                        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.

                              2023 September 13
   A galaxy with blue spiral arms is seen in the image center in the midst
     of numerous foreground stars. This galaxy is surrounded by a white
        envelope, which was found to be hydrogen gas. Please see the
                 explanation for more detailed information.

                  NGC 4632: Galaxy with a Hidden Polar Ring
   Credit: Jayanne English (U. Manitoba), Nathan Deg (Queen's University)
     & WALLABY Survey, CSIRO/ASKAP, NAOJ/Subaru Telescope; Text: Jayanne
                            English (U. Manitoba)

   Explanation: Galaxy NGC 4632 hides a secret from optical telescopes. It
   is surrounded by a ring of cool hydrogen gas orbiting at 90 degrees to
   its spiral disk. Such polar ring galaxies have previously been
   discovered using starlight. However, NGC 4632 is among the first in
   which a radio telescope survey revealed a polar ring. The featured
   composite image combines this gas ring, observed with the highly
   sensitive ASKAP telescope, with optical data from the Subaru telescope.
   Using virtual reality, astronomers separated out the gas in the main
   disk of the galaxy from the ring, and the subtle color gradient traces
   its orbital motion. Why do polar rings exist? They could be material
   pulled from one galaxy as it gravitationally interacts with a
   companion. Or hydrogen gas flows along the filaments of the cosmic web
   and accretes into a ring around a galaxy, some of which gravitationally
   contracts into stars.

                       Tomorrow's picture: open space
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