Computer Modeling of Colliding Galaxies:
Physics and Techniques
Models must capture essential physics:
- Gravity (of stars,
gas, dark matter)
- Hydrodynamics (of
- Star Formation (gas
turns into stars)
- Feedback (star formation
These processes are all coupled
Gravity: N-body Models
- Model galaxy as a system of discrete mass points (``superstars'')
which represent the various mass components of the galaxy:
- Stellar Disk
- Gaseous Disk
- Stellar Bulge
- Dark Matter Halo (mass? size? shape?)
- Calculate gravitational acceleration acting on each
particle from the ensemble:
- Better resolution obtained by reducing gravitational
softening and increasing particle number.
- Use hierarchical ``treecode'' for efficient O(NlogN)
scaling. Current simulations use N=10^5 - 10^6 particles, and achieve resolutions
of ~ few hundred parsecs...
Hydrodynamics: Smoothed Particle Hydrodynamics
- A Lagrangian technique well suited for N-body models.
- Gas particles carry information on the local thermo-
and hydrodynamic properties of the gas.
- Properties updated according to the usual hydrodynamic
conservation laws, including artificial viscosity to capture shocks.
- Interpolation procedure allows the properties of the
fluid to be estimated from neighboring particles.
How to relate local properties of the gas to the star formation rate?
Not well understood, but a simple parametrization is the Schmidt (1959)
How to inject energy from supernovae and stellar winds back into the
- Problems with physics
- Problems with modeling techniques
- Problems with resolution
We simply don't know!
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