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

TL;DR

This study analyzes the distribution of mean electron densities in HII regions of M51 and NGC 4449, revealing a R^{-1/2} relation with region size and exponential decline with galactocentric radius, suggesting pressure equilibrium with surroundings.

Contribution

It introduces a new empirical relation <n_e> ∝ R^{-1/2} for HII regions and compares their density profiles with galaxy structures, providing constraints for future models.

Findings

<n_e> scales as R^{-1/2}

Electron densities decline exponentially with galactocentric radius

HII regions are in pressure equilibrium with their surroundings

Abstract

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. Plotting the densities against the galactocentric radii, r, of the regions we find good exponential fits, with scale lengths of close to 10 kpc for both galaxies. These values are comparable to the scale lengths of the HI column densities for both galaxies, although their optical structures, related to their stellar components are very different. This result indicates that to a first approximation the HII regions can be considered in pressure equilibrium with their surroundings. We also plot the electron density of the HII regions across the spiral arms of M51, showing an envelope which peaks along the ridge lines of the arms.