Astronomical Observatory ZRiFK

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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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