Properties of a (Spiral) Galaxy
Joachim Köppen Praha/Kiel Nov. 2017


Some brief explanations

A spiral galaxy consists of a spherical bulge of old stars, a rather thin disk of stars of all ages and of gas, and spherical halo of dark matter. All these components produce the gravitational potential in which the gas and stars move. The disk of stars and gas rotates around the centre of the galaxy. This tool displays various properties of such a galaxy, like the rotation curve, or the escape speed as a function of distance rho from the rotational axis or the height z above the galactic plane.
This tool also displays the characteristics of the gas disk when it is subjected to a face-on ram pressure when traveling through the hot gas inside a cluster of galaxies. Gunn & Gott (1972) formulated an often used criterion, that gas at a distance from the centre is removed from the disk if the ram pressure exceeds the maximum vertical restoring force at that position.
The density distribution of the spherical components are modelled by Plummer spheres, the disky components by Miyamoto-Nagai disks, and described by several parameters: For comparison, we may also add a point mass at the galactic centre. There is also an enhancement factor for the restoring force which takes into account that in the inner galaxy the presence of molecular cores make it more difficult to remove (neutral) gas from the ISM clouds (cf. Vollmer el al. (2001), Astrophys.J. 561, 708).

The results are shown in the form of curves which give the dependence of either the distance rho from the rotational axis (for a fixed height zpoint above the plane) or of the height for a fixed radius rhopoint:
  • rotation curve: speed of rotation as a function of distance rho from the rotational axis.
  • escape speed: as a function of rho or z. In the display versus z a blue dot indicates the height at which the vertical restoring force is maximum.
  • gas column density: as a function of rho.
  • gas mass fraction outside radius rho.
  • vertical restoring force: as a function of height above the plane and for a fixed radius rho. The maximum value is indicated by a small red dot.
  • maximum restoring force: as a function of radius.
  • height (above the plane) of the maximum restoring force: as a function of radius.
  • centrifugal acceleration: as a function of radius.
  • centrifugal force: as a function of radius.
  • ratio of centrifugal force and maximum restoring force: as a function of radius.

  • stripping radius: as a function of ram pressure. For a constant value of the ram pressure, the Gunn & Gott criterion is used to predict at which radius the ram pressure is just balanced by the maximum restoring force. Thus, outside this radius, all gas can escape from the disk.
  • stripped mass fraction: as a function of ram pressure. This is the mass fraction of the gas that escapes from the disk.
  • deficiency as a function of ram pressure: is a handy quantity to measure the lost gas mass: def = log10(initial gas mass/remaining gas mass).

Operational hints:
  • The user may select among a few models for galaxies. The parameters are displayed in the fields on the right side. Any parameter can be modified by entering a new value and by hitting the enter key the results with the new parameter are shown.
  • The button autoclear ON indicates that any time another galaxy model or another plot is selected or a parameter is changed by the user, the plot will be cleared before the new results are shown. But in order to overplot different results, one can click this button to show autoclear off. Then all following plot are superposed.
  • This feature also allows the user to overplot two different plots, such as rotation curve with escape speed (rho) ... but beware: some other combinations may not be meaningful!
  • The button wipe&plot always clears the plot and shows the current model as a red curve. The other buttons overplot are used to plot the model with modified parameters (but then do NOT hit the Enter key) as a curve in different colour.
  • Set range: In order to use different values for the X and Y axes than the standard value, enter the wanted minimum and maximum values in the two field and click the circular radio button to show the same plot with the new range. Note that we may view any range of values, but meaningful results are not guaranteed when using unreasonable or extreme values.
  • Mouse position: displays the coordinates of the present position of the mouse.

Central point mass
Mass [MSUN]
Bulge ( = Plummer sphere)
Mass [MSUN]
Radial scale [kpc]
Truncation radius [kpc]
Stellar disk ( = Miyamoto-Nagai)
Mass [MSUN]
Radial scale [kpc]
Truncation radius [kpc]
Thickness [kpc]
Gas disk ( = Miyamoto-Nagai)
Mass [MSUN]
Radial scale [kpc]
Truncation radius [kpc]
Thickness [kpc]
Restoring force enhanced by molecular phase
Enhancement factor
Radial scale [kpc]
Dark matter halo ( = Plummer sphere)
Mass [MSUN]
Radial scale [kpc]
Truncation radius [kpc]


rhopoint [kpc]
zpoint [kpc]



X-axis =
Set range: ...
Y-axis =
Set range: ...






Mouse position:

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