Quantifying non-star formation associated 8um dust emission in NGC 628

TL;DR

This study quantifies the fraction of 8um dust emission in NGC 628 that is heated by older stars rather than recent star formation, revealing that 30-43% of the emission is unrelated to current star-forming regions.

Contribution

The paper introduces a method to separate dust heating sources in NGC 628 and quantifies the non-star formation related 8um dust emission, supported by a simple model and Fourier analysis.

Findings

30-43% of 8um dust emission is from older stars.

8um emission is more diffuse than Hα emission.

The 8um dust-to-Ha ratio decreases with galactocentric radius.

Abstract

Combining Ha and IRAC images of the nearby spiral galaxy NGC 628, we find that between 30-43% of its 8um dust emission is not related to recent star formation. Contributions from dust heated by young stars are separated by identifying HII regions in the Ha map and using these areas as a mask to determine the 8um dust emission that must be due to heating by older stars. Corrections are made for sub-detection-threshold HII regions, photons escaping from HII regions and for young stars not directly associated to HII regions (i.e. 10-100 Myr old stars). A simple model confirms this amount of 8um emission can be expected given dust and PAH absorption cross-sections, a realistic star-formation history, and the observed optical extinction values. A Fourier power spectrum analysis indicates that the 8um dust emission is more diffuse than the Ha emission (and similar to observed HI), supporting our analysis that much of the 8um-emitting dust is heated by older stars. The 8um dust-to-Ha emission ratio declines with galactocentric radius both within and outside of HII regions, probably due to a radial increase in disk transparency. In the course of this work, we have also found that intrinsic diffuse Ha fractions may be lower than previously thought in galaxies, if the differential extinction between HII regions and diffuse regions is taken into account.