1. Rotation

Galaxy disks

Recall: not solid-body rotation

(i.e. not like a frisbee)

differential rotation

What determines orbital speed?

Mass enclosed in volume interior to orbit

Recall Kepler's 3rd law:

For Solar System, Sun is virtually ``all'' the mass

Planets' orbital speeds decrease with orbital radius (see lecture 30, section 3):

orbital speed 1/

But!

For a galaxy, there is substantial mass at all radii, so we expect orbital velocities of stars and gas to change differently with orbital radius in a galaxy.

Rotation Curve:

the run of orbital speed with radius in a galaxy

2. Mass

From Kepler's 3rd law:

mass (orbital radius)³ / (orbital period)²

distance³ / time²

Recall:

speed = distance / time

mass (orbital radius) x (speed)²

mass interior to orbital radius

What do we find, using rotation curves and galaxy bulk motions?

Galaxies weigh 10¹¹-10¹² solar masses (M)

Galaxy clusters weigh 10¹³-10¹⁵ M

There is a problem:

We count up the stars, and we don't find this much matter:

. . . not in stars,

. . . not in cold or hot gas

. . . not in dust

There must be dark halos!

3. Dark Matter

``The Dark Side ....''

What makes up dark matter?

brown dwarfs (failed stars -- Jupiters)?

cooled white dwarfs ?

cooled neutron stars?

black holes?

or ...

``Shadow'' matter

exotic fundamental particles that only interact via gravitation

- last refuge of scoundrels (theorists)

Regardless,

90% of matter in the Universe doesn't shine!

An outstanding puzzle.

How do we detect it?

(See Chap. 17.8 for one example:
gravitational micro-lensing)

4. Interactions

Glancing blows

Mergers

Computer simulations:

- initial conditions:

spatial distribution and velocities of stars in galaxies;
bulk velocities of galaxies

- stars modeled as collisionless particles

- pure gravitational interactions

What's missing? (dust, gas ... the sticky stuff)

Results:

Tidal forces and disruption

Tidal tails

``like a hose'' ? .... not quite

Galactic cannabalism: define?

The one way to transform galaxies along the Hubble Sequence (sort of)

Input: any kind of galaxy, as long as there are enough of them . . .

. . .shake well and stir . . .

giant elliptical

Only found at the centers of galaxy clusters.

Why?

(a) stars are following Keplerian orbits

(b) there is substantial mass at large galactic radii

(c) all mass is contained within an interior radius

(d) there is little or no mass at large radii

(e) all mass is contained outside an interior radius

(a) clusters

(b) groups

(c) pairs

(d) isolated in space

(e) isolated in space and in pairs