By Stacy Palen
Over the last year or so, there have been a number of extraordinary images of astronomical objects with an overlay of magnetic fields in the news. One of these, from the Event Horizon Telescope, has caught extra attention, but Sofia’s HAWC+ imager has also been capturing polarization in the far-infrared. And, of course, there are existing famous images, such as the one taken by Planck that maps the magnetic field of the Milky Way.
When I showed these images to my students, I found that I needed to spend some time explaining how to interpret them.
Here are the points that I needed to make explicit to them:
- Places without magnetic fields shown may just be places with no data. The magnetic field is not necessarily zero in those regions.
- The “streamlines” are along the direction of the magnetic field; they are not, for example, contour lines connecting places where the magnetic field strength is constant. These streamlines neglect any component of the magnetic field that is towards or away from the observer. This component cannot be measured using polarization studies of this kind. There are several metaphors that you could use here to help students distinguish these directions. For example, you might reference proper motion versus radial motion; or, you might reference radial velocities from the discussion of exoplanets.
- Places where the streamlines are close together indicate a stronger magnetic field than places where streamlines are farther apart.
- The colors of streamlines are often meaningless. They are chosen to provide contrast with the background image, and also to look pretty.
- Magnetic field lines often parallel flows of material, but not always. For example, in a galaxy, the magnetic field tends to be parallel to the bipolar outflows, but, in a star-forming region, they may be perpendicular to the direction that the infalling material is moving. In brief, this is because, sometimes, the magnetic field is directing the material, and, sometimes, the material is dragging the magnetic field. Untangling the interactions between magnetic fields and the movement of material is the main reason that these kinds of images are interesting to astronomers.
Even physicists have difficulty imagining what magnetic fields look like and how they are distributed, so it is helpful to have these extraordinary images with the magnetic fields in overlay. If we remember to slow down and explain how to interpret these invaluable images in class, we can help our students understand what they are seeing so much better.