New cosmic magnetic field structures discovered in galaxy NGC 4217

Spiral galaxies like our Milky Way Galaxy may have expanding magnetic fields. There are several theories about its formation, but until now the process is not well understood. An international research team has analyzed the magnetic field of the Milky Way-like galaxy NGC 4217 in detail based on radio astronomical observations and has discovered as yet unknown magnetic field structures. The data suggests that star formation and star bursts, called supernovae, are responsible for the visible structures.

The team led by Dr. Yelena Stein of the Ruhr-Universität Bochum, the Center de Données astronomiques de Strasbourg and the Max Planck Institute for Radio Astronomy in Bonn, together with American and Canadian colleagues, published their report in the magazine. Astronomy and astrophysics, released online July 21, 2020.

The data analyzed was compiled in the “Continuous Halos in Nearby Galaxies” project, where radio waves were used to measure 35 galaxies. “Galaxy NGC 4217 is of particular interest to us,” explains Yelena Stein, who began the study at the Ruhr-Universität Bochum Chair of Astronomy with Professor Ralf-Jürgen Dettmar and is currently working at the Données astronomiques Center in Strasbourg. NGC 4217 is similar to the Milky Way galaxy and is only about 67 million light-years away, meaning it is relatively close to it, in the Ursa Major constellation. Therefore, the researchers hope to successfully transfer some of their findings to our home galaxy.

Magnetic fields and origins of star formation.

In evaluating the NGC 4217 data, the researchers found several notable structures. The galaxy has an X-shaped magnetic field structure, which has also been observed in other galaxies, that extends far beyond the galaxy’s disk, that is, more than 20,000 light years.

In addition to the X shape, the team found a helical structure and two large bubble structures, also called superbubbles. The latter originate in places where many massive stars explode as supernovae, but also where stars are formed that emit stellar winds in the process. Therefore, researchers suspect a connection between these phenomena.

“It is fascinating that we discover unexpected phenomena in every galaxy every time we use radio polarization measurements,” says Dr. Rainer Beck of the MPI for Radio Astronomy in Bonn, one of the study’s authors. “Here at NGC 4217, it’s about huge magnetic gas bubbles and a helical magnetic field spiraling up into the galaxy’s halo.”

Furthermore, the analysis revealed large loop structures in magnetic fields throughout the galaxy. “This has never been observed before,” explains Yelena Stein. “We suspect that the structures are caused by the formation of stars, because at these points matter is thrown out.”

The image shows magnetic field structures.

For their analysis, the researchers combined different methods that allowed them to visualize the galaxy’s ordered and chaotic magnetic fields both along and perpendicular to the line of sight. The result was a comprehensive picture of the structures.

To optimize the results, Yelena Stein combined the data evaluated by radio astronomy with an image of NGC 4217 that was taken in the visible light range. The image is available for download on the website. “Visualizing the data was important to me,” says Stein. “Because when you think about galaxies, magnetic fields are not the first thing that comes to mind, although they can be gigantic and show unique structures. The image is supposed to focus more on the magnetic fields.”