MANUAL: INTERGALACTIC GRAVITATIONAL MOTION SIMULATOR

Background

The Intergalactic Gravitational Motion Simulator (version 3.2), or IGMS, was

developed by Daniel Perley in early 2000 for use as part of a science fair project.

Written in Borland C, it models the gravitational interaction between galaxies, which is

thought to be the major cause of peculiar galactic structure seen in the universe. The

simulation model is Newtonian, so all motion is based on Newton's laws, most

fundamentally the universal law of gravitation.

Outline

The simulator models two galaxies, each with a rotating disk of a user-

specified number of tracer particles. The orbit of the galaxies is specified by a

number of parameters, as are the properties of each galaxy.

The simulation consists of three 'pages' (scroll through using F2), each

with a distinct function. The 'properties' page is used to save and load files and

to activate various simulation options, such as coloration and tracking. This page

is the only one from which the simulation can be run (using the F1 command). The

'galaxies' page specifies the essential galactic and intergalactic model parameters

and is used to change the sizes of the galaxies, the shape of their orbit, etc. The

'particles' page, which cannot be accessed if there is an error in the galaxies page

inputs, simply lists the essential properties for each particle.

Page 1 – Properties

File

Simulation Name – the file name under which the simulation will be stored if

Output is set to disk of the file to be loaded if Input is set to file

Input – the source of the coordinate data

simulation – use the simulator to move the particles

file – load positions from a disk file

Output – where to store the coordinate data

none – no data storage

RAM – temporarily hold data in memory (this is unlikely to be used)

disk – save data to a disk file

Record Frequency – data stored to disk is recorded every n timesteps

(not every n sdu.) Since the simulation size is limited to 65

recordings, you should set this to the duration of your intended

simulation (in timesteps) divided by 65.

Load Settings – Load the interface settings from a previously saved file.

Save Settings – Save all entered fields to disk as a .SET file.

Note: The size of an average simulation file is 1 megabyte. If you have a

shortage of disk space, do not store simulations to disk; use the Save

Settings command instead.

Note: When Input is set to file, pressing F1 – Check will load the file's interface

settings from disk and change to F1 – Run. If you want to modify the settings,

change Input back to simulation, and the loaded settings will be retained.

Time

Coarse Timestep – how often the simulator recalculates the position of every

particle. The smaller this value is, the more accurate the simulation, but

the longer it takes to run. Generally a value of 5 is adequate, although

for smaller-scale work this should be set to 1 or less.

Fine Timestep – identical to coarse timestep.

Delay – slows down the simulation with a timing loop. This would only be

useful if the total number of particles is very small or if you are loading

data from disk.

Graphics

Monitoring – non-functional command

Redraw Method – how the simulator erases and redraws the particles

flash – erases all particles, then redraws all particles. This makes the

galaxies appear to scintillate.

sequence – erases and redraws each particle one by one. Adequate

except at regions of high particle density, where one particle

may blacken another.

sequence++ – erases and redraws each particle one by one, then

redraws all particles a second time to make sure none were

erased. This is the best redraw method, although it is slightly

slower than the others.

Particle Style – the appearance of the particles

point – particles are single pixels

cross – particles are five-pixel crosses

circle – particles are large circles (very slow)

varsmall – massive particles are crosses, massless particles are points

varlarge – massive particles are circles, massless particles are crosses

Tracking – fix the camera over the specified point.

none – camera is centered wherever the user specifies.

galaxy 1 – camera is centered over the centroid of galaxy 1

galaxy 2 – camera is centered over the centroid of galaxy 2

center of mass – camera is centered over the galaxies' center of mass

Doppler Mimicry – color-codes particles according to how fast they are

approaching or receding along the line of sight.

Z-Axis Fade – color-codes particles according to their distance along the line

of sight.

Scaling

sdu – the basic unit of distance, defined as one pixel on the screen at 100%

zoom.

smu – the basic unit of mass, defined as the mass required to generate an

acceleration of G smu/stu² at a distance of 1 sdu

stu – the basic unit of time, defined as one timestep at timestep = 1.

G – the gravitational constant from Newton's Law of Universal Gravitation.

F1 – Check: When the galaxies (below) have been set up and the properties are set to

satisfaction, press F1 to have the simulator make sure all inputs are adequate.

If there is an error a message will appear; else this will change to run.

F1 – Run: Initialize the simulation and run.

F2 – Galaxies: Move to the next page.

F3 – Exit: Exit the IGMS.

Page 2 – Galaxies

The galaxy model has three elements: the centroid (massive particle at the center), the

disk (revolving rings of massless particles), and the halo (spherical region of gravitating

dark matter.)

Centroid Properties

x, y, z – the initial x, y, and z coordinates of each galaxy. Generally, this is not

a very good way to set up the simulation; it is much better to use the

retrotrajectory feature (below) to find these automatically.

vx, vy, vz – the initial x, y, and z velocities of each galaxy. Generally, this is not

a very good way to set up the simulation; it is much better to use the

retrotrajectory feature (below) to find these automatically.

Mass – the mass of the centroid of each galaxy. This should always be

entered; if set to zero, the simulator will assume the galaxy does not exist.

Disk Properties

Rmin – the radius of the innermost particle ring of the galaxy.

Rmax – the radius of the outermost particle ring of the galaxy.

