GalCrash Lab Exercise
In this lab, we will use the GalCrash interacting galaxy modeling applet
to study various dynamical aspects of galaxy collisions.
Step 1: Formation of Tidal Tails
Here we will study the formation and evolution of tidal
tails, the long streamers of stars which are ejected when galaxies collide.
Remember that these features are caused by a combination of gravitational
tidal force and the rotation of galaxies.
- Set the companion mass equal to one, so that we have an equal
mass encounter, and run the default simulation. Then, keeping pericenter
distance and galaxy orientation angles constant, change the companion mass
and watch how the tails develop in each case. How does the development
of the tidal tails change as the mass of the companion changes? Why does
this happen?
- Now reset the companion mass to one, and change the pericenter
distance. How do the tails change as you make the encounter more distant?
Why does it change like this?
- Now reset pericenter distance to 10 kpc, and change the inclination
of the galaxy disks (theta, measured with respect to the orbital plane).
Remember now that the simulation is 3-dimensional, so you may need to "grab
and rotate" the simulation to see the tails from different viewing angles.
How does the formation of the tails depend on galaxy inclination? Why does
this happen?
- Many tidal tails appear very thin and linear -- how can you
get those kinds of tails?
Step 2: Merger Dynamics
Now we will look at how the timescale for merging depends
on the parameters of the encounter.
- Set companion mass to one, pericenter distance to 10 kpc,
and inclination of both disks to 0. Run the simulation and write down how
long it takes the galaxies to merge.
- Now change the companion mass, and write down how long it
takes the galaxies to merge, for several different companion masses. How
and why does the merging time depend on mass of the galaxies?
- Reset companion mass to one, and change the pericenter distance.
How and why does the merging time depend on the pericenter distance?
Step 3: (Advanced) Influence of Dark Halos
The dark matter halos have a big effect on the merging
timescale and the evolution of tidal tails in galaxy collisions. The "Big
Halo" option sets the dark matter halo to be four times bigger than in
the standard galaxy models.
- Set companion mass to one, pericenter distance to 10 kpc,
and inclination of both disks to 0. Now click "Big Halo" and run the simulation.
How long does it take the galaxies to merge? Compare this to the standard
encounter -- how has it changed, and why?
- What about the tidal tails -- how have they changed? Why?
Step 4: Real Galaxies
- Here is a collection of real interacting galaxies. Model
3 of them using the GalCrash applet. In each case, explain why you chose
the parameters you did (mass ratio, pericenter, galaxy geometry). Make
sure to make a note of the time at which the simulation matches the observed
galaxy.
- How unique is your solution? What information would you find
helpful to further constrain your solution?
Step 5: Galaxy evolution
- Merge two galaxies together to form an elliptical. What kinds
of mergers make good ellipticals? What kinds make "bad" ones? In what
ways are they good or bad?