T-T plot between the radio–free-free SFR map and the F13 map with the central area masked. The red contours on the top two maps highlight the regions with S/N > 3 in our SFR map (top-left panel), and therefore the pixels that have been considered in this analysis. The plot shows a good correlation between the two maps in the ring region with a correlation parameter of r_ring = 0.75.