MOCSS: Multi-Object Coronagraphy for JWST/NIRSpec Slitless Spectroscopy

We present an experimental JWST/NIRSpec observational method to obtain R 1000 G140M/F100LP slitless spectroscopic observations. This "Multi-Object Coronagraphy for Slitless Spectroscopy" (MOCSS) technique involves opening essentially all microshutters on NIRSpec simultaneously while closing shutters corresponding on-sky with known bright objects. Through observations of the Hubble Ultra Deep Field, we demonstrate the discovery potential of these deep, high resolution observations without the need for target pre-selection. We obtain a catalog of 22 secure multi-line emitters at 1.3<z<4.6, with observations of [OII], [OIII], H α, H β and [NII] lines, accounting for about80% of the emission lines identified directly in the dispersed images. The MOCSS survey provides first time spectroscopic redshifts for 18 of these sources. We also demonstrate that first order traces of the NIRSpec/G140M/F100LP observing mode extend substantially beyond the nominal about1.89 μm red cutoff, as far as 2.94 μm in the most extreme case. Compared to existing NIRISS WFSS of similar exposure times, we find that the MOCSS technique can observe very faint lines while covering the full 1-2 μm regime without gaps in wavelength coverage. This study describes the highest-resolution slitless spectroscopy campaign of the 1-2 μm regime and emphasizes the discovery potential of this unorthodox observational technique.