Properties of the HII region populations of M51 and NGC 4449 from Halpha
  images with ACS on HST
  We have used the images from the ACS on HST in Halpha, and in the neighboring
continuum, to produce flux calibrated images of the large spiral galaxy M51,
and the dwarf irregular NGC 4449. From these images we have derived the
absolute luminosities in Halpha, the areas, and the positions with respect to
the galactic centers as reference points, of over 2600 HII regions in M51 and
over 270 HII regions in NGC 4449. Using this database we have derived
luminosity (L)--volume (V) relations for the regions in the two galaxies,
showing that within the error limits these obey the equation L ~ V^(2/3), which
differs from the linear relation expected for regions of constant uniform
electron density. We discuss briefly possible models which would give rise to
this behavior, notably models with strong density inhomogeneities within the
regions. Plotting the luminosity functions for the two galaxies we find a break
in the slope for M51 at log(L) = 38.5 dex (units in erg s^(-1)) for M51 in good
agreement with the previous ground-based study by Rand, and above this
luminosity NGC 4449 also shows a sharp decline in its luminosity function,
although the number of regions is too small to plot the function well at higher
luminosities. The cumulative diameter distribution for the HII regions of M51
shows dual behaviour, with a break at a radius close to 100 pc, the radius of
regions with the break luminosity. Here too we indicate the possible physical
implications.
