JWST View of the Supernebula in NGC 5253. II. Nebular Lines

The nearby dwarf starburst NGC 5253 is dominated by a compact radio-infrared supernebula powered by a very young and bright embedded super star cluster of ∼10 9 L ⊙ . We observed this source and its surroundings over the 5–25 μ m range with the Mid-Infrared Instrument (MIRI) Medium Resolution Spectrometer (MRS) on JWST, and in Paper I presented the JWST view of the region and its continuum features. We now present the more than 70 emission lines of H I , H 2 , and metal ions detected by MIRI/MRS. We derive the extinction by comparing H I recombination lines to the free–free radio continuum and find that it is very flat, i.e., almost independent of wavelength over this spectral range. Nebular conditions are consistent with young (≲5 × 10 6 yr) and very massive stars. All regions show high excitation, but the spatial distribution of the high-excitation lines suggests that photons with energies close to 50 eV are escaping the supernebula core in spite of 35 mag of visual extinction.