SWS spectroscopy of the starburst galaxy NGC 3256.
Lutz, D.; Genzel, R.; Sturm, E.; Rigopoulou, D.; Egami, E.; Netzer, H.; de Graauw, T.; Feuchtgruber, H.; Sternberg, A.; Moorwood, A. F. M.; Kunze, D.; Bauer, O. H.; Schaeidt, S.
Abstract
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 >=50Msun_ have been forming recently in NGC 3256. The H_2_ observations reveal the presence of warm molecular gas. We estimate that =~10^9^Msun_ of H_2_ 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.