Magnetic fields are thought to play an important role in shaping galaxies, including influencing their initial formation and sculpting structures like spiral arms. Gas in the interstellar medium gathers along magnetic field lines and contributes to the collapse of gas clouds into stars. Detailed mapping of magnetic fields thus help provide a more complete picture of stellar and galactic evolution.
Mapping cosmic magnetic fields, however, presents observational challenges. Using a technique called optical stellar polarimetry, astronomers can characterize the magnetic fields of interstellar clouds by measuring the polarization of starlight that passes through them. For cool, diffuse regions of interstellar gas, the light from distant stars is polarized by absorption and reemission from dust grains aligned with magnetic fields. The problem is that the polarization is cumulative: Light gains additional polarization from the magnetic field of each cloud it passes through. Consequently, the measured polarizations encode the magnetic fields of all the clouds within the line of sight.
To disentangle those overlapping polarizations, Yasuo Doi of the University of Tokyo and colleagues paired polarimetry data with highly accurate stellar distance estimates from the star-mapping European Space Agency spacecraft Gaia. The result, published in January in the Astrophysical Journal, is the first 3D map of the magnetic fields along a spiral arm of the Milky Way.
Doi’s team used the 1.5-meter Kanata telescope at the Higashi-Hiroshima Observatory in Japan to measure the polarizations of light emitted by stars located in the direction of the Milky Way’s Sagittarius Arm. As expected, the polarization vectors showed no overall direction because the light from each star had passed through multiple clouds on its way to Earth.
Using the Gaia star catalog, Doi and colleagues found the precise distances of 184 stars for which they had polarization data. By analyzing the variation of polarization values between stars at different distances, the researchers identified five distinct polarization alignments within specific distance ranges. The shifts signal the magnetic field effects of five interstellar clouds, three of which had never been spotted before.
Finally, the researchers subtracted the contribution of each cloud’s magnetic field along the line of sight and calculated the properties of the individual fields, including their strength and direction and their influence on the plasma turbulence in each cloud. The team determined that the magnetic fields of the clouds are inclined with respect to the galactic plane, a finding that is consistent with previous work that examined magnetic fields in the galactic disk. That could indicate that the galactic magnetic field folds along the spiral arm, which points to complexity in the generation of the magnetic fields.
Rainer Beck, who studies galactic magnetic fields at the Max Planck Institute for Radio Astronomy, says that surveys combining optical polarimetry with stellar distances are an “important and complementary” method to other means of studying galactic fields. Submillimeter observations, for example, can probe the fields of warm dust clouds via radiation emitted from the clouds themselves. The new method, he says, allows the mapping of tenuous, cold clouds, “tracing magnetic fields in dust, independent of temperature.”
Study coauthor Simon Coudé (Worcester State University and the Center for Astrophysics | Harvard & Smithsonian), says that the Gaia data provide a boost to observational astronomers at a wide range of facilities. “In the age of JWST and the Extremely Large Telescope, this work illustrates how smaller facilities like the Higashi-Hiroshima Observatory still provide essential contributions to cutting-edge research in astronomy,” he says. Doi, Coudé, and colleagues are now working to triple the area of their magnetic field survey.
