AUTOMATIC-DETERMINATION OF UNBIASED LUMINOSITY FUNCTIONS FOR H-II REGIONS - II - 4 NEARBY DWARF GALAXIES

Luminosity functions in Ha have been measured for H II regions in the four dwarf galaxies UGCA 86, UGCA 92, UGCA 105, and UGC 4115 using bot h the traditional flux measurement method of fixed-threshold photometr y (FTP) and the new method, percentage-of-peak photometry (PPP). The U GCA galaxies are members of the IC 342-Maffei 1 group. These two metho ds give significantly different results in galaxies in which significa nt numbers of peaks are associated with PI II region complexes. The wo rk demonstrates that fluxes from FTP can lead to biased luminosity fun ctions and that PPP should, in general, be preferred. It is also shown that PPP luminosity functions are not very sensitive to atmospheric t ransparency, 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 sim ilar 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 su rface brightness of the lower luminosity H II regions grows as the one -third power of the flux grows, exactly as predicted for an ensemble i n which the gas density does not vary systematically with the mass of the star cluster. For the brightest H II regions, however, the relatio n steepens, which implies that the most massive star clusters are form ed out of the densest clouds.