In the southern constellation Fornax there is a small part of the sky like any other. Your eyes may have brushed over it a hundred times without thinking about it.
Yet this otherwise mundane point in the cosmos is among the best recorded in the sky. The Hubble Space Telescope observed it for more than 270 hours to obtain its famous ultra deep field (HUDF) – and it contains almost 10,000 galaxies stretching billions of light years into the Universe.
When the James Webb Space Telescope (JWST) was launched, it was only natural that its golden mirrors would look toward the same region of space, using its much more sensitive infrared capabilities to find what Hubble might have missed.
Today it was revealed that a group of space astronomers that astronomers have been interested in for decades could still have surprises in store.
In a small corner of the small field, JWST detected a strange signal – not once, but twice, during observations spaced almost three months apart.
There’s just one problem. In all other previous JWST and Hubble observations, there is absolutely nothing.
This does not mean that there East nothing there – perhaps there was something too faint to detect that briefly set off a powerful flare – but that makes this signal a mystery that, currently, has no satisfactory explanation.
To understand why this signal is so strange, you first need to know a little about how astronomers study the Universe.
When you look through an optical telescope, you see an image formed from the light collected by its mirror. This is pretty much what you see when you look at the HUDF: a composite of images taken through different filters, each capturing a particular range of wavelengths.
But some telescopes can also split light into different wavelengths – think of how a prism can split white light into a rainbow, but on a much more granular scale.
This can reveal features of light that are difficult to distinguish in a multi-wavelength image. For example, if you collect all the light from the Sun, it appears white; but when split into spectrum, you will see dark lines where specific wavelengths have been absorbed by atoms in the solar atmosphere.
Other objects instead produce lines of light, where atoms or molecules release energy in the form of light at specific wavelengths. These are called emission lines.
Astronomers use a technique called spectroscopy to detect these absorption and emission lines, a clever way to determine the chemical composition and other properties of astrophysical objects.
Astrophysicist Mason Huberty of the University of Minnesota and his colleagues wanted to try something a little different.
Rather than using spectroscopy to study already known objects, they wanted to see if JWST’s Near Infrared Spectrograph (NIRSpec) could be used as a tool to find objects that had not yet been found.
Normally, astronomers select known targets from existing observations, then position the instrument’s tiny shutters so that light from the selected target enters the spectrograph. This is a very useful technique, with one limitation: it relies on knowing where the target is first.
Huberty and his team did just the opposite.
They opened almost all the shutters, closing only those covering known foreground objects identified in previous Hubble observations, whose light could contaminate the spectra they wanted to collect.
Then they split the collected light into spectra and searched the resulting data for emission lines.
And they did it twice – two observation windows, 88 days apart in August and November 2023, and – most importantly – from different observation angles. This would help eliminate artifacts and glitches.
The goal was to find faint, distant objects that conventional observations might have missed, using a technique called Multi-Object Coronagraphy for Slitless Spectroscopy (MOCSS).
The HUDF was an ideal testing ground, with the wealth of existing observations allowing researchers to compare their results with known galaxies.
And it worked. After all, six detections that met their strict criteria could be linked to distant galaxies in the HUDF.
The problem was the seventh detection.
It was a single, bright line that appeared in both near-infrared observations at about 1.6 micrometers. In isolation, it’s not so weird; near infrared emission is super normal.
It was the isolation that caused the tragedy. No other emission lines accompanied it – and in previous observations by Hubble and JWST, nothing else could be seen in this part of space.
This also cannot be easily explained away as a problem. It appeared in all four exposure series combined, in both viewing orientations, 88 days apart, and the measurements were consistent.
Here’s the real kicker. The emission line was bright enough that it should have appeared in previous JWST images.
If he had been present during JWST’s JADES survey conducted in October 2022, the telescope could not have missed him.
Which raises the possibility that he may not have been present at the time.
But a single line is very difficult to identify unless you know exactly how far away it is. This is because as space expands, it stretches light towards increasingly red wavelengths.
So an infrared photon could be energetic light coming from very far away, or less energetic light coming from much closer; you need more information than just its wavelength to determine what it is – and how it was produced.
There are several different things that maybe could have produced the show line – with four prime suspects, none of whom quite fit the bill.
A galaxy from the early Universe is first on the list, with two main problems: This emission line would have difficulty passing through the neutral hydrogen that filled the early Universe, and it should have shown up in the JADES observations.
A newer galaxy could also be the cause, but – in addition to the JADES problem – it should have other emission lines, not just one.
It could have been a supernova, but again it should have produced another emission line which was not detected.
And finally, a black hole tearing apart a star can produce a strong emission line, even without other detectable light. But there is not enough evidence to establish that this is what happened.
Which means there is only one thing to do.
“This object is very unusual and probes extreme physical conditions,” the researchers write.
“Follow-up observations are essential to confirm the redshift and determine the physical origin of the emission… Further and more sensitive spectroscopy could reveal additional emission lines and definitively establish the redshift and the nature of the source.”
The research was published in The Astrophysics Journal.
This article was fact-checked by Fiona MacDonald and edited by Fiona MacDonald. Although we are proud of our process, we are only human. If you spot an error, please let us know.
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