Thursday, January 24, 2013

lit: Galactic Dynamics

I'm reading Binney & Tremaine's section on LSR, asymmetric drift and its correction, as I plan to do the corrections for my stellar velocity fields. Some definitions:
LSR is the hypotetical local standard of rest: velocity of some imaginary stars in precisely circular orbits at the Solar radius.
Characteristic thickness is the ratio of surface density to its volume density at the galactic plane: it's different for different stellar populations (like their vertical scale lengths), arising due to interactions w/ molecular clouds and/or spiral arms.
The collisionless Boltzmann equation states that the flow of stellar phase points through phase space is incompressible: phase space density around a phase point of a given star remains constant. A nice example of such incompressible flow is a marathon race where all participants run at constant speeds: at the begining the density of runners is large, but their speed distribution is wide. At the end of the race the density is low, but the speeds of all runners passing a point are very similar. The coordinates of stars in phase space are (x, v).
Asymmetric drift is the difference between the LSR (local circular speed) and the mean rotation velocity of a population. Velocity dispersion and asymmetric drift: see eq. 4.32-4.34 for the correction formulae; also Neistein, Maoz, Rix et al. for their application.

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