Ram pressure stripping of spiral galaxies:
an analytic approach
Joachim Köppen Kiel Nov. 2017

• make first estimates of the outcome of a stripping event on a specific galaxy.
• explore the influence of parameters of the galaxy model or the ram pressure pulse on the stripping outcome.
• explore the dependence and sensitivity of the stripping outcome to parameters of the galaxy model or the ram pressure pulse.
• demonstrate ram pressure stripping.
• use them as educational tools.

• Kicking a test particle from a potential well: What happens to a star or a parcel of gas (represented by a point mass) sitting at rest in the galactic plane, if a pulse-like external force acts on it in the direction perpendicular to the galactic plane? If the force is weak, the particle will execute small vertical oscillations about the rest position. The stronger the force, the larger will be the excursions in height above the plane, and the period of the oscillations will become longer. But if the force is strong enough or lasts long enough, the particle will achieve positive total energy and will escape from the galaxy. This is a very elementary demonstration of how ram pressure stripping can remove gas from a galaxy. The force pulse can be given various shapes, durations, and strength. Various parameters of the particle (position, velocity, energy) can be displayed.


• Properties of a spiral galaxy modelled as the combination of bulge, stellar and gas disk, and a dark matter halo, which are described by their appropriate parameters. Various properties (rotation curve, maximum restoring force, etc) can be displayed for any model. This also allows to compute the effects of ram pressure stripping based on the criterion of Gunn & Gott (1972)


• Simplified application of the criterion of Gunn & Gott (1972) to galaxies with a gas disk of exponential and Miyamoto-Nagai profile, based on a flat rotation curve and using the circumfugal force as a proxy for the maximum restoring force. One may also apply a recipe which takes into account that in the inner galaxy the presence of molecular cores makes it more difficult to remove neutral hydrogen from gas clouds.
• Similar to above but with an additional ram pressure-induced continuous mass loss from the inner parts of the gas disk.


• The analytical description of ram pressure stripping between the long-pulse limit (criterion of Gunn & Gott (1972)) and the short-pulse limit. Since in the long-pulse limit the amount of gas removed from a galaxy depends on the maximum ram pressure this galaxy had experienced so far, but in the short-pulse limit the outcome depends on the time-integrated ram pressure, the complete solution is best displayed in the plane of maximum ram pressure versus time-integrated ram pressure. The galaxy is modelled by the stellar bulge, the disk of stars and gas, and the dark matter halos, with the appropriate paremeters. The gravitational potential is computed from the sum of all four components. The outcome of stripping can be displayed in this parameter plane. Also, the stripped mass fraction, deficiency, or the stripping radius are shown depending on the maximum ram pressure or the time-integrated ram pressure, i.e. in the two limits.
• The analytical description of ram pressure stripping (II) between the long-pulse limit (criterion of Gunn & Gott (1972)) and the short-pulse limit. This is the same as above, but for a simplified characterization of the galaxy by its rotation speed, and the outer radius, radial scale and the mass of the gas disk.


• View of a galaxy undergoing ram pressure stripping is a simulation with test particles initially on circular orbits around the centre of the galaxy. A ram pressure pulse of gaussian shape with a specified strength and FWHM duration is applied face-on with peak pressure at time zero. The particles which represent parcels of the gas disk move only under the influence of the galaxy's gravitational field and the external pressure pulse, without any interaction between particles. The pressure pulse pushes them out of the disk (green dots = remain gravitationally bound) and/or lets them escape from the galaxy (red dots). Blue dots mark particles that remain close to the galactic plane. After running a simulation to generate snapshots of the events, the user can inspect the data in the coordinates of the galaxy as well as view the galaxy and its gas tail from any orientation.


• Radial infall of galaxies into a cluster and the effects of ram pressure stripping on their gas disks, estimated from the long-pulse limit (criterion of Gunn & Gott (1972)).

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