Unwinding the Mysteries of Spiraling Galaxies %

A team of astronomers used NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA) to observe the galaxy M77, some 47 million light-years from Earth, finding its magnetic field to be closely correlated with the galaxy’s star-filled spiral arms.
Galaxies come in various shapes and forms. Some may be spherical, some disk-like, while some spiral around a central orbit – much like our very own galaxy, the Milky Way. Many theories have been presented to explain just why those certain galaxies may be spirals in shape, but only recently did scientists find clues to solve this astronomical mystery.

One of the co-authors of the paper, Terry Jones of the University of Minnesota, explained that the space between galaxies is filled with dust. While the dust may absorb light emitted by the galaxies, its particles heat up and emit far-infrared radiation, which can then be detected. Using the polarization of light by the dust, we can find the direction of the magnetic field.

Since SOFIA flies above water vapor in the atmosphere, it managed to produce a visualization of M77 like never before, using far-infrared radiation. This is not possible from the ground because the atmosphere would have absorbed the faint infrared emitted from dust in distant galaxies, and the previously used machines were not sensitive enough for the task.

While the dust particles themselves are very small, their large-scale alignment made them radiate more brightly, revealing their distribution across M77. This was because that light is “almost entirely radiated by dust grains,” as mentioned by Bruce Draine of Princeton University, who was not involved in the work.

Although M77 and other spiraling galaxies can now be observed better, the question as to how they get their spirals still remains. In the density wave theory, the denser region of a galaxy is believed to rotate slower compared to the surroundings, causing stars to bunch up and form spiral arms that are observable from afar. Jones says “the magnetic field itself is not telling us where the spiral arms should be; the spiral arms are telling us where the magnetic field points”, making the field an effect rather than a cause for the spiraling.

SOFIA’s revelation of very distinct large-scale patterns is novel and important. Ronald Drimmel of the Astrophysical Observatory of Turin in Italy, who was also not involved in the new study, says “It’s showing that the magnetic field in these galaxies isn’t just turbulent or random,” he says. “It’s not obvious that the magnetic field should be ordered in this regular way over large scales. So that is interesting”—and potentially relevant for solving the mystery of why some galaxies have a spiral shape while others do not.