# Interacting Galaxies

 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 "snapshots" of many colliding galaxies computer simulations of colliding galaxies Simulations by Mihos & Hernquist, visualization by Summers. Details here.

## But why the heck do galaxies merge?

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?

## Dynamical Friction

 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?

So as galaxies move through each other's dark matter halos, they feel a frictional force that causes their orbit to decay, and they merge. Put differently the energy and angular momentum of the galaxies orbit gets transferred to the internal energy and spin of the dark matter halos (and to the galaxies' stellar parts, too).

Starbursts: When galaxies interact and merge, their interstellar gas can be compressed and driven to the inner regions through shocks and gravitational torques, triggering intense starbursts.

## And whats's with these tails?

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.

 The Antennae (NGC 4038/39) The Cartwheel

Galaxy Formation and Evolution: Linking the pieces

• Disk galaxies can collide and merge
• When they do so, gas clouds in the disk are driven into the central regions
• This inflow can feed a central black hole, causing it to grow
• The inflow can also drive a central starburst and starburst wind
• These processes convert cold gas into stars and hot gas
• The merging process also scrambles up stars on circular orbits into random orbits

Major mergers are thought to be closely tied to formation of elliptical galaxies. What types of mergers these were is still not clear. An example of a merger-spawned elliptical galaxy in the making:

## Satellite infall

Because of the mass function of galaxies -- little galaxies are much more common than big galaxies -- the typical interaction does not involve two big galaxies, but a galaxy interacting with a smaller companion.

If the satellite is massive, dynamical friction can lead to orbital decay and merging (Hernquist & Mihos 1995):