On the Form of the H II Region Luminosity Function
M.S. Oey & C.J. Clarke
Observed variations in the H II region luminosity function (HII LF) seen
in spiral arm vs. interarm regions, and different galactic Hubble
type, can be explained simply by evolutionary effects and maximum
number of ionizing stars per cluster.
We present Monte Carlo simulations of the HII LF, drawing the number of
ionizing stars N_* from a
power-law distribution of constant slope, and the stellar masses from a
Salpeter IMF with an upper-mass limit of 100 M_sun. We
investigate the evolution of the HII LF, as determined by stellar
main-sequence lifetimes and ionizing luminosities, for a single burst
case and continuous creation of the nebular population. Shallower
HII LF slopes measured for the arms
of spiral galaxies can be explained as a composite slope, expected for
a zero-age burst population, whereas the interarm regions tend to be
dominated by evolved rich clusters described by a single, steeper
slope. Steeper slopes in earlier-type galaxies can be explained
simply by a lower maximum N_* cutoff found for the parent OB
associations. The form of the HII LF can reveal features of the most recent
(~< 10 Myr) star formation history in nearby galaxies.
AJ, 115, 1543
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