A Bright Line in Empty Sky, Seen Twice

The Hubble Ultra Deep Field is a small patch of southern sky in Fornax that astronomers have stared at for decades. Mason S. Huberty, Claudia Scarlata and colleagues used JWST's NIRSpec in a slitless mode they call MOCSS: nearly all the microshutters open, the known bright foreground sources closed, and a search through whatever lines remained.

Six lines that passed their cuts matched distant galaxies. The seventh did not. It is a single bright line at about 1.60 micrometres. No second line beside it. No object in the deep Hubble frames. No object in the JWST pictures either, including the JADES survey of October 2022. The line was bright enough that JADES should have seen it if it had already been there.

They saw it twice. August and November 2023, 88 days apart, in four dither positions and two spacecraft orientations. The measurements agreed. Reported flux is about 2.5 times 10 to the minus 18 erg per second per square centimetre. The line is narrow. Signal to noise is about 8.4. If the source had been sitting there the whole time, the equivalent width would have to be enormous, on the order of 7,620 angstroms, because the continuum was never detected.

The paper is in The Astrophysical Journal, DOI 10.3847/1538-4357/ae9a94. ScienceAlert carried it on 15 October. A technical writeup followed on 19 October. It is inside the feature window, and it is still the strangest verified item in today's file.

Key Evidence

• One line at about 1.60 micrometres, no companion lines, no imaging counterpart (Huberty et al., ApJ).
• Detected in four dithers and two position angles, 88 days apart.
• Absent from JADES imaging in October 2022, despite being bright enough to have appeared.
• Six other lines in the same program matched catalogued galaxies.

The Rational Explanation

Slitless spectra can put a line on the wrong bit of sky. A signal-to-noise of 8 is a detection, not a new class of object. A flare, a supernova, a black hole tearing up a star, or a galaxy with one unusually strong line can all produce a lonely feature. "Empty" means undetected at the depth of the pictures we have.

What We Don't Know

None of those readings fit cleanly. A nearby galaxy usually shows more than one line. A supernova usually does too. An early galaxy at that wavelength has to get the line through neutral hydrogen, and the earlier Webb images should have caught a persistent source. The line may have turned on between October 2022 and August 2023. Nobody has said what turned it on.

The Rabbit Hole

The Ultra Deep Field was supposed to be the patch where surprises had already been catalogued. A blank coordinate that emits once, then again, is the opposite of that promise.