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The HII region luminosity function (LF) is an important tool for deriving thebirthrates and mass distribution of OB associations, and is an excellent tracerof the newly formed massive stars and associations. To date, extensive work(predominantly in H{\alpha}) has been done from the ground, which is hinderedby dust extinction and the severe blending of adjacent (spatially or inprojection) HII regions. Reliably measuring the properties of HII regionsrequires a linear resolution <40 pc, but analyses satisfying this requirementhave been done only in a handful of galaxies, so far. As the first space-basedwork using a galaxy sample, we have selected 12 galaxies from our HST NICMOSPa{\alpha} survey and studied the luminosity function and size distribution ofHII regions both in individual galaxies and cumulatively, using a virtuallyextinction-free tracer of the ionizing photon rate. The high angular resolutionand low sensitivity to diffuse emission of NICMOS also offer an advantage overground-based imaging by enabling a higher degree of de-blending of the HIIregions. We do not confirm the broken power-law LFs found in ground-basedstudies. Instead, we find that the LFs, both individual and co-added, follow asingle power law dN(L)/dlnL ~ L^-1, consistent with the mass function of starclusters in nearby galaxies, and in agreement with the results of the existinganalyses with HST data. The individual and co-added size distribution of HIIregions are both roughly consistent with dN(D)/dlnD ~ D^-3, but the power-lawscaling is probably contaminated by blended regions or complexes.

The Pa{\alpha} Luminosity Function of HII Regions in Nearby Galaxies from HST/NICMOS