Astronomy Picture of the Day

    Discover the cosmos! Each day a different image or photograph of our
      fascinating universe is featured, along with a brief explanation
                    written by a professional astronomer.

                               2021 January 20

                 The Magnetic Field of the Whirlpool Galaxy
    Image Credit: NASA, SOFIA, HAWC+, Alejandro S. Borlaff; JPL-Caltech,
              ESA, Hubble; Text: Jayanne English (U. Manitoba)

   Explanation: Do magnetic fields always flow along spiral arms? Our
   face-on view of the Whirlpool Galaxy (M51) allows a spectacularly clear
   view of the spiral wave pattern in a disk-shaped galaxy. When observed
   with a radio telescope, the magnetic field appears to trace the arms'
   curvature. However, with NASA's flying Stratospheric Observatory for
   Infrared Astronomy (SOFIA) observatory, the magnetic field at the outer
   edge of M51's disk appears to weave across the arms instead. Magnetic
   fields are inferred by grains of dust aligning in one direction and
   acting like polaroid glasses on infrared light. In the featured image,
   the field orientations determined from this polarized light are
   algorithmically connected, creating streamlines. Possibly the
   gravitational tug of the companion galaxy, at the top of the frame, on
   the dusty gas of the reddish star-forming regions, visible in the
   Hubble Space Telescope image, enhances turbulence -- stirring the dust
   and lines to produce the unexpected field pattern of the outer arms.

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       Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (UMCP)
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