Learning about the Recent Star Formation History of Galaxy Disks by Comparing their Far-Infrared and Radio Morphologies: Cosmic-Ray Electron Diffusion after Star Formation Episodes

TL;DR

This study compares far-infrared and radio images of 29 galaxies to understand cosmic-ray electron diffusion and star formation history, revealing that recent star formation episodes significantly influence cosmic-ray populations and galaxy morphology.

Contribution

It introduces a two-component infrared smoothing method to analyze cosmic-ray electron diffusion and star formation history in galaxies, extending previous approaches.

Findings

Late-type spirals with high star formation benefit from the two-component method.

Disk component dominates in galaxies with low star formation activity.

Recent star formation episodes lead to younger cosmic-ray electron populations.

Abstract

We present results on the interstellar medium (ISM) properties of 29 galaxies based on a comparison of {\it Spitzer} far-infrared and Westerbork Synthesis Radio Telescope radio continuum imagery. Of these 29 galaxies, 18 are close enough to resolve at $\la$1 kpc scales at 70 $\micron$ and 22 cm. We extend the \citet{ejm06a,ejm06b} approach of smoothing infrared images to approximate cosmic-ray (CR) electron spreading and thus largely reproduce the appearance of radio images. Using a wavelet analysis we decompose each 70 $\micron$ image into one component containing the star-forming {\it structures} and a second one for the diffuse {\it disk}. The components are smoothed separately, and their combination compared to a free-free corrected 22 cm radio image; the scale-lengths are then varied to best match the radio and smoothed infrared images. We find that late-type spirals having high amounts of ongoing star formation benefit most from the two-component method. We also find that the disk component dominates for galaxies having low star formation activity, whereas the structure component dominates at high star formation activity. We propose that this result arises from an age effect rather than from differences in CR electron diffusion due to varying ISM parameters. The bulk of the CR electron population in actively star-forming galaxies is significantly younger than that in less active galaxies due to recent episodes of enhanced star formation; these galaxies are observed within $\sim10^{8}$ yr since the onset of the most recent star formation episode. The sample irregulars have anomalously low best-fit scale-lengths for their surface brightnesses compared to the rest of the sample spirals which we attribute to enhanced CR electron escape.