1. Announcements Starry Night Saint-Rémy: June, 1889 Van Gogh Out in the sky the great dark clouds are massing; I look far out into the pregnant night … Paul Laurence Dunbar Ships That Pass in the Night Assigned studying: Chapter 16. Syllabus updates: 1) Details of Final Exam posted. 2) HW #8 will be posted on Thursday 4; due on Thursday 11.

2. Galaxies

3. Survey Questions 1. 1. What are the key characteristics of the four basic types of galaxies? 2. the Hubble Tuning Fork Scheme? 2. What is the Hubble Tuning Fork Scheme? 3. 3. Does the scheme really tell you a formation sequence of galaxies (NO)? 4. 4. How do galaxies form?

4. Galaxies take one of four different morphologies

5. SPIRALS Spirals have a disk with dust and gas: star formation

6. The tightness of a spiral galaxy’s arms is correlated to the size of its nuclear bulge Type SaType SbType Sc The tighter the arms, the bigger the bulge

7. Variety of Spiral Arms Flocculent spirals (fleecy) Grand-design spirals (highly organized)

8. We easily see these spiral arms because they contain numerous bright O and B stars which illuminate dust in the arms. However, stars in total are very evenly distributed throughout the disk.

9. BARRED SPIRALS

10. The bar is not a spiral arm. It is made of old stars, like those in the bulge

11. Bars of stars run through the nuclear bulges of barred spiral galaxies Type SBaType SBbType SBc

12. ELLIPTICALS Type E0Type E3Type E7

13. ELLIPTICALS Ellipticals: have no dust and no cold gas, hence no star formation. Made mostly of old, metal- enriched stars. They like to cluster a lot

14. Discussion Question Given their color, do you expect elliptical galaxies to have more or less new star formation than spirals? Why? 1) ellipticals have more new star formation 2) ellipticals have less new star formation

15. Elliptical galaxies display a variety of sizes and masses Giant elliptical galaxies can be 20 times larger than the Milky Way Giant elliptical galaxies can be 20 times larger than the Milky Way Dwarf elliptical galaxies can be as small as a big globular cluster. They are extremely common and can contain as few as a million stars Dwarf elliptical galaxies can be as small as a big globular cluster. They are extremely common and can contain as few as a million stars

16. IRREGULAR Irregulars mostly have gas, dust and star formation. They contain both old and young stars

17. The classification scheme of galaxy morphology is known as the Hubble Tuning Fork Scheme

18. Galaxies like to cluster: eg. M81 group Optical Image Radio Image

19. Hercules Cluster

20. The ideas of galaxy formation Spiral galaxies form from the collapse of spinning gas cloud inside Dark Matter halos Spiral galaxies form from the collapse of spinning gas cloud inside Dark Matter halos Elliptical galaxies form from the mergers of disk galaxies, or from clouds with low spinning (probably wrong for big galaxies; might work for early galaxies) Elliptical galaxies form from the mergers of disk galaxies, or from clouds with low spinning (probably wrong for big galaxies; might work for early galaxies) Peculiar galaxies are formed through the interactions of galaxies Peculiar galaxies are formed through the interactions of galaxies Most popular theory: CDM (Cold Dark Matter)hierarchical cosmology. Structures form from bottom to top: small ones first, bigger ones later by merging. Most popular theory: CDM (Cold Dark Matter)hierarchical cosmology. Structures form from bottom to top: small ones first, bigger ones later by merging. CDM predicts hierarchical assembly of cosmic structures. Many of its predictions are verified. CDM predicts hierarchical assembly of cosmic structures. Many of its predictions are verified.

23. Billions of stars all tug on each other instead of just one planet tugged by the gravity of the Sun. Galaxy Merging: evolution of morphology, trigger of star formation

24. How two spirals collide and merge, making tidal tails and, at the end, an elliptical galaxy

27. Survey Questions 1. 1. What are the key characteristics of the four basic types of galaxies? 2. the Hubble Tuning Fork Scheme? 2. What is the Hubble Tuning Fork Scheme? 3. 3. Does the scheme really tell you a formation sequence of galaxies? NO!!!!! It is just a morphological classification scheme

28. How did we find Dark Matter: The mass of galaxies and rotation curves M v2 A rotation curve is just a plot of rotational velocity vs distance for objects in (roughly) circular orbits. For a set of objects orbiting a common point, what is the orbital velocity for different objects at different distances from the common point? v1 If you think about it, this is just Kepler’s Third Law applied to galaxies

29. Rotation Curve for a rigid body no gravity involved, no information on mass Straight because objects are rigidly held.

30. Rotation Curve for our Solar System gravity involved, information on mass Curve decreases because mass that generates the gravity is all at the center (Sun is much, much greater than all planets together.

31. Rotation curve of the MW disk (and of all other galaxies) It is essentially Kepler’s Third Law One uses it to measure the mass of galaxies

32. Rotation Curves of Galaxies Observe frequency of spectral lines across a galaxy. From blue / red shift of spectral lines across the galaxy  infer rotational velocity Plot of rotational velocity vs. distance from the center of the galaxy: Rotation Curve

33. Determining the Masses of Galaxies Based on rotation curves, use Kepler’s 3 rd law to infer masses of galaxies.

34. Evidence for Dark Matter Evidence #1 - The amount of mass that we can “see” by counting the stars, the gas and the dust in a galaxy is not enough compared to the observed gravitational mass measured with the rotation curves. Evidence #1 - The amount of mass that we can “see” by counting the stars, the gas and the dust in a galaxy is not enough compared to the observed gravitational mass measured with the rotation curves.

