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