14-03 Rotation of the Milky Way Galaxy

  • Motion of the Sun in the Galaxy
  • The orbital speed of the Sun


    The Sun moves relative to local halo stars at a speed of 250 km/s.
    The Sun moves relative to the LSR (Local Standard of Rest) at a speed of 20 km/sec in the direction 18h08m +30°.
    (LSR moves at the center-of-mass velocity of stars in the solar neighberhood in the galactic disk.)

    The LSR moves around the galactic center nearly circularly at a speed of 240 km/sec.
    (Schonrich 2012, MNRAS 427, 274; Kushniruk et al. 2017, A&A 608 A73)
     
  • The period of the Sun's revolution is about

    2πr / v = 2.0 x 108 years

    The mass inner to the solar orbit can be estimated as

    M = (v2/r) (r2/G) = 1.0 x 1011 M⊙
  • The rotation curve of the Milky Way galaxy



    Orbital speeds approach a constant in the outer part of the Galaxy, instead of a Keplerian fall-off. (Should one expect such a fall-off?)

    The luminous mass in our Galaxy can account for only 10% of the total mass inferred from the orbital speeds, which exceeds 1012 M⊙.


    [All spiral galaxies show similar rotation curves.]


    [An example of adding dar matter to explain the rotation curves of galaxies.]

  • Candidates for dark matter

    MACHOs (MAssive Compact Halo Objects, like brown dwarfs, white dwarfs, black holes)? --- way less than required.



    Neutrinos? --- hot and less than required.

    WIMPs (Weakly Interacting Massive Particles)? --- still mysterious.

    Other alternatives? Modified Newtonian Dynamics (MOND)? New theory for gravity?