Residual images computed by subtracting an elliptical symmetric model obtained by fitting a synthetic image GALFIT, following the same procedure as with NGC 4104 (see Fig. 10). The synthetic image was generated using only the stellar component of simulation S3 at t = 5 Gyr. From left to right: three different projection angles of the collision axis on the plane of the sky, 10°, 20°, and 30°. Middle panel: our preferred projection, where the number and intensity of the shells on the residual image are closer to what we observe on NGC 4104.