Abstract
With ACS-HST images we produced continuum subtracted photometric maps in Hα for the galaxies M51 and NGC 4449. From the Hα luminosity (L), equivalent radius (R) and coordinates of the HII regions with respect to the galaxy centers (over 2000 regions for M51, over 200 regions for NGC 4449), we derived the mean luminosity weighted electron density, <n_e>, for each region.
