with their own problems.

“Regardless of its true nature…this object is highly unusual and probes extreme physical conditions,” the scientists wrote in the study.