Galaxies Lecture 18

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Oort Constants

  • Oort Constants
    • Our tangential velocity with respect to the galactic center can be expressed as:

where is a measurement of shear (determined from radial velocities), and is a measurement of the local curl (determined from tangential velocities).

    • Measurements show and . This implies and .
    • We are on the inside edge of a spiral arm. Gas falling into the arm provides a shear beyond normal, so our A is abnormally high.
    • There are additionally Oort Constants C and K which measure shear along the tangential axis, and the divergence of the local velocities.
  • Measurements of Constants
    • is determined from RR Lyrae stars, globular clusters, proper motion of masers in the galactic center, and from statistical parallax. Statistical parallax is done using VLBI to get the position of masers with extreme accuracy, and then watching the masers move. We find the sun is about 8 kpc from the galactic center.
    • is determined from A, B measurements (as described before) and by measuring the proper motion of Sgr . We find that , but changes, so we deduce that it’s motion is our motion.
  • Gas Flow
    • CO measurements at the galactic anticenter show inflow at 2-3 km/s.
    • This gas flow replentishes gas which is depleted from star formation.
    • Star formation proceedes at 1-5 , from a reservoir of , so star formation would end in yrs without this inflow.