BRACKETT LINE SPECTROSCOPY OF BURSTS OF STAR FORMATION IN THE NUCLEI OF GALAXIES

We have observed Brα (4.05 μm) and Brγ (2.17 μm) lines with 7″.2 spatial and 350 km s-1 spectral resolution in seven infrared-bright galaxies. From these measurements, we derive the Lyman continuum flux and the luminosity resulting from young stars. The usual problems of extinction and contamination by nonthermal emission are avoided. The contributions of OB stars to the total luminosities of these galaxies can be estimated and range from insignificant (in NGC 3079) to dominant (NGC 1614 and others). The ratios of Brackett to total infrared luminosity are used to deduce the initial mass function of the star formation activity and thus an age for the star formation process. The activity in the program galaxies can be truly described as bursts because they are surprisingly concentrated in both space and time. Most of the infrared emission appears to be generated in an area on the order of the beam size, that is, in the inner few hundred parsecs of the nuclear region. In several of the galaxies, the star-forming regions may be as young as a few million years. Extremely young starbursts may also be indicated by galaxies where many H II regions appear to be optically thick in the radio continuum. The observed ionization is insufficient to produce the observed 10 μm flux by Lyman heating of dust. Heating by nonionizing photons or nonradiative mechanisms such as shocks may be important.