The galaxy-wide distributions of mean electron density in the HII regions of M51 and NGC 4449

Understand

Using ACS-HST images to yield continuum subtracted photometric maps in H\alpha of the Sbc galaxy M51 and the dwarf irregular galaxy NGC 4449, we produced extensive (over 2000 regions for M51, over 200 regions for NGC4449) catalogues of parameters of their HII regions: their H\alpha luminosities, equivalent radii and coordinates with respect to the galaxy centers.

• From these data we derived, for each region, its mean luminosity weighted electron density, <n_e>, determined from the H\alpha luminosity and the radius, R, of the region.
• Plotting these densities against the radii of the regions we find excellent fits for <n_e> varying as R^{-1/2}.
• This relatively simple relation has not, as far as we know, been predicted from models of HII region structure, and should be useful in constraining future models.