35. Evidence for Dark Matter Evidence #2 - The amount of mass that we can “see” in a cluster of galaxies does not account for the observed gravitational pull on the galaxies in the cluster (as evidenced by the large variation in velocities of the galaxies). Evidence #2 - The amount of mass that we can “see” in a cluster of galaxies does not account for the observed gravitational pull on the galaxies in the cluster (as evidenced by the large variation in velocities of the galaxies).

36. Hercules Cluster Galaxies in a cluster have more erratic velocities than we expect from the amount of matter we can “see” in the cluster

37. Evidence for Dark Matter Evidence #3 - The intracluster gas is too hot to be retained by the gravity of the visible matter in a cluster. Evidence #3 - The intracluster gas is too hot to be retained by the gravity of the visible matter in a cluster. If all the mass there really were only that of visible matter, its gravity would not be enough to retain the hot gas, which would evaporate rapidly. Since the gas is there, there must be more gravity, hence dark matter. If all the mass there really were only that of visible matter, its gravity would not be enough to retain the hot gas, which would evaporate rapidly. Since the gas is there, there must be more gravity, hence dark matter.

38. Cluster of galaxies: Hydra A OpticalX-ray

39. Evidence #4: gravitational lensing Just as black holes and other massive objects curve space around them, galaxies and galaxy clusters curve space. When the alignment of a large mass and background objects is right, a dramatic effect is observed. This is known as a gravitational lens. gravitational lens

40. Lensing by a dense cluster of galaxies. Using Einstein’s laws of general relativity, we can estimate the mass of the lens. About 10 times more mass is present than one would estimate by summing the mass of the visible galaxies.

41. Foreground Cluster of Galaxies Background lensed galaxies

42. Our picture of the mass around galaxies now looks something like this Dark matter outweighs Normal matter on average by 10 to 1

43. So what is this missing mass? The only way to explain the rotation curve of our galaxy is to say that there is lots and lots of mass that is not emitting light. The only way to explain the rotation curve of our galaxy is to say that there is lots and lots of mass that is not emitting light. The halo of our galaxy must be full of it. The halo outweighs the disk by a factor of 10. The halo of our galaxy must be full of it. The halo outweighs the disk by a factor of 10. As far as we can tell, this mass doesn’t emit any light at any frequency. As far as we can tell, this mass doesn’t emit any light at any frequency. What is the form of the missing mass?

44. Dark Matter Possibilities Here is the first lists of candidate materials for the dark matter that dominates the mass in our galaxy. Here is the first lists of candidate materials for the dark matter that dominates the mass in our galaxy. Black Holes Black Holes Black Dwarfs Black Dwarfs Brown Dwarfs Brown Dwarfs Unknown particles Unknown particles Baryonic Matter (e, p, n … the same stuff that we’re made of) Non baryonic matter

45. Does dark matter matter?

46. Yes! Dark Matter Rules the Universe Since dark matter outweighs other matter by a factor of 10, nearly all the large scale gravitational interactions in the Universe are dominated by the dark matter: Since dark matter outweighs other matter by a factor of 10, nearly all the large scale gravitational interactions in the Universe are dominated by the dark matter: Dark matter holds galaxies together – without it the outer stars would escape Dark matter holds galaxies together – without it the outer stars would escape Dark matter holds clusters of galaxies together – without it, the clusters would disperse Dark matter holds clusters of galaxies together – without it, the clusters would disperse Dark matter causes these structures to form in the first place! Dark matter causes these structures to form in the first place!

47. Which of the following is NOT a good reason for learning more about dark matter? dark matter provides the gravitational force that holds galaxies together the total amount of dark matter determines the fate of our Universe the Earth would be destroyed if it ever ran into a clump of cold dark matter dark matter is responsible for the formation of structure in our Universe our understanding of dark matter may reveal a new, undiscovered form of matter

48. What is the best evidence for dark matter in galaxies? 1) the orbital speeds of gas clouds and stars 2) the orbital distances of gas clouds and stars 3) the low density of stars in spiral arms 4) the large orbital eccentricities of disk objects 5) the lack of heavy elements in globular cluster stars

49. Dark Matter Summary 90% of the universe is made of matter that does not emit detectable radiation at any wavelength. 90% of the universe is made of matter that does not emit detectable radiation at any wavelength. Evidence to date suggests that only a very small fraction of the dark matter is made of familiar matter (baryons). Evidence to date suggests that only a very small fraction of the dark matter is made of familiar matter (baryons). Dark matter is not stars or stellar remnants, galaxies, dust clouds, or anything else made of protons, neutrons, or electrons – as far as we can tell, it is an as of yet undiscovered form of matter. Dark matter is not stars or stellar remnants, galaxies, dust clouds, or anything else made of protons, neutrons, or electrons – as far as we can tell, it is an as of yet undiscovered form of matter. This is one of the biggest mysteries facing astronomers – What is our Universe made of?

50. Survey Questions 1. What is a rotation curve of a galaxy? 2. How can a rotation curve be used to measure the mass of the galaxy? 3. Where is most of the gravitational mass in a galaxy? 4. What are the lines of evidence for dark matter in the universe?