Electron densities and filling factors of extragalactic HII regions: NGC 2403 and NGC 628

Abstract

Measurements of the electron density of populations of extragalactic HII regions in nearby galaxies remain limited, despite the relevance of this quantity for characterizing the porosity of the interstellar medium and the escape of the ionizing radiation. We initiated a project aimed at analyzing the root-mean-square electron density ne_rms, the in-situ density ne and the volume filling factor (phi) of extragalactic HII regions, investigating the dependence of these attributes on nebular and host galaxy properties. We present an image-segmentation methodology for constructing homogeneous HII region catalogues, and apply it to two pilot galaxies: NGC 2403 and NGC 628. We derive ne_rms from their Halpha luminosities and equivalent radii (R_eq), and obtain ne and phi for spectroscopic subsamples. While ne is below 300 cm$^{-3}$, ne_rms is typically one to two orders of magnitude lower, implying that phi is in the range ~$10^{-4}$ to $10^{-1}$. The two galaxies exhibit a similar size-density relation, which breaks for R_eq >~ 50 pc, show at most a weak dependence of ne_rms on galactocentric radius for NGC 2403, and no clear dependence of ne or phi on these parameters. Combining these results with published data, ne_rms presents tentative scaling relations with the median HII region size, the fraction of large regions in the parent galaxy, and the star formation rate surface density. These trends, if confirmed, would provide new constraints for massive cluster formation models and important clues for interpreting dependencies observed at high redshift, underscoring the necessity of consistently extending this analysis to larger samples.

Figures (16)

• Figure 1: Continuum-subtracted α H$\alpha$ image of NGC 2403 (see Sect. \ref{['sec:halpha_im']}). The H ii regions of NGC 2403 for which optical spectroscopy has been obtained in this work are marked with orange circles, together with the corresponding identification (see Sect. \ref{['sec:spec']}). H ii regions for which emission-line fluxes are available from previous work are also indicated: blue diamonds for data from the CHAOS collaboration Rogers21, and green squares for data collected in PaperI from several authors, as indicated in Sect. \ref{['sec:compiled_data']}. The colour bar shows the α H$\alpha$ surface brightness (in units of erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$).
• Figure 2: Same as Fig. \ref{['fig:finderN2403']} for NGC 628.
• Figure 3: Flow chart diagram illustrating the method used to create a map of segments or structures in the input α H$\alpha$ image from which the H ii region catalogue is derived. See Sect \ref{['sec:cat_method']} for details. We include the name of the algorithms in the photutils package used to carry out the different steps.
• Figure 4: Portion of NGC 2403 showing images employed in the different stages of the H ii region cataloguing process. (a) and (e) show the α H$\alpha$ continuum-subtracted image (with degraded resolution), with and without background subtraction, respectively (Sect. \ref{['background']}). (b) and (f) are segmentation maps before and after the deblending procedure, respectively (Sect. \ref{['segmentation']}). (c) and (g) are maps of the _Hα $\Sigma_{H\alpha}$ gradient from the original and degraded α H$\alpha$ image (Sect. \ref{['background']}), and (d) and (h) are images used to evaluate the definition of the segments and the corresponding deblending, an image of the continuum emission next to the α H$\alpha$ emission line (d), and a FUV/$GALEX$ image of the same zone of the galaxy (h).
• Figure 5: Differential H ii region luminosity functions for NGC 2403 (top) and NGC 628 (bottom). The dotted line shows a power-law fit to the data points lying above the completeness limit, that yields similar indices for the two galaxies ($\eta=1.68\pm0.08$ for NGC 2403, $\eta=1.7\pm0.1$ for NGC 628). The full lines represent fits to a double power law (see text for details).