J/MNRAS/547/G480ivo://CDS.VizieR/J/MNRAS/547/G480CDSZurita A.Bresolin F.Florido E.Verley S.Relano M.Beckman J.E.2026-04-29T08:40:31Z2026-04-29T09:30:30ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.frh-ii-regionschemical-abundancesvisible-astronomyMeasurements 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 <n_e_>_rms_, the in-situ density (n_e_) 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 <n_e_>_rms_ from their Halpha luminosities and equivalent radii (R_eq_), and obtain n_e and phi for spectroscopic subsamples. While n_e_ is below 300cm^-3^, <n_e_>_rms_ is typically one to two orders of magnitude lower, implying that phi is in the range 0.0001-0.1. The two galaxies exhibit a similar size-density relation, <n_e_>_rms_{prop.to}R_eq_^-0.3^, which breaks for R_eq_>50pc, show at most a weak dependence of <n_e_>_rms_ on galactocentric radius for NGC 2403, and no clear dependence of n_e_ or phi on these parameters. Combining these results with published data, <n_e_>_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.2026MNRAS.547ag480Zhttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/MNRAS/547/G480CatalogResearchIsServedByTAP VizieR generic serviceIsServedByConesearch servicehttps://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/MNRAS/547/G480https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/MNRAS/547/G480http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/MNRAS/547/G480https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/MNRAS/547/G480https://vizier.iucaa.in/viz-bin/votable?-source=J/MNRAS/547/G480http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/MNRAS/547/G480GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapCone search capability for table J/MNRAS/547/G480/n2403hii (Catalogue of HII regions in NGC 2403)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/MNRAS/547/G480/n2403hii?https://vizier.iucaa.in/viz-bin/conesearch/J/MNRAS/547/G480/n2403hii?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/MNRAS/547/G480/n2403hii?GETtext/xml+votable180.050000true114.4307111165.478410.005555555555555556Cone search capability for table J/MNRAS/547/G480/n628hii (Catalogue of HII regions in NGC 628)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/MNRAS/547/G480/n628hii?https://vizier.iucaa.in/viz-bin/conesearch/J/MNRAS/547/G480/n628hii?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/MNRAS/547/G480/n628hii?GETtext/xml+votable180.050000true24.2846161115.651366940.005555555555555556Cone search capability for table J/MNRAS/547/G480/n2403lf (Reddening-corrected emission-line fluxes for HII regions in NGC 2403 (CAHA/CAFOS observations))https://vizier.cds.unistra.fr/viz-bin/conesearch/J/MNRAS/547/G480/n2403lf?https://vizier.iucaa.in/viz-bin/conesearch/J/MNRAS/547/G480/n2403lf?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/MNRAS/547/G480/n2403lf?GETtext/xml+votable180.050000true114.3617919465.625811110.0055555555555555566/7741 7743 18398 18420https://cdsarc.cds.unistra.fr/viz-bin/moc/J/MNRAS/547/G480?format=asciiOpticaldefaultJ/MNRAS/547/G480/n2403hiiCatalogue of HII regions in NGC 2403recnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintHIIHII region identifier (1)meta.id;meta.mainintRAJ2000Right ascension (J2000) (2)degpos.eq.ra;meta.mainDEJ2000Declination (J2000) (2)degpos.eq.dec;meta.mainlogLHaLog10 H-alpha luminosity (erg/s) with local background subtractionlog(1e-07W)phys.luminosity;em.line.Halphafloate_logLHaUncertainty in logLHalog(1e-07W)stat.errorfloatlogLHanlbsLog10 H-alpha luminosity (erg/s) without local background subtractionlog(1e-07W)phys.luminosity;em.line.Halphafloate_logLHanlbsUncertainty in logLHa_nlbslog(1e-07W)stat.errorfloatAreaArea of regionarcsec**2phys.area;obs.fieldfloatasigmaSemimajor axis (3)arcsecphys.angSize;srcfloatbsigmaSemiminor axis (3)arcsecphys.angSize.sminAxisfloatPAPosition angle of major axis (N->E)degpos.posAngfloatReqEquivalent radius (4)arcsecphys.angSize;srcfloatReqpcEquivalent radius in parsecs (4)pcphys.size.radiusfloatJ/MNRAS/547/G480/n628hiiCatalogue of HII regions in NGC 628recnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintHIIHII region identifier (1)meta.id;meta.mainintRAJ2000Right ascension (J2000) (2)degpos.eq.ra;meta.mainDEJ2000Declination (J2000) (2)degpos.eq.dec;meta.mainlogLHaLog10 H-alpha luminosity (erg/s) with local background subtractionlog(1e-07W)phys.luminosity;em.line.Halphafloate_logLHaUncertainty in logLHalog(1e-07W)stat.errorfloatlogLHanlbsLog10 H-alpha luminosity (erg/s) without local background subtractionlog(1e-07W)phys.luminosity;em.line.Halphafloate_logLHanlbsUncertainty in logLHa_nlbslog(1e-07W)stat.errorfloatAreaArea of regionarcsec**2phys.area;obs.fieldfloatasigmaSemimajor axis (3)arcsecphys.angSize;srcfloatbsigmaSemiminor axis (3)arcsecphys.angSize.sminAxisfloatPAPosition angle of major axis (N->E)degpos.posAngfloatReqEquivalent radius (4)arcsecphys.angSize;srcfloatReqpcEquivalent radius in parsecs (4)pcphys.size.radiusfloatJ/MNRAS/547/G480/n2403lfReddening-corrected emission-line fluxes for HII regions in NGC 2403 (CAHA/CAFOS observations)recnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintIDHII region identifier (sN.N)meta.id;meta.maincharF3727? Reddening-corrected [OII] (3727{AA}) flux (1)phot.fluxfloatnullablee_F3727? Uncertainty in [OII]stat.errorfloatnullableF3970? Reddening-corrected Hepsilon (3970{AA}) flux (1)phot.fluxfloatnullablee_F3970? Uncertainty in Hepsilonstat.errorfloatnullableF4101? Reddening-corrected Hdelta (4101{AA}) flux (1)phot.fluxfloatnullablee_F4101? Uncertainty in Hdeltastat.errorfloatnullableF4340? Reddening-corrected Hgamma (4340{AA}) flux (1)phot.fluxfloatnullablee_F4340? Uncertainty in Hgammastat.errorfloatnullableF4363? Reddening-corrected [OIII] (4363{AA}) flux (1)phot.fluxfloatnullablee_F4363? Uncertainty in [OIII]stat.errorfloatnullableF4861Reddening-corrected Hbeta (4861{AA}) flux (1)phot.fluxfloate_F4861Uncertainty in Hbetastat.errorfloatF4959? Reddening-corrected [OIII] (4959{AA}) flux (1)phot.fluxfloatnullablee_F4959? Uncertainty in [OIII]stat.errorfloatnullableF5007Reddening-corrected [OIII] (5007{AA}) flux (1)phot.fluxfloate_F5007Uncertainty in [OIII]stat.errorfloatF5755? Reddening-corrected [NII] (5755{AA}) flux (1)phot.fluxfloatnullablee_F5755? Uncertainty in [NII]stat.errorfloatnullableF5876? Reddening-corrected HeI (5876{AA}) flux (1)phot.fluxfloatnullablee_F5876? Uncertainty in HeIstat.errorfloatnullableF6300? Reddening-corrected [OI] (6300{AA}) flux (1)phot.fluxfloatnullablee_F6300? Uncertainty in [OI]stat.errorfloatnullableF6312? Reddening-corrected [SIII] (6312{AA}) flux (1)phot.fluxfloatnullablee_F6312? Uncertainty in [SIII]stat.errorfloatnullableF6364? Reddening-corrected [OI] (6364{AA}) flux (1)phot.fluxfloatnullablee_F6364? Uncertainty in [OI]stat.errorfloatnullableF6548? Reddening-corrected [NII] (6548{AA}) flux (1)phot.fluxfloatnullablee_F6548? Uncertainty in [NII]stat.errorfloatnullableF6563Reddening-corrected Halpha (6563{AA}) flux (1)phot.fluxfloate_F6563Uncertainty in Halphastat.errorfloatF6583Reddening-corrected [NII] (6583{AA}) flux (1)phot.fluxfloate_F6583Uncertainty in [NII]stat.errorfloatF6678? Reddening-corrected HeI (6678{AA}) flux (1)phot.fluxfloatnullablee_F6678? Uncertainty in HeIstat.errorfloatnullableF6716Reddening-corrected [SII] (6716{AA}) flux (1)phot.fluxfloate_F6716Uncertainty in [SII]stat.errorfloatF6731Reddening-corrected [SII] (6731{AA}) flux (1)phot.fluxfloate_F6731Uncertainty in [SII]stat.errorfloatF7065? Reddening-corrected HeI (7065{AA}) flux (1)phot.fluxfloatnullablee_F7065? Uncertainty in HeIstat.errorfloatnullableF7136? Reddening-corrected [ArIII] (7136{AA}) flux (1)phot.fluxfloatnullablee_F7136? Uncertainty in [ArIII]stat.errorfloatnullableF7320? Reddening-corrected [OII] (7320{AA}) flux (1)phot.fluxfloatnullablee_F7320? Uncertainty in [OII]stat.errorfloatnullableF7330? Reddening-corrected [OII] (7330{AA}) flux (1)phot.fluxfloatnullablee_F7330? Uncertainty in [OII]stat.errorfloatnullableRAJ2000Right ascension (J2000)degpos.eq.ra;meta.mainDEJ2000Declination (J2000)degpos.eq.dec;meta.mainEWabsequivalent width of the stellar absorption0.1nmspect.line.eqWidthfloatcHbExtinction coefficient at Hbetaphys.absorption.coefffloate_cHbUncertainty in cHbstat.errorfloatFHbExtinction-corrected Hbeta flux (in 10^-15^erg/s/cm^2^)1e-18W.m**-2phot.fluxfloate_FHbUncertainty in FHb (in 10^-15^erg/s/cm^2^)1e-18W.m**-2stat.errorfloatApAperturearcsecphys.angSize;instr.fovfloat