The spatially-resolved correlation between [NII] 205 {\mu}m line emission and the 24 {\mu}m continuum in nearby galaxies

TL;DR

This study investigates the correlation between [NII] 205 μm line emission and 24 μm continuum in nearby galaxies, revealing it is linked to star formation rate and influenced by ionization conditions, across different galaxy orientations.

Contribution

It demonstrates that the [NII] 205 μm and 24 μm emission correlation exists in face-on galaxies and is governed by star formation rate, extending previous findings beyond edge-on galaxies.

Findings

Correlation observed in face-on galaxies, not just edge-on.

Star formation rate influences the [NII] and 24 μm emission relationship.

Regions with intense star formation show higher ionization parameters.

Abstract

A correlation between the 24 {\mu}m continuum and the [NII] 205 {\mu}m 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 {\mu}m 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, M51 and M83, 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 M83 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 {\mu}m far-infrared colour, that suggest the presence of higher ionization lines that may become more important for gas cooling, thereby reducing the observed [NII] 205 {\mu}m line emission in regions with higher star formation rates. Finally, we present a general relation between the [NII] 205 {\mu}m 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.