Article

# On the Form of the HII Region Luminosity Function

01/1998;
Source: arXiv

ABSTRACT Observed variations in the HII 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_sol. 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.

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### Keywords

continuous creation

different galactic Hubble type

evolutionary effects

HII LF

HII region luminosity function

interarm regions

ionizing luminosities

ionizing stars

ionizing stars N_*

lower maximum N_* cutoff

Observed variations

parent OB associations

power-law distribution

rich clusters

Salpeter IMF

Shallower HII LF slopes

single burst case

spiral galaxies

steeper slope

zero-age burst population