Detalles de publicación
PP 026034
Electron densities and filling factors of extragalactic Hii regions: NGC 2403 and NGC 628
1Departamento de Física Teórica y del Cosmos, Campus de Fuentenueva, Edificio Mecenas, Universidad de Granada, E18071–Granada, Spain
2Instituto Carlos I de Física Teórica y Computacional, Facultad de Ciencias, E18071–Granada, Spain
3Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
4Instituto de Astrofísica de Canarias, E38205–La Laguna, Tenerife, Spain
5Departamento de Astrofísica, Universidad de La Laguna, Tenerife, Spain
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.
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.