In the southern constellation of Fornax lies a tiny patch of sky just like any other. Your eyes may have brushed past it a hundred times without a thought.
Yet this otherwise unremarkable speck of the cosmos is among
the most well-cataloged in the sky. The Hubble Space Telescope stared at it for
more than 270 hours to obtain its famous Ultra Deep Field (HUDF) – and
contained therein are nearly 10,000 galaxies extending billions of light-years
into the Universe.
When the James Webb Space Telescope (JWST) launched, it was
only natural that its golden mirrors would turn to the same region of space,
using its vastly more sensitive infrared capabilities to find what Hubble may
have missed.
Now, it has revealed that a patch of space astronomers have
been poring over for decades may yet hold some surprises.
In a tiny 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 every other prior observation
from both JWST and Hubble, there's absolutely nothing there.
That doesn't mean that there is nothing there – there may be
something otherwise too dim to detect that briefly unleashed a powerful flare –
but it does make that signal a mystery that, currently, has no satisfying
explanation.
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The Hubble Ultra Deep Field. |
To understand why this signal is so strange, though, first
you need to know a little more about how astronomers study the Universe.
When you look into an optical telescope, you see an image
formed from the light collected by its mirror. That's pretty much what you're
looking at when you gaze upon 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 in the light that are hard 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 specific wavelengths have been absorbed by atoms in the solar
atmosphere.
Other objects produce bright lines instead, where atoms or
molecules release energy as 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 colleagues wanted to try something a little bit 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 hadn't been found yet.
Normally, 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 relies on knowing where the target is
first.
Huberty and his team did sort of the complete opposite.
They opened up nearly 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 light collected 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 – crucially – at different observation
angles. This would help eliminate artifacts and glitches.
The aim was to find faint, distant objects that conventional
observations might have missed, using a technique called Multiobject
Coronagraphy for Slitless Spectroscopy (MOCSS).
The HUDF was an ideal testing ground, with its wealth of
existing observations allowing the researchers to check their results against
known galaxies.
And it worked. After all was said and done, six detections
that passed their strict criteria could be matched 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, that's not
so weird; near-infrared emission is super normal.
It's the isolation that caused the drama. There was no other
emission line accompanying it – and in prior observations from Hubble and JWST,
nothing else could be seen in that patch of space.
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JWST imaging of the location of the mysterious emission
line. The colored rectangles show the positions of the spectrograph's shutters
during four sets of observations, while the pink circles mark three nearby
galaxies. At the center of the rectangles, where the emission line was
detected, no source is visible. (Huberty et al., ApJ, 2026) |
It can't easily be explained away as a glitch, either. It
appeared in all four sets of combined exposures, across both observing
orientations, 88 days apart, and the measurements were consistent.
Here's the real kicker. The emission line was bright enough
that it should have shown up in the earlier JWST images.
Had it been present during JWST's JADES survey conducted in
October 2022, there's no way the telescope could have missed it.
Which raises the possibility that maybe it wasn't present
then.
But just one line is really tricky to identify, unless you
know exactly how far away it is. That's because, as space expands, it stretches
light into redder and redder wavelengths.
So an infrared photon could be energetic light from really
far away, or less energetic light from much closer; you need more information
than just its wavelength to figure out which it is – and how it was produced.
There are several different things that maybe could have
produced the emission line – with four main suspects, none of which fit the
bill cleanly.
A galaxy in the early Universe is the first in the lineup,
with two main problems – that emission line would struggle to travel through
the neutral hydrogen that filled the early Universe, and it should have
appeared 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
the one.
It could have been a supernova, but again, it should have
produced another emission line that wasn't detected.
And finally, a black hole tearing apart a star can produce a
strong emission line, even without much other detectable light. But there's not
enough evidence to establish that this is what happened.
Which means that there's only one thing to do.
"This object is highly unusual and probes extreme physical conditions," the researchers write.