This image shows the huge extent of a spiral galaxy’s magnetic field. The galaxy NGC 4217 is a star-forming, spiral galaxy, similar to our own Milky Way, 67 million light-years from Earth in the constellation Ursa Major. The galaxy is seen edge-on in a visible-light image from the Sloan Digital Sky Survey and Kitt Peak National Observatory, and the magnetic field lines, shown as green, are revealed by the National Science Foundation’s Karl G. Jansky Very Large Array (VLA) radio telescope.

The magnetic field lines extend as much as 22,500 light-years beyond the galaxy’s disk. Scientists know that magnetic fields play an important role in many processes, such as star formation, within galaxies. However, it is not fully understood how such huge magnetic fields are generated and maintained. A leading explanation, called the dynamo theory, suggests that magnetic fields are generated by the motion of plasma within the galaxy’s disk. Ideas about the cause of the kinds of large vertical extensions seen in this image are more speculative, and astronomers hope that further observations and more analysis will answer some of the outstanding questions.

“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,” said Yelena Stein, of the Centre de Données astronomiques de Strasbourg, leader of the study.

The scientists who produced the image are reporting their results in the journal Astronomy & Astrophysics.

CREDIT: Composite image by Yelena Stein of the Centre de Données astronomiques de Strasbourg (CDS) with the support of Jayanne English (University of Manitoba). VLA radio data from Yelena Stein and Ralf-Juergen Dettmar (Ruhr University Bochum). The observations are part of the project Continuum HAlos in Nearby Galaxies — an EVLA Survey (CHANG-ES) led by Judith Irwin (Queen’s University, Canada). The optical data are from the Sloan Digital Sky Survey. The ionized hydrogen data (red) are from the 0.9m telescope of the Kitt Peak National Observatory, collected by Richard J. Rand of the University of New Mexico. The software code for tracing the magnetic field lines was adapted by Y. Stein from Linear Integral Convolution code provided by Arpad Miskolczi of Ruhr University Bochum.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

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“CHANG-ES XXI. Transport processes and the X-shaped magnetic field of NGC 4217: off-center superbubble structure” (DOI: https://www.aanda.org/10.1051/0004-6361/202037675)

New cosmic magnetic field structures discovered in galaxy NGC 4217

Superbubbles, giant loops and X-shaped magnetic field structures - this galaxy boasts a veritable wealth of shapes

RUHR-UNIVERSITY BOCHUM

 

Spiral galaxies such as our Milky Way can have sprawling magnetic fields. There are various theories about their formation, but so far the process is not well understood. An international research team has now analysed the magnetic field of the Milky Way-like galaxy NGC 4217 in detail on the basis of radio astronomical observations and has discovered as yet unknown magnetic field structures. The data suggest that star formation and star explosions, so-called supernovae, are responsible for the visible structures.

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

The analysed data had been compiled in the project "Continuum Halos in Nearby Galaxies", where radio waves were utilised to measure 35 galaxies. "Galaxy NGC 4217 is of particular interest to us," explains Yelena Stein, who began the study at the Chair of Astronomy at Ruhr-Universität Bochum under Professor Ralf-Jürgen Dettmar and who currently works at the Centre de Données astronomiques de Strasbourg. NGC 4217 is similar to the Milky Way and is only about 67 million light years away, which means relatively close to it, in the Ursa Major constellation. The researchers therefore hope to successfully transfer some of their findings to our home galaxy.

Magnetic fields and origins of star formation

When evaluating the data from NGC 4217, the researchers found several remarkable structures. The galaxy has an X-shaped magnetic field structure, which has also been observed in other galaxies, extending far outwards from the galaxy disk, namely over 20,000 light years.

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

"It is fascinating that we discover unexpected phenomena in every galaxy whenever we use radio polarisation measurements," points out Dr. Rainer Beck from the MPI for Radio Astronomy in Bonn, one of the authors of the study. "Here in NGC 4217, it is huge magnetic gas bubbles and a helix magnetic field that spirals upwards into the galaxy's halo."

The analysis moreover revealed large loop structures in the magnetic fields along the entire galaxy. "This has never been observed before," explains Yelena Stein. "We suspect that the structures are caused by star formation, because at these points matter is ejected outward."

Image shows magnetic field structures

For their analysis, the researchers combined different methods that enabled them to visualise the ordered and chaotic magnetic fields of the galaxy both along the line of sight and perpendicular to it. The result was a comprehensive image of the structures.

To optimise the results, Yelena Stein combined the data evaluated by means of 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. "Visualising the data was important to me," stresses Stein. "Because when you think about galaxies, magnetic fields is not the first thing that comes to mind, although they can be gigantic and display unique structures. The image is supposed to shift the magnetic fields more into focus."

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