Infrared spectroscopy of star formation in galaxies: extreme conditions and very short time scales

We have measured Brackett α and γ with a 7.2 min beam and 350 km/sec spectral resolution in 11 infrared-bright galaxies and used the fluxes as diagnostics of the population of young OB stars. The star-formation activity in the program galaxies is surprisingly concentrated in both space and time. The OB stars present in 7.2 min are sufficient to generate the bulk of the infrared emission observed in the much larger area measured by IRAS, and the OB stars in several galaxies are no older than a few million years. In some galaxies the ratios of Brackett to radio continuum flux are unusual and could indicate the presence of ultra compact HII regions, very low electron temperatures, or widespread departures from Case B recombination. In all galaxies the 10 μm continuum flux exceeds that which can be produced by the Lyman heating of dust. Our conclusions are that conditions in star-formation sources can be quite extreme, and that Brackett lines are flexible and powerful probes of conditions in external galaxies