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

Abstract

A correlation between the 24 µm continuum and the [N] 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 [N] 205−24 µm emission correlation has some physical origin or is merely an artefact of line-ofsight 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 [N] 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 [N] 205 µm line emission in regions with higher star formation rates. Finally, we present a general relation between the [N] 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.

Regulating Star Formation in Nearby Dusty Galaxies: Low Photoelectric Efficiencies in the Most Compact Systems

“…To best match this area, we begin by using [C II] and [O I] line fluxes measured through the central spaxel of the PACS/IFU with dimensions of ∼ 9 4 × 9 4 (See Section 3.2 in Díaz-Santos et al 2017). Next, we project the effective SL slit and PACS central-spaxel aperture onto the Spitzer/IRAC 8 μm images, assuming that it traces the co-spatial PAH and far-IR line emission from star-forming regions in LIRGs, which is reasonable for the 2-7 kpc-scale observations in this work (Peeters et al 2004;Alonso-Herrero et al 2010;Pereira-Santaella et al 2010;Hughes et al 2016;Salgado et al 2016). Smith et al (2007) show the fractional power of PAHs to the IRAC 8 μm band to have a median value of 70% in normal star-forming galaxies, which increases only marginally when a more careful subtraction of stellar continuum is done using the IRAC 3-5 μm channels (e.g., Helou et al 2004).…”

Regulating Star Formation in Nearby Dusty Galaxies: Low Photoelectric Efficiencies in the Most Compact Systems

“…3.2 in Díaz-Santos et al 2017). Next, we project the effective SL slit and PACS central-spaxel aperture onto the Spitzer /IRAC 8 µm images, assuming that it traces the co-spatial PAH and far-IR line emission from star-forming regions in LIRGs, which is reasonable for the 2 − 7 kpc-scale observations in this work (Peeters et al 2004;Alonso-Herrero et al 2010;Pereira-Santaella et al 2010;Salgado et al 2016;Hughes et al 2016). Smith et al (2007) show the fractional power of PAHs to the IRAC 8 µm band to have a median value of 70% in normal star-forming galaxies, which increases only marginally when a more careful subtraction of stellar continuum is done using the IRAC 3−5 µm channels (e.g., Helou et al 2004).…”

Probing the Baryon Cycle of Galaxies with SPICA Mid- and Far-Infrared Observations

“…The ratio of the [N iii ] 57 μm and [O iii ] 52 μm lines is a good measure of the N/O abundance ratio, which is an indicator of the degree of stellar processing of the ISM through gas-phase metallicity (Rubin et al 1988; Nagao et al 2011; Pereira-Santaella et al 2017). The [N ii ] emission at 205 μm has also been advocated as a good tracer of the SFR (Zhao et al 2013; Wu et al 2015; Hughes et al 2016; Herrera-Camus et al 2016), as well as the combined [Ne ii ] + [Ne iii ] line emission (Ho & Keto 2007).…”