The history of star formation in II Zw 40

We present new spectra and images in the near-infrared and in the optical of the dwarf galaxy II Zw 40. We find that the H-2 IR lines are probably excited predominantly by fluorescence, not supernova-driven shocks, supporting the evidence from the radio that this galaxy has a very small number of supernova remnants. The [Fe II] emission may be driven by shocks, but the faintness of the [Fe II] lines also supports the evidence for few supernovae. We show that the He I 4 D-3-3 P-3(o) line at 1.70 mu m can be used to place a lower limit on the temperatures of the ionizing stars. This line should be very useful as a temperature indicator for the stars in dusty H II regions and starbursts. The observations along with data from the literature are used to constrain starburst models. We find that the current activity results from a burst of star formation that began no more than 4 million years ago. There is evidence against any similar events occurring in this galaxy for at least the past 10(9) years. We detect an old stellar population in the outer part of the galaxy; we estimate that the mass of stars that initially formed in the galaxy exceeded the mass of the present starburst by an order of magnitude.