Example spectral index map . Each pixel value corresponds to the slope in Fig. 1 in that pixel for a single combined low-frequency emission component (using a single component to represent the “Free-free”, “Synchrotron”, and “Spinning dust” emission types indicated in Fig. 1. In reality, each pixel is the sum of multiple slightly different spectral behaviours from different clouds of particles behind one another, but we instead work with a single compromise value for the dominating emission in the given direction. The mixing maps of this paper, , are taken to be proportional to , where each component k has a different β-map. This figure is reproduced directly from Planck Collaboration X (2015).