The extraordinary infrared spectrum of NGC 1222 (Markarian 603)

The infrared spectra of starburst galaxies are dominated by low-excitation lines of [Ne II] and [S III], and the stellar populations deduced from these spectra appear to lack stars larger than about 35 M-circle dot. The only exceptions to this result until now were low-metallicity dwarf galaxies. We report our analysis of the mid-infrared spectra obtained with the Infrared Spectrograph on Spitzer of the starburst galaxy NGC 1222 (Mrk 603). NGC 1222 is a large spheroidal galaxy with a starburst nucleus that is a compact radio and infrared source, and its infrared emission is dominated by the [Ne III] line. This is the first starburst of solar or near- solar metallicity known to us that is dominated by high-excitation lines and is a likely host of high-mass stars. We model the emission with several different assumptions as to the spatial distribution of high-and low-excitation lines and find that the upper mass cutoff in this galaxy is 40- 100 M circle dot.