ON THE FORM OF THE H-II REGION LUMINOSITY FUNCTION

Observed variations in the H II region luminosity function (H II LF) s een in spiral arm versus interarm regions, and different galactic Hubb le type, can be explained simply by evolutionary effects and maximum n umber of ionizing stars per cluster. We present Monte Carlo simulation s of the H rr LF, drawing the number of ionizing stars (N) from a pow er-law distribution of constant slope, and the stellar masses from a S alpeter initial mass function with an upper mass limit of 100 M-circle dot. We investigate the evolution of the H II LF, as determined by st ellar main-sequence lifetimes and ionizing luminosities, for a single- burst case and continuous creation of the nebular population. Shallowe r H II LF slopes measured for the arms of spiral galaxies can be expla ined as a composite slope, expected for a zero-age burst population, w hereas the interarm regions tend to be dominated by evolved rich clust ers described by a single, steeper slope. Steeper slopes in earlier-ty pe galaxies can be explained simply by a lower maximum N cutoff found for the parent OB associations. The form of the H II LF can reveal fe atures of the most recent (less than or similar to 10 Myr) star format ion history in nearby galaxies.