1996A&A...315L.125R

We present 2.5-40µm ISO SWS spectra of the starburst galaxy NGC 3256. We have observed many fine-structure lines of various atomic species as well as three rotational transitions of molecular hydrogen. From the [SIII] 18µm/33µm line ratio we infer low electron densities of ≃300cm^–3 in the HII regions. The observed [NeIII]/[NeII] (15µm/12µm), [ArIII]/[ArII] (8.9µm/6.9µm), and [SIV]/[SIII] (10.5µm/18.7µm) line ratios are consistent with an ionizing radiation field with an effective temperature of ≃41000K. Comparison of the observed fine-structure line ratios with theoretical models of nebular emission from evolving starbursts shows that stars with masses ≥50M_☉ have been forming recently in NGC 3256. The H₂ observations reveal the presence of warm molecular gas. We estimate that ≃10⁹M_☉ of H₂ gas is present at temperatures close to 150K. This mass corresponds to a few percent of the total cold molecular mass as estimated from CO studies.