Spiral galaxies look pretty and orderly, with most of their stars and gas confined to a carefully arranged disk in swirling spiral arms. But there is much more to a galaxy than we can see, as a new image of invisible phenomena skillfully demonstrates.
The image shows a galaxy called NGC 4217, about 67 million light-years from the Milky Way, viewed from the edge, and depicted amid a mapped visualization of the galaxy’s vast and complex magnetic field, spanning about 22,500 light years in space around NGC 4217.
Since we don’t know much about how galactic magnetic fields are generated and maintained, astronomers hope that the lessons learned from this new map can be applied to our galaxy, the Milky Way.
“Galaxy NGC 4217 is of particular interest to us,” said astronomer and physicist Yelena Stein, formerly of the Ruhr-Universität Bochum in Germany, and now at the Center de Données astronomiques de Strasbourg in France.
“This image clearly shows that when we think of galaxies like the Milky Way, we should not forget that they have magnetic fields throughout the galaxy,” he added.
Magnetic fields are invisible fields that exert a force on particles that are magnetically sensitive. They can be generated by magnets, but also by changing electric currents and electric fields.
However, the fact that magnetic fields are invisible does not mean that they are undetectable. Astronomers have a very smart way to detect magnetic fields in distant galaxies, starting with cosmic rays, which are subatomic particles that travel at significant fractions of the speed of light as they flow through space.
When cosmic ray electrons accelerate at the shock fronts of supernova remnants, they can accelerate to almost the speed of light. These relativistic electrons then spiral along magnetic field lines, generating radio waves called synchrotron emission across a wide range of wavelengths. A synchrotron is an electron accelerator.
It is this synchrotron emission that can be detected here on Earth to reconstruct a magnetic field. However, it is not only the strength of the emission that astronomers use, but also the polarization or the way radio waves twist. This polarization shows how the lines of the magnetic fields are oriented.
This is the technique the team used to map the magnetic field around NGC 4217, using the Karl G. Jansky Very Large Array radio astronomy observatory in New Mexico, and the Low Frequency Matrix Radio Telescope network, based in the Countries Low.
Composite image of NGC 4217. (Y. Stein, NRAO, SDSS, KPNO 0.9m, J. English, R.-J. Dettmar, A. Miskolczi, RJ Rand and J. Irwin)
Their results showed a large X-shaped magnetic field. It is not very strong, with an average total force of 9 microgauss, or millionths of a gauss, compared to the average 0.5 gauss force of Earth’s magnetic field.
However, it is huge and extends up to 22,500 light years above and below the galactic disk. This is not unusual; Several edge-forming star-forming galaxies have a similarly shaped magnetic field.
In addition, the team found a helix-shaped structure and two large “superbubble” structures. These superbubbles form in two types of regions: those where many massive stars end their lives in supernovae; and those where stars are born, a process that generates intense stellar winds. The same structures at the two different locations suggest that the processes could be linked.
“It is fascinating that we discover unexpected phenomena in every galaxy every time we use radio polarization measurements,” said astronomer Rainer Beck of the Max Planck Institute for Radio Astronomy in Germany.
“Here at NGC 4217, it’s about huge magnetic gas bubbles and a helical magnetic field spiraling up into the galaxy’s halo.”
The researchers also found something really strange: large loops in the magnetic field throughout the galaxy.
“This has never been observed before,” Stein said. “We suspect that the structures are caused by the formation of stars, because at these points matter is thrown out.”
Although the mechanism behind galactic magnetic fields is unclear, the main hypothesis is that it is generated and maintained by a dynamo. This is an electrically conductive, convective, rotating fluid that converts kinetic energy into magnetic energy.
On Earth, that fluid is molten iron in the outer core. In the sun, that fluid is plasma. In disk galaxies, the dynamo fluid is also believed to be plasma. According to the researchers, it is possible that supernova explosions and Coriolis force combine with the shear motion to create large-scale regular magnetic fields, while the gas drop can create turbulence resulting in asymmetry.
Deeper observations of the galaxy could provide more detailed data about its magnetic field, which in turn would help understand the processes that drive both order and chaos in its magnetic field.
The research has been published in Astronomy and astrophysics.
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