14-02 Spiral Arms

  • Mapping out the spiral arms
  • HI regions -- observation of the 21-cm lines
    Spin-flip transition of the electron in H atom: hyperfine structure





    Distribution of HI regions in the disk





  • Molecular clouds
    CO (2.6 mm lines) as tracers of H2 (NCO / NH2 ~ 10-4)



  • OB stars, HII regions, open clusters, dust



  • Spiral arms in our Milky Way galaxy
     
    4 major arms: Sagittarius, Perseus, Centaurus, and Cygnus arms
     
    Our sun is located in a minor arm (some call it a spur) -- the Orion arm.


    [The nomenclature of these arms has yet to be unified. This is a map more or less similar to the following ones in some sense.]


    [A map in year 1626.]


    [A map in year 1742.]


    [A map in year 1899.]

    Spiral arms are full of dust, molecular clouds, OB stars.
    However, stars are actually all over the disk.
     
  • Spiral arms are less obvious in redder color


    [M83 in optical, radio and infrared bands.]
     
  • The density wave model
     
    What are spiral arms actually?
     
    The density-wave description -- like traffic jam on a high way



    Dissipation and excitation of the density-wave?
    The density-wave model produces clear, well-defined spiral arms. But there are galaxies with fuzzy, poorly-defined arms -- flocculent spiral galaxies.