Astronomers Have Spotted The Enormous Magnetic Field Surrounding A Spiral Galaxy

Edge-on view of the spiral galaxy NGC 4217, showing magnetic field lines (green) extending far above and below the plane of the galaxy. Credit: Y. Stein (CDS), NRAO, SDSS, KPNO 0.9m, J. English (U. Manitoba), R.-J. Dettmar and A. Miskolczi (Ruhr U.), R.J. Rand (U.N.M.), and J. Irwin (Queen’s U.).

An international team of astronomers analyzing the enormous magnetic field that surrounds spiral galaxy NGC 4217 has discovered as yet unknown magnetic field structures. It's unclear how the magnetic field is generated, but the new observations provide insights into where the incredible magnetism comes from.

As reported in Astronomy & Astrophysics, NGC 4217's magnetic field extends 22,500 light-years from the plane of the galaxy, which we observe edge-on. The researchers observed a wealth of shapes, including an X-shaped structure that has been seen in other galaxies, a helical structure, superbubbles, and shells, as well as never before seen large loops in the magnetic field across the entire galaxy.

“It is fascinating that we discover unexpected phenomena in every galaxy whenever we use radio polarisation measurements,” said Dr Rainer Beck from the MPI for Radio Astronomy in Bonn in a statement. “Here in NGC 4217, it is huge magnetic gas bubbles and a helix magnetic field that spirals upwards into the galaxy’s halo.”

The loop structures, however, were unexpected. “This has never been observed before,” added lead author Dr Yelena Stein, from the Centre de Données Astronomiques de Strasbourg. “We suspect that the structures are caused by star formation, because at these points matter is ejected outward.”

The superbubbles are believed to originate where several supernovae have exploded, ejecting their plasma into and around the galaxy. But the team state that the role of regular stars releasing their stream of charged particles, known as the stellar wind, also plays a role in these structures. Their research suggests that the motion of plasma within the disk of the galaxy could act as a dynamo and support the vast magnetic field.

The image (above) of NGC 4217 shows the extent of the huge galaxy-wide magnetic field. This galaxy is located 67 million light-years from us and is similar to our own despite what the edge-on view might suggest. The Milky Way may have something similar going on, and the researchers hope their findings may be applicable to our galaxy.

“This image clearly shows that when we think of galaxies like the Milky Way, we should not forget that they have galaxy-wide magnetic fields,” Dr Stein said.

The image is a mixture of optical observations from the Sloan Digital Sky Survey and radio observations from the National Radio Astronomy Observatory. Using radio waves, it is possible to determine the intensity and direction of magnetic fields, which allowed for this kind of work.  

“Visualising the data was important to me because when you think about galaxies, magnetic fields are not the first thing that comes to mind, although they can be gigantic and display unique structures," explained Stein. "The image is supposed to shift the magnetic fields more into focus.”

Although it is still unclear how magnetic fields are generated and maintained, and the work is speculative about what causes the large vertical extensions seen here, the researchers hope further analysis will answer some of these questions.  

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