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.

Jiangtao Li, assistant research scientist with the Department of Astronomy at the University of Michigan, was a member of the team responsible for the capture of this image. Li has been a member of the CHANG-ES consortium since 2010. In addition to scientific discussions, he is conducting multi-wavelength observations of the sample galaxies, including X-ray observations tracing hot gas (10^6-7K), optical emission line observations tracing warm ionized gas (10^4-5K), and radio observations tracing cold molecular gas (<~10^2K). The optical and radio observations are conducted based on University of Michigan's privileged access to the MDM observatory and the IRAM 30m telescope.

“Characterizing the large scale magnetic field structure is very important in our understanding of how galaxies co-evolve with their gaseous environment,” said Li. “Based on similar studies conducted by the CHANG-ES group, including the observation of magnetic field as shown in this image, as well as comparison to the multi-wavelength data tracing multiple gas phases, we found that the magnetic field is often strong enough to play an important role in the global motions of gas flows around the galaxy. Its overall effect is as important as other phases such as hot gas, cosmic ray, radiation pressure, etc.”

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.


University of Michigan Contact: Jiangtao Li

Link to Original Release: National Radio Astronomy Observatory