The relations among 8, 24, and 160 micron dust emission within nearby spiral galaxies

TL;DR

This study explores the complex relationships among 8, 24, and 160 micron dust emissions in nearby spiral galaxies, revealing spatial variations and correlations that inform our understanding of interstellar dust and radiation fields.

Contribution

It provides new insights into how PAH and dust emissions vary spatially and relate to each other within spiral galaxies, highlighting the association with diffuse cold dust and radiation field effects.

Findings

PAH 8 micron emission correlates with 160 micron emission on ~2 kpc scales.

The (PAH 8 micron)/24 micron ratio varies significantly across regions.

PAH emission is linked to diffuse, cold dust heated by the interstellar radiation field.

Abstract

We investigate the relations among the stellar continuum-subtracted 8 micron polycyclic aromatic hydrocarbon (PAH 8 micron) emission, 24 micron hot dust emission, and 160 micron cold dust emission in fifteen nearby face-on spiral galaxies in the Spitzer Infrared Nearby Galaxies Survey sample. The relation between PAH 8 and 24 micron emission measured in ~2 kpc regions is found to exhibit a significant amount of scatter, and strong spatial variations are observed in the (PAH 8 micron)/24 micron surface brightness ratio. In particular, the (PAH 8 micron)/24 micron surface brightness ratio is observed to be high in the diffuse interstellar medium and low in bright star-forming regions and other locations with high 24 micron surface brightness. PAH 8 micron emission is found to be well-correlated with 160 micron emission on spatial scales of ~2 kpc, and the (PAH 8 micron)/160 micron surface brightness ratio is generally observed to increase as the 160 micron surface brightness increases. These results suggest that the PAHs are associated with the diffuse, cold dust that produces most of the 160 micron emission in these galaxies, and the variations in the (PAH 8 micron)/160 micron ratio may generally be indicative of either the intensity or the spectrum of the interstellar radiation field that is heating both the PAHs and the diffuse interstellar dust.