Phase Diagram

A typical phase diagram for a spiral galaxy: the X-axis shows the azimuthal angle in the disk (CCW from the of the major axis) while the Y-axis shows the natural logarithm of the galactocentric distance. Both quantities are corrected for the inclination angle, i.e reduced to the face-on position. The advantage of this graph is that the logarithmic spiral appears as a straight line inclined to the X-axis exactly by the value of a spiral pitch angle. In this diagram for the Virgo Cluster spiral NGC 4254, showing the distribution of polarized intensity and apparent B-vectors (VLA at 6 cm) you can see that:

The galaxy has an azimuthally oriented highly polarized region between azimuths 70 and 150 degrees.
The magnetic field is nearly azimuthal there with some localized jumps.
The radial disk scale length is considerably smaller than on the opposite disk side (a pushed-in disk?).
Between azimuths 240-330 deg. the opposite is true: the disk is more extended and the B-vectors are inclined by some 40-60 deg.

Here is another example of the phase diagram, this time for the interacting galaxy NGC 3627. Again it shows the distribution of polarized intensity and B-vectors (VLA 3.6cm). Red solid lines show the optical dust lanes, tracing the density wave compression. One can see:

The western spiral arm (azimuths 250-330 degs) apparently pushed-in at azimuth of about 270 deg. This is accompanied by a bright polarization spot and some decrease of the pitch angle.
The eastern magnetic spiral arm (azimuths 100-150 deg.), is thought to be "anomalous" as it crosses the optical dust lane. Nevertheless, in contrast to the western arm, the eastern one keeps a constant pitch angle of both the polarization distribution and magnetic pitch angle along its whole length. This phenomenon is currently under an intense investigation.

The purple spot at azimuths 150 - 180 deg. is the region of peculiar HI and CO velocities discussed by Zhang et al. 1993.

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