PROPERTIES OF THE HII REGION POPULATIONS OF M51 AND NGC 4449 FROM Ha IMAGES WITH THE ADVANCED CAMERA FOR SURVEYS ON THE HUBBLE SPACE TELESCOPE

We have used images from the Advanced Camera for Surveys on the Hubble Space Telescope in H alpha, 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 H alpha, 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-volume (L-V) relations for the regions in the two galaxies, showing that within the error limits these obey the equation L similar to V-2/3, which differs from the linear relation expected for regions of constant uniform electron density. We briefly discuss 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 behavior, with a break at a radius close to 100 pc, the radius of regions with the break luminosity. Here too we indicate possible physical implications.