Comparing Gaseous and Stellar Orbits in a Spiral Potential
  It is generally assumed that gas in a galactic disk follows closely non
self-intersecting periodic stellar orbits. In order to test this common
assumption, we have performed MHD simulations of a galactic-like disk under the
influence of a spiral galactic potential. We also have calculated the actual
orbit of a gas parcel and compared it to stable periodic stellar orbits in the
same galactic potential and position. We found that the gaseous orbits approach
periodic stellar orbits far from the major orbital resonances only. Gas orbits
initialized at a given galactocentric distance but at different azimuths can be
different, and scattering is conspicuous at certain galactocentric radii. Also,
in contrast to the stellar behaviour, near the 4:1 (or higher order) resonance
the gas follows nearly circular orbits, with much shorter radial excursions
than the stars. Also, since the gas does not settle into a steady state, the
gaseous orbits do not necessarily close on themselves.
