Mapping the Magnetic Field of the Galaxy

The Galactic magnetic field that permeates the interstellar regions of the Milky Way is believed to play important roles in interstellar processes, such as stellar birth.   The dust grains that are mixed in with the interstellar gas are often aligned by the Galactic fields, and as a result the grains' thermal emission is linearly polarized.  This polarized emission comes out mostly in the submillimeter band, so by carrying out submillimeter polarization measurements we can trace out Galactic field lines.   At most places on Earth, the atmosphere is nearly opaque at submillimeter wavelengths, but the SPARO polarimeter was operated on the Antarctic plateau, where skies are exceptionally transparent to submillimeter waves during the cold Antarctic winter.

Our Main Results

Most stars are born in relatively dense and massive interstellar gas clouds called Giant Molecular Clouds, or GMCs.   Thus, the first step in the stellar life-cycle is the formation of a GMC.  SPARO observations made in 2003 show that magnetic fields in GMCs tend to be parallel to the large-scale Galactic magnetic field, which indicates that the Galactic field is strong enough to play important roles in the process of GMC formation.   To learn more about this result, see our report in SPIE Newsroom and our scientific papers.

During observations made with SPARO in 2000, we discovered large-scale toroidal magnetic fields at the center of our Galaxy, where a super-massive black hole is located.   Earlier observers had seen a poloidal field, but the SPARO observations show that the field has a more complex geometry, possibly including a large-scale twist.   For more information about this Galactic Center result, see our February 2003 update or our scientific papers.

Details of the Experiment

SPARO was developed at Northwestern University's Department of Physics and Astronomy, with several key components developed at the University of Chicago Engineering Center.   Submillimeter polarimetry is also being carried out at locations where the atmosphere is less transparent than South Pole, but using larger telescopes.   An example is the CSO on Mauna Kea (see SHARP site).   The larger size of such telescopes in comparison with SPARO's South Pole telescope means that they obtain better angular resolution.   But SPARO observations are characterized by much better sensitivity to the relatively fainter, more extended emission.

SPARO's novel design was optimized for ease-of-use during the harsh South Pole winters, when experiments must be operated remotely with help from only a small winter-over crew.   Our magnetic field map of the Galactic center (see above) was the first astronomical result to be obtained during a South Pole winter-over using detectors operated below 1 Kelvin.   Novel features that were incorporated into SPARO's design include dual vapor-cooled radiation shields, that provide long ⁴He hold time, and a simplified process for condensing the liquid ³He that makes use of a separate, continuously pumped, capillary-fed ⁴He reservoir.   A photo of graduate students Hua-bai Li and Megan Krejny working on SPARO's telescope at South Pole in 2003 appears below.   (Drs. Li and Krejny now work as postdoctoral researchers at Harvard and the University of Minnesota, respectively.)