88 days apart, twice: Mystery signal detected coming from empty region of space

James Webb Space Telescope (JWST) has detected a strange signal - not once but twice - coming from a region of the universe that is empty. Previously, the Hubble Space Telescope had spent more than 270 hours peering into that same region to obtain an Ultra Deep Field (HUDF). It detected nearly 10,000 galaxies extending billions of light-years into the Universe. Then Webb's sensitive infrared capabilities were used to see if Hubble had missed something. The research has been published in The Astrophysical Journal.

JWST picked up a signal from one tiny corner of this field in observations gathered three months apart. But the strange thing is both Webb and Hubble data show there is absolutely nothing there. Scientists admit that there still could be something there which is too dim to detect. However, until something is actually found, the signal remains an anomaly.

Telescopes combine light through different filters to show an object in the universe. Some telescopes act like prisms, splitting light into a rainbow to reveal chemical makeup. There are also absorption lines, dark gaps where atoms in an object absorb specific wavelengths of light. Emission lines are bright stripes where atoms release energy at specific wavelengths.

How the observation was made

Astronomers use spectroscopy to detect these absorption and emission lines. However, astrophysicist Mason Huberty of the University of Minnesota wanted to try something new and used JWST's Near-Infrared Spectrograph (NIRSpec) to find objects that hadn't been found yet. Huberty decided to cover known foreground objects identified by Hubble to prevent their light from contaminating the spectra they wanted to collect.

Then they searched for emission lines twice, 88 days apart in August and November 2023. The aim was to detect faint, distant objects that had never been picked up before. Six detections were revealed that could be matched to distant galaxies in the HUDF. The problem was the seventh detection. This single, bright line appeared in both near-infrared observations at about 1.6 micrometres.

Not a glitch

There was no other emission line with it, and Hubble and JWST had also seen nothing else in that patch of space. Since it appeared in all four sets of combined exposures, it could not be dismissed as a glitch. Scientists are confused since this emission line was so bright it should have been detected by JWST.

Astronomers tried to explain this line using different possibilities, none of which fit. It could have been a galaxy in the early Universe, a more recent galaxy, a supernova or a black hole tearing apart a star. "This object is highly unusual and probes extreme physical conditions," the researchers write. More observations are needed to determine what could have produced this signal.