T11: colliding Galaxis

We are going to use live galaxies and simulate there evolution (e.g. galaxy-galaxy mergers).

Before the tutorials:

Do, think about, and research the following:

Bring your own program to the level of the example given for the last tutorial, so that you can start from a clean and working version.
What are the main components a galaxy ?
What would be typical values for a collision, e.g. distances and velocities ?
What do you expect to see when two galaxies collide ?

During the tutorials:

Program it!

Read two galaxies (can be still the same ones) and set them up to a merger. You can take some example galaxies (e.g. best start first with the smallest ones):
v160_c12_l05_1e3.ic.ascii, makegalaxy_1e3.par
v160_c12_l05_3e3.ic.ascii, makegalaxy_3e3.par
v160_c12_l05_1e4.ic.ascii, makegalaxy_1e4.par
Hint: Switch the internal units to
m_unit = 1.989e43; // 1e10 Msol
l_unit = 3.085678e21; // kpc
v_unit = 1e5; // km/s
t_unit = l_unit / v_unit;
You can try different impact parameters.
Try to plot the different components of the galaxies.
What's about rotating the galaxies to create more general merger scenarios ?

Example programs:

These are just examples.

C/C++:
- Current version (making extensively use of C++ features):
  example.cc (colorized), example.h (colorized), Makefile (colorized) (rename to “Makefile” or use with “make -f Makefile_c”)
F90:
- Simple version (this is sufficient for the moment, but it doesn’t hurt you to start improving it as suggested above):
  T10.f90 (colorized), Makefile (colorized) (rename to “Makefile” or use with “make -f Makefile_gfortran”)
- More complex version:
  (To be added later. We will reorganize the program structure when we get to multiple particles; think about how you would do this on your own.)

Discussion of the results:

Plot your results with gnuplot and plot the behaviour of the systems. Can you explain what you see ?

Example plots:

Extra programming goals for T11:

Your program now propagates the whole system always at the lowest time-step, which is dominated by the most dense part of your simulation. How to speed up your program ?

First think about how to re-structure your program to have the particles on individual time-steps. It is not to complicated, but you have to adapt the different subroutines.
example2.cc, example2.h
However, this does not really help, as the drift of the system now has to be called extremely often (compared to before). So next thing is to introduce integer time-steps, so that again you group particles at different time-steps. Here it gets even useful to define integer time-steps as power of two values to even better group your different time-steps.
example3.cc, example3.h

Further references:

We used a public available program makegalaxy to setup the galaxies, which can be downloaded from here.
To do more elaborated simulation, you can use a public version of P-Gadget, see here