The Nuclear Starburst in NGC 253

We have obtained long-slit spectra of NGC 253 in the J, H, K, and N bands, broadband images in the J, H, and Ks bands, narrowband images centered at the wavelengths of Brγ and H2(1, 0) S(1), and imaging spectroscopy centered on [Ne II] (12.8 μm). We have subtracted a composite stellar spectrum from the galaxy spectrum to measure faint emission lines that otherwise would be buried in the complicated continuum structure. We use these data and data from the literature in a comprehensive reassessment of the starburst in this galaxy. We confirm that the [Fe II] emission is predominantly excited by supernova explosions and show that the rate of these events can be derived from the strength of the infrared [Fe II] lines. Although the H2 emission superficially resembles a thermally excited spectrum, most of the H2 infrared luminosity is excited by fluorescence in low-density gas. We confirm the presence of a bar and also show that this galaxy has a circumnuclear ring. The relation of these features to the gaseous bar seen in CO is in agreement with the general theoretical picture of how gas can be concentrated into galaxy centers by bars. We derive a strong upper limit of ~37,000 K for the stars exciting the emission lines. We use velocity-resolved infrared spectra to determine the mass in the starburst region. Most of this mass appears to be locked up in the old, preexisting stellar population. Using these constraints and others to build an evolutionary synthesis model, we find that the initial mass function (IMF) originally derived to fit the starburst in M82 also accounts for the properties of NGC 253; this IMF is similar to a modified Salpeter IMF. The models indicate that rapid massive star formation has been ongoing for 20-30 million yr in NGC 253; that is, it is in a late phase of its starburst. Its optical spectrum has characteristics of a transitional H II/weak-[O I] LINER. We model the emission-line spectrum expected from a late phase starburst and demonstrate that it reproduces these characteristics.