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Description
Luminosity functions in H{alpha} have been measured for H II regions in the four dwarf galaxies UGCA 86, UGCA 92, UGCA 105, and UGC 4115 using both the traditional flux measurement method of fixed-threshold photometry (FTP) and the new method, percentage-of-peak photometry (PPP). The UGCA galaxies are members of the IC 342-Maffei 1 group. These two methods give significantly different results in galaxies in which significant numbers of peaks are associated with H II region complexes. The work demonstrates that fluxes from FTP can lead to biased luminosity functions and that PPP should, in general, be preferred. It is also shown that PPP luminosity functions are not very sensitive to atmospheric transparency, whereas those constructed from FTP can be, based on data taken under different atmospheric conditions. Results to date for six galaxies show that spirals and dwarfs have luminosity functions of similar shape, which implies that the distribution of the masses of star formation sites is largely independent of the mass, and by implication, the metallicity, of the host galaxy. As measured by PPP, the mean surface brightness of the lower luminosity H II regions grows as the one-third power of the flux grows, exactly as predicted for an ensemble in which the gas density does not vary systematically with the mass of the star cluster. For the brightest H II regions, however, the relation steepens, which implies that the most massive star clusters are formed out of the densest clouds.
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