next up previous contents
Next: 10.3 Corrections to Non-Photometric Up: 10.2 Corrections to the Previous: 10.2.2 Correction for the

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 [*].

  \begin{figure}% latex2html id marker 22976\makebox[\textwidth]{
...he shown observation is 1.8 arcsec,
i.e.\ log($r$ /arcsec) = 0.25.

Properties of E and S0 Galaxies in the Clusters HydraI and Coma
Master's Thesis, University of Copenhagen, July 1997

Bo Milvang-Jensen (