John Lacy: Gas Motions in Inner 2 pc of SgrA West

NeII 12um line

Observationally, little has changed. Interpretation has changed a lot
  • Ne II and 6cm continuum look identical
  • data avilable from Irons et al erratum
Two models
  • elliptical orbit
  • spiral density wave + both assume same plane as CND
  • northern arm offset in PV diagram, but matched slope + similar in western arm
  • model assumes gas moving along ellipse + reality: probably moving at some angle to the ellipse
  • spiral model same as infalling streamer, something close to circle... + linear spiral + Eastern arm doesn't fit the model
Is there a physical explanation for a 1-arm spiral density wave?

  • 1-armed spiral is dominant instability in a keplerian disk

  • perturber needed: gas not self-gravitating, so can't be same as disk + B-field perturber?

  • dynamical model + orbits precess with power-law mass distribution + orbit crowding near apocenter -> spiral formation + 2e5 yrs to develop

    • persists for a long time: the precession timescale
    • fitting with Keplerian velocity is wrong
      • gas is seen at the apocenter of its elliptical orbit

  • inflow rate is small

[Exercise: reproduce his simulation & movie]

Questions

  • Q: Clumps in CND interacting with each other?
  • A: Spiral density wave fails at 2pc from center. Should not persist outside of 2pc, but accidental spirals should persist because there is no differential precession
  • Q Fred Lo: Trying to explain gas, that is not collisionless.
  • A: People here know how to do HD, MHD.
  • Q: Not a complete explanation. What about western arm?
  • A: Maybe magnetically controlled structure? Not in the same plane.
  • Q: Not a density wave in the Western arm
  • Q: What is the source of ionization?
  • A: Assumed massive stars. But, too bright. Maybe shock ionization?

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