Rings – the total number of particle rings in the galaxy.

Particles – the total number of particles per ring.

Tilt – the angle of x-z inclination of the galaxy above the x-y plane.

Skew – the angle of x-y rotation of the galaxy within the x-y plane.

Color – the color of the disk particles.

Note: the simulation limits the total number of particles to 502, including the

centroids. This means that (particles x rings) for galaxy 1 + (particles x rings)

for galaxy 2 cannot exceed 500, or the program will not allow you to proceed

to the next page.

Note: Tilt and skew for galaxy 1 should always be set to zero if retrotrajectory

is used.

Retrotrajectory

Distance – the distance between the two centroids at perigalacticon (the point

of closest approach in their orbits) in sdu.

Eccentricity – the shape of the galaxies' orbit (a function of velocity). 0 = e is

a circle, 0<e<1 is an ellipse, 1 = e is a parabola, and e>1 is a hyperbola.

A parabolic orbit (e = 1) is standard.

Inclination – the angle the orbital plane the second galaxy makes with the

rotational plane of the first galaxy, in degrees. 0 describes a perfectly

flat interaction, 90 defines a vertical interaction in which the second

galaxy rises directly above the first galaxy, and 180 defines a flat but

retrograde interaction.

Argument – the rotation of galaxy 2's orbit within its orbital plane, in degrees.

Initial Time – how long the simulation starts before closest approach, in stu.

This should always be negative. The simulation is not realistic unless this

number is relatively large, at least 300 in most cases.

Halo Properties

Cent. Density – The relative density of dark matter in the center of the galaxy.

Edge Density – The relative density of dark matter at the edge of the halo.

Massive Radius – The radius of the spherical halo, in sdu.

F2 – Particles: Upon pressing this the simulator will first check if inputs are OK; if not,

an error will be generated. If so, the simulator will proceed to set up the

particles in the simulation. If you plan to use retrotrajectory, make sure you

press F4 before F2, as the inputs in the coordinate/velocity fields are used.

F3 – Exit: Exit the IGMS.

F4 – Retroproject: Based on the retrotrajectory and centroid mass inputs, the

simulator will run the simulation with the centroids in reverse time to determine

the coordinate/velocity data for the centroids. Always run this before continuing

to the next page if you plan to use the retrotrajectory fields.

Page 3 – Particles

From here the user can scroll through a listing of all particles in the simulation

(although there should be no need for this)

Simulator Commands

[Esc] Terminate the simulation and return to the interface.

[space] Pause or unpause the simulation.

[?] Pause and display simulation header information.

[–] Decrease the delay.

[+] Increase the delay.

[ I ] Zoom in by 33%.

[ O ] Zoom out by 50%.

[ ] Move camera down.

[ ¯ ] Move camera up.

[®] Move camera left.

[¬] Move camera right.

[P] Rotate image along y-z plane.

[{] Rotate image along y-z plane.

[L] Rotate image along x-z plane.

[ ; ] Rotate image along x-z plane.

[<] Rotate image along x-y plane.

[>] Rotate image along x-y plane.

[R] Recenter image.

[J] Decrease color intercept (rest vel).

[K] Increase color intercept (rest vel).

[N] Decrease contrast value.

[M] Increase contrast value.

[T] Toggle tracers.

[C] Clear tracers and headers

[S] Save visual data as snapshot file (requires reader software to view)

[A] Pause and display as monochrome color scheme for easy printing.

[PrintScrn] Save on-screen data in memory as a bitmap (load using Paint)

Walkthrough of a basic simulation.

1. Double-click on the IGMS icon.

2. Maximize the screen (if this is not done automatically)

3. Press [à] to open the 'Simulation Name' field for input.

4. Delete the default name and give the simulation a name.

5. If you have a slow computer, scroll down using [¯] to 'Timestep' and

increase the timestep value to 3 by pressing [à] twice. Otherwise

don't change it.

6. Press [F2] to go to the galaxies screen.

7. Scroll down to 'Mass' in the first column and enter '200' (this is a typical

mass for a spiral galaxy)

8. Scroll down to 'Rmin' in the first column and enter '15'.

9. Scroll down to 'Rmax' and enter '50'.

10. Scroll down to 'Rings' and enter '10'.

11. Scroll down to 'Particles' and enter '50'.

12. Scroll to 'Mass' in the second column and enter '100'.

13. Leave the disk property fields empty (0) for now for the second galaxy.

14. Scroll over to the 'Retrotrajectory' fields.

15. Scroll to 'Distance' and enter '90'.

16. Scroll to 'Eccentricity' and enter '1'.

17. Leave 'Inclination' and 'Argument' at zero for now.

18. Scroll to 'Initial Time' and enter '-600'

19. Leave the halo model fields (below) at zero for now.

20. Press [F4] to retroproject. Various numbers should appear in the

coordinate fields for each galaxy (x, y, z, vx, vy, vz). If not, correct

any errors and retry.

21. Press [F2] to set up the particles and go to the next screen. No errors

should appear, if they do, correct any mistakes.

22. Press [F2] again to go back to the 'Properties' page.

23. Press [F1] to check all inputs.

24. Press [F1] to launch the simulation.

25. Press [space] to unpause. A simulation should proceed.

26. When the simulation appears to be over, press [Esc] to terminate.