What we care about is the \(S_{2(signal)}\), i.e. the structure function from signal only. This is found simply by calculating \(S_2\) from noise only, and subtracting it from \(S_2\) overall.
The above image displays (from left to right) the \(T_{max}\) map of the region of M51 that we’re looking at (at speeds from \(200 km/s\) to \(400 km/s\) ), a 2D map of \(S_{2(signal)}\) where \(\delta v=0\), and a 1D plot of azimuthally-averaged \(S_{2(signal)}\) versus radius (for various values of \(\delta v\)).