Discussion Most bright galaxies are spirals. What do you think causes these galaxies to have a spiral pattern?

What makes the spiral pattern? At first glance spiral structure in rotating disk galaxies seems fairly easy to explain. Any disturbance in the gas disk can cause the gravitational collapse of surrounding material. This disturbance will be stretched into a spiral pattern by the rotation of the galaxy.

Density waves The spiral arms are disturbances in the stars and gas and do not carry the stars or gas with them. Instead, the stars and gas rotate faster than the spiral pattern.

Discussion What happens when gas collides with the inside edge of a spiral density wave?

Discussion What observations of spiral galaxies could you make to help confirm that spiral arms are caused by these slow moving density waves?

Galaxy merger simulation

Centaurus A

NGC 5128

Discussion What does the rotation curve of the Milky Way galaxy look like?

Discussion How can we measure the rotation curve for spiral galaxies other than the Milky Way?

Rotation curves of spiral galaxies In spiral galaxies, atomic hydrogen extends to a radius about twice that of the visible stars. By using 21 cm radio emission, we can follow the rotation curve beyond the visible disk of a galaxy.

Discussion It’s rather easy to measure the rotation speed of edge-on disk galaxies. How do you think we might measure the mass of an elliptical galaxy which does not rotate?

Dark matter 90% of the matter in galaxies is in a form we cannot see.

Discussion Can we explain this dark matter in galaxies by assuming that there is a lot of mass in the central black hole? Explain.

MACHO project Massive Compact Halo Objects If the object is compact and massive it will bend and focus background star light. Thus, if one of these happens to pass in front of a background star, that star will appear to brighten and then dim back to normal.

Discussion How would you go about looking of these things?

The results There are not enough MACHO events to explain the missing mass in our galaxy. WIMPs? Weakly Interacting Massive Particles.

Discussion Does the solar system contain large amounts of dark matter? How can we tell?

The Local Group The Local Group contains about 3 dozen galaxies within 1 Mpc. 90% of the mass is in two large spirals, the Andromeda galaxy, the Milky Way and a dwarf spiral M33. The rest of the galaxies are dwarf galaxies.

Fornax

Local Group dynamics While it is easy to get radial velocities of Local Group galaxies, plotting their orbits is impossible. Proper motions are simply too small to measure.

Discussion Is there possibly a way we can measure the amount of matter in the local group using only the radial motions of the galaxies? Hint: assume the galaxies have random orbits

Magellanic Stream We can estimate the orbit of the Magellanic Clouds because they appear to be leaving behind a trail of HI. This gas is most likely tidally pulled from the clouds as the orbit the Milky Way.

LMC a disrupted spiral?

SMC dust tail in IR

Discussion The Magellanic clouds will eventually merge with the Milky Way galaxy. What effect will this have on the obits of the stars in the Milky Way’s disk?

Andromeda galaxy

Warped galaxy disk

Discussion Why do you think the neutral hydrogen gas forms a disk that is much thinner than the disk formed by the stars?

HI neutral hydrogen map

Discussion What is the likely result of a supersonic collision between two clouds of gas? Note that gas clouds do collide in galaxy collisions.

Major Mergers Collisions between similar massed spiral galaxies can convert gas into stars, and randomize orbits, turning a spiral galaxy into an elliptical.