10.2.3 PA-profiles that Cross $0^\circ$ or $180^\circ$

The position angles determined from galmag are in the range $0^\circ$-$180^\circ$. They do not go all the way to $360^\circ$, since the major axis of a galaxy does not have a positive and a negative direction, as opposed to for example a coordinate axis.

If a galaxy has an isophote twist, i.e. the position angle changes with the radius r, the position angle might cross the upper or lower limit of $180^\circ$ or $0^\circ$, respectively, and thereby appearing discontinuous. This is in a way a cosmetical problem. Since the position angle is only determined to within a factor of $180^\circ$, we can add or subtract $180^\circ$ at certain radii to make the profile look continuous. This was done using the task pa1shift. An example of this is shown in Figure .