15-02 Spiral Arms

  • mapping out the spiral arms
     
  • HI regions -- the 21-cm lines
    spin-flip transition of the electron in H atom





    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 are less obvious in redder color



  • 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.]

    [maps in 1626, 1742, 1899]

    Spiral arms are full of dust, molecular clouds, OB stars.
    However, stars are actually all over the disk.
     
  • 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.