CHANG-ES X: Spatially-resolved Separation of Thermal Contribution from Radio Continuum Emission in Edge-on Galaxies

TL;DR

This study develops and applies spatially-resolved methods to separate thermal and non-thermal radio emissions in edge-on galaxies, revealing spectral index steepening and the impact of dust extinction on infrared-based SFR calibrations.

Contribution

It introduces a new calibration for correcting Hα luminosity for dust in edge-on or dusty galaxies and demonstrates the spatially-resolved separation of thermal radio emission using combined IR and Hα data.

Findings

Non-thermal spectral index steepens with height above the galactic disk.

Edge-on galaxies show lower IR flux ratios due to dust extinction.

New dust correction calibration for Hα luminosity in dusty galaxies.

Abstract

We analyze the application of star formation rate (SFR) calibrations using H$\alpha$ and 22 micron infrared imaging data in predicting the thermal radio component for a test sample of 3 edge-on galaxies (NGC 891, NGC 3044, and NGC 4631) in the Continuum Halos in Nearby Galaxies -- an EVLA Survey (CHANG-ES). We use a mixture of H$\alpha$ and 24 micron calibration from Calzetti et al. (2007), and a linear 22 micron only calibration from Jarrett et al. (2013) on the test sample. We apply these relations on a pixel-to-pixel basis to create thermal prediction maps in the two CHANG-ES bands: L- and C-band (1.5 GHz and 6.0 GHz, respectively). We analyze the resulting non-thermal spectral index maps, and find a characteristic steepening of the non-thermal spectral index with vertical distance from the disk after application of all methods. We find possible evidence of extinction in the 22 micron data as compared to 70 micron Spitzer Multband Imaging Photometer (MIPS) imaging in NGC 891. We analyze a larger sample of edge-on and face-on galaxy 25 micron to 100 micron flux ratios, and find that the ratios for edge-ons are systematically lower by a factor of 1.36, a result we attribute to excess extinction in the mid-IR in edge-ons. We introduce a new calibration for correcting the H$\alpha$ luminosity for dust when galaxies are edge-on or very dusty.