2. Homework #10 is due
Wednesday, April 25, 9:00 pm.
Review session
Wednesday, April 25, 7:15 pm.

3. What are quasars?
Active galactic nuclei are very bright objects seen in the centers of some galaxies, and quasars are the most luminous type
What is the power source for quasars and other active galactic nuclei?
The only model that adequately explains the observations holds that supermassive black holes are the power source
Do supermassive black holes really exist?
Observations of stars and gas clouds orbiting at the centers of galaxies indicate that many galaxies, and perhaps all of them, have supermassive black holes

4. Dark matter and dark energy

5. Unseen Influences
Dark Matter: An undetected form of mass that emits little or no light but whose existence we infer from its gravitational influence on galaxies and groups of galaxies
Dark Energy: An unknown form of energy that seems to be the source of a repulsive force causing the expansion of the universe to accelerate

6. Contents of Universe “Normal” Matter: ~ 4.4% Dark Matter: ~ 23%
Normal Matter inside stars: ~ 0.6%
Normal Matter outside stars: ~ 3.8%
Dark Matter: ~ 23%
Dark Energy ~ 73%

7. The Evidence for Dark Matter

8. Galaxy Motions in a Cluster of Galaxies: The “Missing Mass” Problem
Clusters of galaxies are gravitationally bound - they do not “evaporate” due to galaxies moving at greater than the escape velocity.
Motions of galaxies tell us how much mass is needed to gravitationally “bind” the cluster
Adding up all the visible matter seen in the cluster does not yield enough mass to bind it.
The Coma Cluster 8

9. Although a problem, it didn’t seem serious,
Motions of nearby stars in a direction perpendicular to the disk of the Milky Way also indicated more mass than is seen.
The Missing Mass problem remained a curiosity for the next four decades.
Although a problem, it didn’t seem serious,
Then along came Vera Rubin and things became much more serious. 9

10. Rotation Curves: rotational (orbital) velocity versus radius
“solid body” rotation
Solar system’s (“Keplerian”) rotation curve declines because Sun has almost all the mass
Spiral galaxy’s rotation curve becomes flat at distances from the center where little to no light is seen 10

11. The rotational (orbital) velocity of a spiral galaxy at a given radius gives a measure of the mass of the galaxy interior to that radius.
Mass within Sun’s orbit:
1.0 x 1011 MSun
Total mass:
~1012 MSun

12. M31 – “Flat” Rotation Curve
We expected that the rotation curve of spiral galaxies would become Keplerian outside the visible range of the galaxy, since it was assumed that the visible edge marked the limit of where mass existed in these galaxies.
Twasn’t so!!
M31 – “Flat” Rotation Curve

13. Flat Rotation Curves are found in almost all spiral galaxies
NGC 3198
Rotational velocities remain roughly constant far beyond where stars are visible 13

14. Most of the mass in spiral galaxies seems to be dark matter!
CONCLUSION: Mass in spiral galaxies is spread out over a larger region than the stars
Most of the mass in spiral galaxies seems to be dark matter!

15. The visible portion of a galaxy lies deep in the heart of a large halo of dark matter

16. Broadening of spectral lines in elliptical galaxies tells us how fast the stars are orbiting
These galaxies also have dark matter

17. Dark Matter in Elliptical Galaxies –
Details of X-ray emissions from hot gas show how strong the gravity must be to keep this gas from escaping from the galaxy
Broadening of spectral lines tells us how fast the stars are orbiting
Both support the existence of dark matter in elliptical galaxies
NGC 4697 optical
X-ray 17

18. Dark matter in clusters:
We can measure the velocities of galaxies in a cluster from their Doppler shifts

19. The mass we find from galaxy motions in a cluster is about
50 times larger than the mass in stars!

20. Clusters contain large amounts of X-ray emitting hot gas
Temperature of hot gas (particle motions) tells us cluster mass:
85% dark matter
13% hot gas
2% stars

21. Gravitational lensing, the bending of light rays by gravity, can also tell us a cluster’s mass

22. All methods of measuring cluster mass indicate similar amounts of dark matter

23. Our Options
Dark matter really exists, and we are observing the effects of its gravitational attraction
Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter

24. Our Options
Dark matter really exists, and we are observing the effects of its gravitational attraction
Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter
Because gravity is so well tested, most astronomers prefer option #1

25. Some observations of the universe are very difficult to explain without dark matter

26. What is dark matter?

27. Two Basic Options: MACHOS & WIMPS
Ordinary Dark Matter (MACHOS)
Massive Compact Halo Objects:
dead or failed stars in halos of galaxies
Extraordinary Dark Matter (WIMPS)
Weakly Interacting Massive Particles:
mysterious neutrino-like particles

28. Two Basic Options Ordinary Dark Matter (MACHOS)
Massive Compact Halo Objects:
dead or failed stars in halos of galaxies
Extraordinary Dark Matter (WIMPS)
Weakly Interacting Massive Particles:
mysterious neutrino-like particles
The Best Bet

29. Why Believe in WIMPs? There’s not enough ordinary matter
WIMPs could be left over from Big Bang
Models involving WIMPs explain how galaxy formation works

30. What is the evidence for dark matter in galaxies?
Rotation curves of galaxies are flat, indicating that most of their matter lies outside their visible regions
What is the evidence for dark matter in clusters of galaxies?
Masses measured from galaxy motions, temperature of hot gas, and gravitational lensing all indicate that the vast majority of matter in clusters is dark

31. Does dark matter really exist?
Either dark matter exists or our understanding of our gravity must be revised
What might dark matter be made of?
There does not seem to be enough normal (baryonic/MACHO) matter to account for all the dark matter, so most astronomers suspect that dark matter is made of (non-baryonic) particles that have not yet been discovered

32. Dark matter and galaxy formation

33. Gravity of dark matter is what caused protogalactic clouds to begin their contract early in time, i.e., dark matter provided the seeds for galaxy formation

34. WIMPs can’t contract to centers of protogalaxies because they don’t radiate away their orbital energy
They produce dark matter halos within which the luminous galaxy is built

35. Dark matter is still pulling things together
After correcting for Hubble’s Law, we can see that galaxies are flowing toward the densest regions of space
This produces immense structures

36. Maps of galaxy positions reveal extremely large structures: superclusters and voids

37. Time in billions of years
0.5
2.2
5.9
8.6
13.7 13 35 70 93
140
Size of expanding box in millions of lt-yrs
Models show that gravity of dark matter pulls mass into denser regions – universe grows lumpier with time

38. Structures in galaxy maps look very similar to the ones found in models in which dark matter is WIMPs

39. What is the role of dark matter in galaxy formation?
The gravity of dark matter seems to be what drew gas together into protogalactic clouds, initiating the process of galaxy formation
What are the largest structures in the universe?
Galaxies appear to be distributed in gigantic chains and sheets that surround great voids

40. Will the universe continue expanding forever?

41. Does the universe have enough kinetic energy to escape its own gravitational pull?

42. Fate of universe depends on the amount of dark matter

43. Observations indicate that the amount of dark matter is ~25% of the critical density, suggesting that the fate of the Universe is eternal expansion

44. But expansion appears to be speeding up!
Dark
Energy?

45. Estimated age of universe depends on both dark matter and dark energy