Properties of a (Spiral) Galaxy

Joachim Köppen Praha/Kiel Nov. 2017

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:

- Mass: is the total within the truncation radius
- Radial scale length in radial direction
- Truncation radius: the outer radius of the matter distribution
- Thickness of a disky component is the vertical scale parameter

The results are shown in the form of curves which give the dependence of either the distance

- 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.

Mass [M

Mass [M

Radial scale [kpc]

Truncation radius [kpc]

Mass [M

Radial scale [kpc]

Truncation radius [kpc]

Thickness [kpc]

Mass [M

Radial scale [kpc]

Truncation radius [kpc]

Thickness [kpc]

Enhancement factor

Radial scale [kpc]

Mass [M

Radial scale [kpc]

Truncation radius [kpc]