Molecular Hydrogen Emission in the Wolf-Rayet Nebula NGC 2359

Abstract

We report the first direct detection of molecular hydrogen emission in the interstellar medium in the vicinity of a Wolf-Rayet (W-R) star. The spatial distribution of the excited molecular gas associated with NGC 2359 is filamentary and lies mainly on the border of the ionized gas, as traced by optical emission lines such as Hα or [O III] λ5007. The typical 1–0 S(1) H2 brightness in the filaments is 5 × 10-5 ergs s-1 cm-2 sr-1 and the total 1–0 S(1) H2 luminosity detected is ∼4 L☉. The detected line flux in the 1–0 S(1) transition of H2 at λ = 2.122 μm could equally be explained by shock excitation or by fluorescence from the strong ultraviolet flux of the W-R star. The morphological distribution of the H2 filaments is not inconsistent with either mode of excitation. Although the ubiquity of this phenomenon needs to be confirmed, the relatively high level of 1–0 S(1) H2 emission detected in this W-R nebula indicates that hot stars could potentially contribute a significant fraction of the total H2 emission of young starburst galaxies.