A Single Line Emitter Identified in an Unbiased Search with JWST/NIRSpec: Multiobject Coronagraphy for Slitless Spectroscopy
Journal article, 2026
We report the detection of a previously unknown object with a single emission line within the Hubble Ultra Deep Field, using a new JWST NIRSpec technique called Multiobject Coronagraphy for Slitless Spectroscopy (MOCSS) with the G140M/F100LP observing mode. The MOCSS technique involves opening almost all the shutters on the NIRSpec microshutter array, while simultaneously closing the shutters that lie on top of known low-redshift objects, facilitating a wide-field slitless spectroscopic search without contamination from bright objects. This observational technique has enabled the detection of a strong emission line in four different dither positions and in two position angles, and thus cannot be artificial. This line is detected at 1.60 μm with a line flux of [2.5 ± 0.3] × 10−18 erg s−1cm−2 and an FWHM of 13.2 ± 2.0 Å, and does not correspond to any known source with some of the deepest Hubble Space Telescope and JWST imaging in existence. Based upon the absence of imaging detection, we place a lower limit on the observed frame equivalent width (EW) of 7620 Å on this object, which strongly limits the number of possible lines that this line can be. Given the wavelength, an EW limit that is near the ceiling of expectation, and the lack of other lines in the bandpass, this line cannot be easily explained by [O ii] at z = 3.29, [O iii] at z = 2.20, Hα at z = 1.44, and Lyα at z = 12.16. We discuss possible active galactic nucleus and transient phenomena that could explain the finding.
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Author
Mason S. Huberty
University of Minnesota
Claudia Scarlata
University of Minnesota
Matthew J. Hayes
Oskar Klein Centre
Alaina Henry
Space Telescope Science Institute (STScI)
Danielle A. Berg
University of Texas
Vieri Cammelli
University of Modena and Reggio Emilia
R. S. Ellis
University College London (UCL)
Anne E. Jaskot
Williams College
Yu Heng Lin
California Institute of Technology (Caltech)
Vihang Mehta
California Institute of Technology (Caltech)
Matilde Mingozzi
Space Telescope Science Institute (STScI)
Pierluigi Monaco
University of Trieste
Osservatorio Astronomico di Trieste
Alberto Saldana-Lopez
Oskar Klein Centre
A. Saxena
University of Oxford
Jonathan Tan
University of Virginia
Chalmers, Physics, Subatomic, High Energy and Plasma Physics
Alice R. Young
Oskar Klein Centre
Astrophysical Journal
0004-637X (ISSN) 1538-4357 (eISSN)
Vol. 1009 2 117Subject Categories (SSIF 2025)
Atom and Molecular Physics and Optics
Astronomy, Astrophysics, and Cosmology
DOI
10.3847/1538-4357/ae9a94