The spatially resolved correlation between [NII] 205 μm line emission and the 24 μm continuum in nearby galaxies

T. M. Hughes, M. Baes, M. R. P. Schirm, T. J. Parkin, R. Wu, I. De Looze, C. D. Wilson, S. Viaene, G. J. Bendo, A. Boselli, D. Cormier, E. Ibar, O. L. Karczewski, N. Lu, L. Spinoglio

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    A correlation between the 24 μm continuum and the [Nii] 205 μm line emission may arise if both quantities trace the star formation activity on spatially-resolved scales within a galaxy, yet has so far only been observed in the nearby edge-on spiral galaxy NGC 891. We therefore assess whether the [Nii] 205−24 μm emission correlation has some physical origin or is merely an artefact of line-of-sight projection effects in an edge-on disc. We search for the presence of a correlation in Herschel and Spitzer observations of two nearby face-on galaxies, M 51 and M 83, and the interacting Antennae galaxies NGC 4038 and 4039. We show that not only is this empirical relationship also observed in face-on galaxies, but also that the correlation appears to be governed by the star formation rate (SFR). Both the nuclear starburst in M 83 and the merger-induced star formation in NGC 4038/9 exhibit less [Nii] emission per unit SFR surface density than the normal star-forming discs. These regions of intense star formation exhibit stronger ionization parameters, as traced by the 70/160 μm far-infrared (FIR) colour. These observations suggest the presence of higher ionization lines that may become more important for gas cooling, thereby reducing the observed [Nii] 205 μm line emission in regions with higher star formation rates. Finally, we present a general relation between the [Nii] 205 μm line flux density and SFR density for normal star-forming galaxies, yet note that future studies should extend this analysis by including observations with wider spatial coverage for a larger sample of galaxies.
    Original languageUndefined
    Article numberA45
    JournalAstronomy & Astrophysics
    Publication statusPublished - 1 Mar 2016


    • galaxies: star formation, galaxies: spiral, galaxies: ISM, infrared: galaxies, ISM: lines and bands

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