NASA’s flying observatory reveals magnetic fields of galaxies

Magnetic fields are difficult to detect. For decades, astronomers have studied the factors that shape the interiors of galaxies—gravity, kinetic energy, stellar radiation, gas pressure—but magnetic fields remain largely unknown. Magnetic fields are a fundamental factor in the evolution of any galaxy: they direct interstellar gas toward the supermassive black hole at its center, regulate the rate at which new stars form, influence the formation of molecular clouds of gas, and even influence the motions of disks in spiral galaxies. Magnetic field lines permeate the space between stars and the gas clouds that make up galaxies, but they do not emit light themselves, so magnetic fields are difficult to map.

Just like dense clouds in the atmosphere of the Earth bring rain, dense interstellar molecular clouds form stars. Magnetic fields are amplified within these dense star-forming clouds, which optical telescopes (such as the Hubble Space Telescope) cannot penetrate. So astronomers must use specialized telescopes and instruments to detect the magnetic fields.

One of those instruments is the High Resolution Polarimetric Camera (HAWC+) on NASA's Stratospheric Observatory for Infrared Astronomy (SOFIA), a flying observatory. Its sensitivity to the far-infrared region of the electromagnetic spectrum allows HAWC+ to observe the effects of magnetic fields deep in the cold, dark molecular clouds. HAWC+ observed polarized light emitted in the far infrared from magnetically aligned dust grains.

The 15 neighboring galaxies observed by the HAWC+ polarimeter on NASA's SOFIA flying observatory. The relief pattern shows the magnetic field morphology in the far infrared, which reveals the dense interstellar clouds.

An international team of scientists, including Kostas Tassis, a Professor at the Univ. of Crete and affiliated faculty of IA-FORTH, iworking on a project called SALSA (Survey on extragALactic magnetiSm with SOFIA) used HAWC+ to observe 15 galaxies in the neighborhood of our own Milky Way galaxy. They mapped the far-infrared magnetic fields and compared their structures to those obtained in radio waves with the Very Large Array in New Mexico and the Effelsberg Telescope in Germany, both of which are sensitive to the galaxies' less dense gas.

SALSA finds that the magnetic fields in the turbulent, dense clouds of star-forming gas are more chaotic and have a more irregular morphology than those in the diffuse interstellar gas detected by radio telescopes. Different spectral regions (far infrared, radio waves) reveal different layers of the structure of magnetic fields. High-resolution observations of galaxy polarization in the far infrared, such as those that were provided by SOFIA/HAWC+ until its final flyby in September 2022, are crucial to our understanding of the role of magnetic fields in the evolution of the Universe.

Article: Borlaff et al. "Extragalactic magnetism with SOFIA (SALSA Legacy Program) -- V: First results on the magnetic field orientation of galaxies", The Astrophysical Journal (2023 in press)