Volume 587, March 2016
|Number of page(s)||7|
|Published online||15 February 2016|
The spatially resolved correlation between [NII] 205 μm line emission and the 24 μm continuum in nearby galaxies
Instituto de Física y Astronomía, Universidad de Valparaíso,
Avda. Gran Bretaña 1111,
2 Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281-S9, 9000 Gent, Belgium
3 Department of Physics & Astronomy, McMaster University, Hamilton, Ontario L8S 4M1, Canada
4 Department of Astronomy, the University of Tokyo, Bunkyo-ku, 113-0033 Tokyo, Japan
5 UK ALMA Regional Centre Node, Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK
6 Laboratoire d’Astrophysique de Marseille, Université d’Aix-Marseille and CNRS, UMR7326, 13388 Marseille Cedex 13, France
7 Institut für theoretische Astrophysik, Zentrum für Astronomie der Universität Heidelberg, Albert-Ueberle 2, 69120 Heidelberg, Germany
8 Department of Physics & Astronomy, University of Sussex, Brighton, BN1 9QH, UK
9 Infrared Processing and Analysis Center, California Institute of Technology, MS 100-22, Pasadena, CA 91125, USA
10 Istituto di Astrofisica e Planetologia Spaziali, INAF-IAPS, Via Fosso del Cavaliere 100, 00133 Roma, Italy
Received: 26 October 2015
Accepted: 30 December 2015
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.
Key words: galaxies: star formation / galaxies: spiral / galaxies: ISM / infrared: galaxies / ISM: lines and bands
© ESO, 2016
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