What we care about is the \(S_{2(signal)}\). This is found simply by calculating \(S_2\) using only noise data, and subtracting it from \(S_2\) (which is calculated using the overall data).
The above images display (from left to right) the \(T_{max}\) map of the region of M33 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 \(\delta v\)).