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We have used the images from the ACS on HST in Halpha, and in the neighboringcontinuum, to produce flux calibrated images of the large spiral galaxy M51,and the dwarf irregular NGC 4449. From these images we have derived theabsolute luminosities in Halpha, the areas, and the positions with respect tothe galactic centers as reference points, of over 2600 HII regions in M51 andover 270 HII regions in NGC 4449. Using this database we have derivedluminosity (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), whichdiffers from the linear relation expected for regions of constant uniformelectron density. We discuss briefly possible models which would give rise tothis behavior, notably models with strong density inhomogeneities within theregions. Plotting the luminosity functions for the two galaxies we find a breakin the slope for M51 at log(L) = 38.5 dex (units in erg s^(-1)) for M51 in goodagreement with the previous ground-based study by Rand, and above thisluminosity 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 higherluminosities. The cumulative diameter distribution for the HII regions of M51shows dual behaviour, with a break at a radius close to 100 pc, the radius ofregions with the break luminosity. Here too we indicate the possible physicalimplications.

Properties of the HII region populations of M51 and NGC 4449 from Halpha images with ACS on HST