Webb Finds an Unusual Infrared Signal With No Identified Source

Even a familiar patch of sky can still surprise us.

Astronomers using the James Webb Space Telescope have detected an unusual infrared emission line in the Hubble Ultra Deep Field. It appears in observations taken 88 days apart, but there’s no corresponding source in earlier deep images from Webb’s JADES survey.

The researchers have explored several explanations. None fits comfortably yet.

A different way of looking

The discovery came from a new observing strategy called Multiobject Coronagraphy for Slitless Spectroscopy, or MOCSS.

Normally, astronomers choose targets before observing them with Webb’s Near-Infrared Spectrograph, known as NIRSpec. For this survey, the team instead left more than 90% of the instrument’s microshutter area open, blocking selected foreground objects to reduce interference.

That let them search for emission lines without first requiring a source to appear in an image.

Emission lines are concentrations of light at particular wavelengths. They can reveal information about the gas producing them and, when identified, help astronomers measure a source’s redshift and distance.

One detection stood apart

The researchers identified seven secure emission-line candidates that matched across two observing orientations. Six had identifiable galaxy hosts in JADES.

The seventh did not.

Its emission line appeared at roughly 1.6 micrometers in the infrared and was detected in all four combined exposure groups.

The observations were taken on August 23 and November 19, 2023. The measured line brightness was consistent between those two visits, separated by approximately 88 days.

That repeat detection is important. It gives the team more evidence to work with than a feature appearing in just one exposure.

Why the missing image matters

The comparison images were taken in October 2022, roughly a year before the spectroscopy.

According to the researchers’ calculations, the emission line alone should have been bright enough to register through the relevant imaging filter if it had been present at the same strength.

But no corresponding source was detected.

One explanation is that the source changed between the imaging and spectroscopic observations. Another is that it has unusually extreme physical properties.

There’s also no detected continuum, the broader background of light surrounding an emission line. If the source is persistent, that makes the line remarkably strong compared with its underlying light.

Could it be a distant galaxy?

The researchers compared the detection with familiar hydrogen and oxygen emission lines. Each identification would place the source at a different redshift.

Some interpretations would require unusually extreme conditions. Others would normally produce additional lines that weren’t detected, although some expected features would have been close to the observations’ sensitivity limits.

One possibility is Lyman-alpha emission from a galaxy at a redshift of about 12.2, placing it in the early universe.

But that interpretation is not confirmed. The combination of strong line emission and no detected continuum would make it a highly unusual source.

Could a black hole be involved?

The team also considered an active galactic nucleus, a galaxy’s central region powered by matter falling toward a supermassive black hole.

That explanation has difficulties too. Several possible line identifications would predict accompanying features that the observations don’t show.

The paper also discusses a fading active nucleus or a separate gas cloud illuminated by a nearby active galaxy. These are possibilities to investigate, not conclusions.

What about a temporary event?

A transient event could explain why the source was absent from the earlier images.

One candidate is a Type IIn supernova, whose interaction with surrounding material can produce narrow emission lines. However, an additional line expected under that interpretation wasn’t detected. Dust could potentially weaken it.

Another possibility is a tidal disruption event, when a supermassive black hole tears apart a star.

Under certain conditions, such an event could produce strong hydrogen emission with weak surrounding light and no detectable companion lines within the observed wavelength range.

The similar brightness across 88 days doesn’t rule that out. Some tidal disruption events evolve over longer periods.

Still, the available evidence doesn’t establish that a black hole disrupting a star caused this detection.

Another look is needed

The researchers say deeper spectroscopy is essential to identify the line and determine the source’s nature.

They highlight Webb’s NIRSpec integral field unit as a useful next step. It could gather spectra of the emitter and nearby galaxies together, helping clarify whether they’re connected.

For now, the result is a well-supported detection with an uncertain explanation.

That’s what I find interesting about this story. A new way of examining a deeply studied field has uncovered something that doesn’t fit neatly into the usual categories.

We don’t know what it is yet. But now astronomers know where to look.

Source: Research published in The Astrophysical Journal, DOI: 10.3847/1538-4357/ae9a94.