Astrophysicist Mason Huberty of the University of Minnesota was hunting for ancient objects formed shortly after the Big Bang when his team stumbled on something unexpected: a single bright emission line coming from an unidentified source in the southern constellation of Fornax.

Using Webb's Near InfraRed Spectrograph, the researchers blocked light from known objects previously identified by Hubble, then searched the remaining data for emission lines. Six of seven detections could be matched to distant galaxies.

The seventh could not — and it appeared twice, during observation windows 88 days apart in August and November 2023. The findings are detailed in a new paper in The Astrophysical Journal.

The surprise is that both Hubble and Webb had already stared at that exact corner of the sky and seen nothing visible at the location.

If the source was present, the researchers argue it should have been bright enough to appear in the JADES survey conducted in October 2022 — and yet the wavelength showed up multiple times, from different angles, 'corresponding to no known source.'

The technique behind the discovery, called Multi-Object Coronagraphy for Slitless Spectroscopy (MOCSS), involves opening essentially all of NIRSpec's thousands of microshutters simultaneously while closing shutters aligned with known bright objects, letting faint signals that would otherwise be drowned out by 'brighter show-offs' emerge.

The team has ruled out known sources but cannot pin down the origin, in large part because distance is unknown: cosmic expansion stretches light toward redder wavelengths, so an infrared photon could be high-energy light from a very distant object or low-energy light from a closer one.

The candidates are not garden-variety space objects. It could be a primitive galaxy from a few hundred million years after the Big Bang — but such an object should have shown up earlier, and its light would struggle through the 'fog' of neutral hydrogen in the early universe.

It could be an active galactic nucleus in its dying phase, or an AGN in one of three neighboring sources illuminating a separate gas cloud — but then other emission lines, particularly from metals, should have been caught.

A short-lived event like a star being shredded by a black hole would explain the earlier blank images, but carries its own problems.

Whatever it turns out to be, the team concludes the object probes extreme physical conditions.

If the line is Lyman-alpha emission from neutral hydrogen, the light would have been emitted soon after the Big Bang and stretched over billions of years, implying a galaxy at redshift around z ∼ 12.2 — though galaxies have been found at higher redshifts, making that reading awkward.

The alternative is an extreme metal-poor source much closer, at z ∼ 1.4. 'Regardless of its true nature… this object is highly unusual and probes extreme physical conditions,' the scientists wrote in the study.

Either branch — a primeval, extremely low-metallicity galaxy or a fading distant AGN — would make the source a genuinely strange member of the known cosmos.

The mystery stands until astronomers can take another look at the region to establish the source's redshift and confirm what produced the line.

Determining distance is the key first step: once astronomers know how far away the emitter is, they can work backward to what kind of object could produce a single, unaccompanied emission line that bright.

The case also showcases the payoff of the MOCSS approach, which lets Webb search for signals that have no known source at all rather than only revisiting cataloged targets — a capability that could surface more such anomalies in archival and future data.

For now, the source of the unusual signal remains unknown.