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

3. 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.

9. 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.

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

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

19. Galaxy merger simulation

21. Centaurus A

22. NGC 5128

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

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

30. 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.

33. 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?

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

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

37. 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.

40. Discussion
How would you go about looking of these things?

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

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

44. 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.

52. Fornax

54. 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.

55. 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

56. 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.

59. LMC a disrupted spiral?

60. SMC dust tail in IR

61. 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?

64. Andromeda galaxy

65. Warped galaxy disk

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

71. HI neutral hydrogen map

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

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