Astronomers have photographed the Hubble Ultra Deep Field more than almost any other patch of sky. It sits in the southern constellation Fornax and holds about 10,000 catalogued galaxies. That makes it a strange place to find light coming from a spot where nothing shows up. A team led by Mason Huberty, an astrophysicist at the University of Minnesota, did exactly that with the James Webb Space Telescope.
The signal is a single bright emission line at about 1.6 micrometers in the near-infrared. Webb's Near-Infrared Spectrograph (NIRSpec) picked it up in August 2023 and again in November 2023, 88 days apart. It showed up in all 4 sets of combined exposures, from both observing orientations, and the measurements matched each time. Earlier Hubble and Webb observations of that location show no source at all.
The find came from a new way of using the instrument. Most spectroscopy points at objects astronomers already know about. The team's technique, Multiobject Coronagraphy for Slitless Spectroscopy (MOCSS), does the reverse: it opens nearly all of NIRSpec's micro-shutters and closes only the ones covering known foreground objects. Then it waits to see what turns up. In the Ultra Deep Field, that search produced a catalog of 22 secure galaxies showing multiple emission lines, at redshifts between 1.3 and 4.6. It also produced this one object that showed a single line and nothing else.
The results appeared October 15 in The Astrophysical Journal under the title "A Single Line Emitter Identified in an Unbiased Search with JWST/NIRSpec." Co-authors include Claudia Scarlata of the Minnesota Institute for Astrophysics, plus researchers from Stockholm University, the Space Telescope Science Institute and the University of Texas at Austin. The paper says the line was seen in 4 dither positions and 2 position angles, so it "cannot be artificial." In other words, it's not a glitch in the detector.
So what is it? The line's observed-frame equivalent width is more than 7,600 Ångströms, which means the line is extremely bright compared with any background light from its source. The authors tested whether it could be ionized oxygen, hydrogen-alpha or Lyman-alpha emission at different distances. Each option would require physical conditions they call implausible. A very early galaxy would have trouble getting that light through the neutral hydrogen of the young universe, and it should have shown up in the JADES survey. A closer galaxy should show other emission lines too. A supernova should have produced a second line that wasn't detected. A tidal disruption event, where a black hole tears a star apart, can make one strong line with little other light, but the team says the evidence isn't strong enough to back it.
Some options are still open. The paper lists a transient event, an active galactic nucleus, or the signature of an extremely young, metal-poor galaxy as possibilities still under investigation. The researchers wrote that "this object is highly unusual and probes extreme physical conditions" and that follow-up observations are essential.
The bigger takeaway for the Minnesota team may be the method. A blind search of one of the most studied fields in the sky still found something no one had catalogued. That suggests a population of faint, distant objects that targeted surveys miss because they don't know to look for them.
