In the southern constellation of Fornax lies a small patch of sky like any other. Your eyes may have passed it hundreds of times without thinking about it. Yet this otherwise nondescript speck of the cosmos is among the best cataloged 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 that extend billions of light years into the Universe. When the James Webb Space Telescope (JWST) launched, it was only natural that its golden mirrors would rotate toward the same region of space, using its much more sensitive infrared capabilities to find what Hubble may have missed. Now, it has revealed that a patch that space astronomers have been studying for decades may still contain some surprises. In a small corner of the tiny field, JWST detected a strange signal not once, but twice, in observations nearly three months apart. There’s just one problem. In all other previous observations from both JWST and Hubble, there is absolutely nothing there. That doesn’t mean there’s nothing there (there may have been something too faint to detect that briefly unleashed a powerful flare), but it does make that signal a mystery that currently has no satisfactory explanation. Hubble’s ultra-deep field. (NASA, ESA and S. Beckwith (STScI) and the HUDF team) However, to understand why this signal is so strange, you first need to know a little more 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. That’s pretty much what you’re seeing when you look at the HUDF: a combination of images taken through different filters, each of which captures a particular range of wavelengths. But some telescopes can also split light into different wavelengths; Think about how a prism can split white light into a rainbow, but on a much more granular scale. This can reveal features in the light that are difficult to distinguish in a multi-wavelength image. For example, if you collect all the light from the Sun, it looks white; But when split into a spectrum, you’ll see dark lines where atoms in the sun’s atmosphere have absorbed specific wavelengths. Instead, other objects produce bright lines, 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. Instead of 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. Typically, astronomers select known targets from existing observations and then position the instrument’s tiny shutters so that light from the selected target enters the spectrograph. It’s a really useful technique, with one limitation: it depends on knowing where the target is first. Huberty and his team did the complete 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. They then 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, more 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 its large number of existing observations allowing researchers to compare their results with known galaxies. And it worked. After all was said and done, six detections that passed their strict criteria could match 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, that’s not so strange; The near infrared emission is super normal. It’s the isolation that caused the drama. There was no other accompanying emission line, and in previous Hubble and JWST observations, nothing else could be seen in that portion of space. JWST images of the location of the mysterious emission line. Colored rectangles show the positions of the spectrograph shutters during four sets of observations, while pink circles mark three nearby galaxies. In the center of the rectangles, where the emission line was detected, no source is visible. (Huberty et al., ApJ, 2026) It also cannot be easily explained as a technical problem. It appeared in all four sets of combined exposures, in both viewing orientations, 88 days apart, and measurements were consistent. Here’s the real trick. The emission line was bright enough to have appeared in previous JWST images. Had it been present during JWST’s JADES survey in October 2022, there is no way the telescope would have missed it. Which raises the possibility that he may not have been present at the time. But a single line is really difficult to identify, unless you know exactly how far away it is. This is because as space expands, it stretches light toward increasingly redder wavelengths. Therefore, an infrared photon could be energetic light coming from very far away, or less energetic light coming from much closer; More information than just its wavelength is needed to determine what it is and how it was produced. There are several different things that could perhaps have produced the broadcast line, with four main suspects, none of whom fit perfectly. A galaxy in the early Universe is first on the list, with two main problems: That emission line would have difficulty traveling through the neutral hydrogen that filled the early Universe, and it should have shown up in the JADES observations. A more recent galaxy could also be the culprit, 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 that went undetected. And finally, a black hole tearing apart a star can produce a strong emission line, even without much other detectable light. But there is not enough evidence to establish that this is what happened. Which means there’s only one thing to do. “This object is very unusual and explores extreme physical conditions,” the researchers write. “Follow-up observations are essential to confirm the redshift and determine the physical origin of the emission… Deeper, more sensitive spectroscopy could reveal additional emission lines and definitively establish the redshift and the nature of the source.” The research has been published in The Astrophysical Journal. This article was fact-checked by Fiona MacDonald and edited by Fiona MacDonald. While we take pride in our process, we are human. If you spot an error, please let us know.
