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 H₂ IR lines are probably excited predominantly by fluorescence, not supernovadriven 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 4³D-3³P° line at 1.70 μm 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 10⁹ 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. Subject.