Analysis of Radio Spectral Indices in the CHANG-ES Galaxy Sample
The spectral index of a galaxy provides insight on the processes occurring within the galaxy, and can give hints to how the cosmic ray electrons propagate through the galaxy. Band-to-band spectral index maps provide a method for seeing features within a galaxy that are not visible in the in-band spectral index maps due to the increased signal to noise ratio. With data from the CHANG-ES project 126 observed band-to-band spectral index maps have been made from the 35 galaxies in the sample. These maps are created by taking advantage of the matching resolutions of the C-array L-band and D-array C-band data sets, as well as the B-array L-band and C-array C-band sets. The band-to-band spectral index maps assume a straight line in log space from the L-band to the C-band, thus increasing the signal to noise ratio in the maps but ignoring any curvature within the bands. Further, using the method by Vargas et al. (2018), thermal prediction and non-thermal band-to-band spectral index maps have been created for 22 of the 35 galaxies in the CHANG-ES sample. This is the largest set of spatially resolved maps of its kind. From this set of maps we have concluded that the spectral index steepens towards the edges of a galaxy as the cosmic rays propagate away from their source and experience energy losses. We find that nearly all of the galaxies have a mean non-thermal spectral index that falls within the observed range of -0.5 to -1.1. We have also produced some of the clearest images of the jets in NGC 660 that have ever been seen. These jets are not even visible in the total intensity radio maps. The maps produced for this thesis will be released to the public for further analysis in an upcoming data release by the CHANG-ES collaboration.
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