The circle (in black) with a FWHM radius encompasses the emission peak of the blue- and redshifted outflow. To reduce noise, the outer shape (fat line) of the outflow contours is then used to cut out the outflow inside the circle. This allows to focus only on the actual outflow located inside the circle and reduces the noise that is not part of the outflow. The white circle in the top left indicates the beamsize.