Dynamics in Disk Galaxies

Dynamics in Disk Galaxies - Dynamics in Disk Galaxies A...

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Dynamics in Disk Galaxies A detailed understanding of the internal working of galaxies must incorporate their dynamics - just how the gravitational potential and internal motions balance to give galaxies their forms. This is all from Newton's laws at work, but distributed masses can give results that violate intuitions honed on the inverse-square law. All of this is treated well and exhaustively by Binney and Tremaine. The key is that observable stars and gas move in response to the total potential of the mass. Major surprises have come from recent observations both of spirals and ellipticals. Disks Local density: Close to the disk plane, if we adopt a reference frame sharing the local rotational velocity, stellar motions consist mainly of small oscillations about the mean location (or guiding center) - shades of Ptolemaic astronomy! Radial, tangential, and vertical oscillations may occur, with the vertical frequency not in general matching the in-plane motion as it would for small perturbations to a Keplerian orbit (because galaxies have extended mass distributions). Small radial oscillations (epicycles) have an axial ratio 2:1 (tangential:radial extent - we'd have been in real trouble if Hipparchus had tried that for the planets instead of perfect circles). The epicyclic frequency is given by in which R g is the mean radius of the orbit's guiding center. Vertical oscillations through the disk plane are of particular interest, with an instantaneous period for a given z -velocity set by the surface density within the region covered by the orbit. In the limit of a disk with infinitely thin
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This note was uploaded on 11/10/2011 for the course AST AST1002 taught by Professor Emilyhoward during the Fall '10 term at Broward College.

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Dynamics in Disk Galaxies - Dynamics in Disk Galaxies A...

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