|When we look in the
sky, we see many
galaxies which come in pairs and are often morphologically very
peculiar. Time scale for major interactions ~ 500 Myr - 1 Gyr or so.
Can't watch individual galaxies collide and evolve; to understand how they change, we need to rely on
Simulations by Mihos & Hernquist, visualization by Summers. Details here.
Galaxies can orbit each other just like comets orbit the sun. Comets don't spiral into the sun, but galaxies can spiral together to merge. What's the difference?
- Why don't comets spiral into the Sun?
- Why do low-orbit satellites eventually spiral in towards the Earth?
- How does this relate to merging galaxies?
Imagine a massive object moving through a background "sea" of low mass objects. As it moves through, it creates a trailing "wake" -- an excess in the density of the low mass objects behind it.
Why would this act like a frictional force in the motion of the massive object?
What is the massive object?
What is the sea?
Think back to gravitational tidal forces: they act to radially stretch anything passing near a massive object. Couple that with the fact that galaxies are only bound by gravity, and that they are rotating, and we can see that tails for from material "spun off" by gravitational forces during collisions.
The shape of tails can be affected by:
- Mass ratio of the two galaxies
- Time since interaction
- Encounter/disk geometry
- Viewing angle
Therefore, the tails hold a "archeological record" of the encounter, although they can be tricky to decipher.
Look at the GalCrash applet to explore these effects. Try making these galaxies:
|The Antennae (NGC 4038/39)
Galaxy Formation and Evolution: Linking the pieces