Monday March 1st 1999
Announcements:

Lecture notes:

Class 22
So far we have talked about our Galaxy and other galaxies out in space.  But we have not talked about overall distribution of
galaxies in space.

Are they just randomly scattered about or is there some
organization?

larger sale - need megaparsec 1 Mpc = 1x10⁶ pc.
Tend to be clustered together in collections ranging from a few to thousands.

Interesting and raises important questions.
How did galaxies get that way?
are the galaxy populations of small clusters the same as those of large clusters?
What holds galaxy clusters together?

Rich galaxy clusters
- contain more than 1000 galaxies in volume that is about 3 Mpc in diameter.
- mostly contain elliptical galaxies - often have a giant elliptical in center.
Examples
coma cluster - 1000 galaxies about 100 Mpc away
Virgo Cluster - 2500 galaxies - giant elliptical in center = M87
18 Mpc away
- gravity holds together - galaxies not stationary.
- also have hot X-ray emitting gas - 1 million K

Poor galaxy clusters
- contain less than 1000 galaxies.
- often irregularly shaped with subcondensations.
Example - the Local Group
- our group of galaxies.
- about 21 galaxies in volume 1 Mpc in diameter.

3 spirals - Milky Way M31 M33
14 elliptical
4 irregular - LMC SMC

Astronomers have noticed that rich clusters have higher proportions of eliptical galaxies than do poor clusters.  An important
clue to why galaxies are what they are - environment affects galaxy structure.

When Galaxies Collide
Galaxies are not stationary in clusters.  They don't move much in a human lifetime but do move quite a bit on timescales of about 1 billion years.  Since universe is about 15 billion years old galaxies will have made about 15 'orbits' in a cluster.
If galaxies tightly packed and moving can they ever smash into each other?  Yes there is strong evidence that galaxies can
collide and that this can strongly affect their evolution.
(Slides shown of colliding galaxies)

When galaxies collide the individual stars almost never hit each other.  Stars separated by about 1 x10⁷ times their diameter.  Like 2 gnats in a baseball stadium.  Galaxies are mostly empty space.

Gas clouds do collide - causes star formation in some cases.

galaxy collisions take 100's of millions of years - cannot watch in real time.  Can make computer models to study how galaxy
collisions work though.

Galactic cannibalism - small galaxies can be captured and pulled apart by big galaxies - galaxy gets spread throughout the big
galaxy.

Our own Galaxy is a cannibal.  1994 - Rodrigo Ibata and collaborators discovered a galaxy being eaten by the
Milky Way called the Sagittarius dwarf about 16 Kpc from center on the far side of the Galaxy.

Also MW is interacting strongly with LMC SMC - irregular since deformed by tidal forces.  Orbital period is about 1x10⁹ years.  Will probably be eaten by MW eventually.

A possible origin for the types of galaxies. It is theorized that - elliptical galaxies are products of
collisions
- spirals have had relatively few collisions.
- average elliptical build up by merger of a number of smaller galaxies.
Can account for so many E in rich clusters - more collisions due to high galaxy density.

Irregulars are smashed remains of galaxies that have just recently gone through collisions.

Complex issue - ideas above probably have a grain of truth but are probably incomplete.