1. Large Scale Structure and Dark Matter
Most Galaxies are in Clusters
Clusters are in Superclusters, Sheets and Filaments
Most of the Matter in the Universe is Dark -- not Baryons

2. CLUSTERS OF GALAXIES
Recall that in clusters of galaxies: E's and S0's dominate the central parts (90% or so) but S's and SB's dominate the outskirts of clusters.
Why this segregation within clusters?
Central regions are denser, more collisions between galaxies
Such collisions easily turn S's into E's but rarely turn E's into S's
Also, ram pressure stripping by the intracluster medium or ICM removes much gas from galaxies moving through the central cluster.
The ICM can also bend radio jets by ram pressure yielding Wide Angle Tail or Head-Tail sources.

3. Head-Tail Radio Galaxy NGC 1265

4. Intracluster Medium Properties
VERY HIGH temperatures: 50 to 100 million K
VERY LOW densities: n ~ cm-3
BUT volumes are so large (cubic Mpc’s) that the ICM contains A LOT of MASS -- often greater than the mass in the cluster’s galaxies!
Also, it is enriched, having significant iron content, implying much of the ICM was once in stars in galaxies: processed material.
This gas can be detected through the X-rays it emits.
Start here on 11/17/09

5. X-rays from Virgo Cluster ICM (optical plus ROSAT)

6. Virgo Cluster in Optical

7. X-rays from Galaxy Clusters:Lots of ICM (with IR images of galaxies)

8. Distant Rich Cluster: Abell 1689 ~2 Gpc away

9. Cluster Merger Movie
When galaxies or clusters merge, stars don’t collide, but gas does form shocks and heats up

10. CLUSTER MASSES Found in FOUR WAYS
Sum of Galaxies Mass: add up all individual galaxy masses based on their luminosities.
Virial Mass, determined from velocities of galaxies and size of cluster: how much mass needed to hold it together.
Gravitational Lensing Mass: look for distorted images of quasars or galaxies BEHIND a cluster and use their shapes to figure out the total mass of the cluster
Temperature of Hot Gas: tells us how much mass is needed to hold the gas in the cluster

11. Virial Masses for Clusters
The Virial Mass always greatly exceeds the Sum of Galaxies Mass.
Even when ICM mass (comparable to Sum of Galaxies Mass) is added, the total of KNOWN MASSES is MUCH LESS THAN VIRIAL MASS: say ~1014Mluminous vs ~1015Mvirial
This implies LOTS OF DARK MATTER or MISSING MASS on this large scale

12. Gravitational Lensing
Double image: theory and observed--quasar AC114

13. Galaxy Cluster Lensing: Weigh All of It
A 2218 produces over 100 arc images of more distant galaxies
produces multiple (blue) images of 1 background galaxy Graviational Lensing Movie

14. Mapping Extended DM through Gravitational Lensing
Small cluster near center; blue smears are distorted distant
galaxies. Deduced DM distribution in and around cluster.

15. 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

16. Dark Matter in Individual Galaxies
Flat rotation curves also let us know that the outer parts of individual galaxies are dominated by Dark Matter: Mass ~proportional to Radius

17. Thought Question
What would you conclude about a galaxy whose rotational velocity rises steadily with distance beyond the visible part of its disk?
A. Its mass is concentrated at the center
B. It rotates like the solar system
C. It’s especially rich in dark matter
D. It’s just like the Milky Way

18. Thought Question
What would you conclude about a galaxy whose rotational velocity rises steadily with distance beyond the visible part of its disk?
A. Its mass is concentrated at the center
B. It rotates like the solar system
C. It’s especially rich in dark matter
D. It’s just like the Milky Way

19. Mostly Dark Galaxies?
The stream to the right of UGC may have been ripped out by a low mass DM “galaxy”

20. Our Options for “Missing Mass”
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

21. Our Options for “Missing Mass”
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

22. Two Basic DM Possibilities
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

23. Two Basic DM Possibilities
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

24. Why Believe in WIMPs? There’s not enough ordinary matter
WIMPs could be left over from Big Bang, when many exotic particles are expected, though details aren’t clear
Models involving WIMPs explain how galaxy formation works

25. LARGE-SCALE STRUCTURE
Clusters of galaxies are mostly part of SUPERCLUSTERS.
Local SC
Hydra-Centaurus SC
Perseus-Pisces SC
The Great Wall
These SCs are often arrayed in FILAMENTS, basically the intersections of SHEETS of galaxies, which surround VOIDS, with very few galaxies.
These structures have sizes of Mpc: the structure of VISIBLE MATTER is FROTHY-- a good analogy is the head on a glass of beer.

26. Local Supercluster Our galaxy and
Local Group would be ~20 Mpc above the page

27. 3-D Structure from Redshift Surveys: Hydra-Centaurus & Perseus-Pegasus SCs

28. Filaments and Voids: 10-50 Mpc Scales
Great Wall -- a 6 degree slice of CfA survey

29. The Largest Scales Mapped Las Campanas Survey shows voids and walls out to Mpc scales Structure Simulation

30. WIMPs can’t contract to center because they don’t radiate away their orbital energy.
So most DM on outskirts of visible galaxy
This agrees with rotation curves, clusters and lensing

31. 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

32. 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

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

34. Galaxy Formation and Evolution
Some stars formed w/in 400 Myr after Big Bang
Some quasars also formed very early, with SMBH at their centers
The radiation of stars and quasars reheated gas between galaxies
Smaller galaxies often formed first, in densest concentrations of DM
Usually these smaller lumps merged into bigger galaxies (“bottom-up” growth of structures)
MOST bigger galaxies today resulted from mergers
Clusters and then superclusters grew via gravity, most of it due to DM

35. Hubble Deep Field: “Young Galaxies”
Immense no. of small irregular galaxies have high redshifts: so most distant and seen at earliest stages of formation

36. Galactic Growth, illustrated

37. Starbursts From Mergers: often Spirals turn into Ellipticals
Very often these mergers can also feed the SMBH at the center, (re)igniting the AGN.
Typically these starbursts and active episodes last < 100 Myr

38. Quasar Host Galaxies
At redshifts > 3, when the universe was only a few Gyr old, many quasars were already in irregular, forming galaxies

39. Merging Galactic Nuclei
2 galaxies being eaten by cD in Abell 2199
Big BHs at cores of merged galaxies NGC 6420 (Hubble +Chandra)

40. Possible AGN/Galaxy Co-Evolution

41. Most (all?) Galaxies Have Central SMBHs
Roughly, SMBH mass is of the BULGE mass;
for ellipticals, the bulge mass is the entire star mass; for most spirals, just a fraction.
So BH and galaxy probably grow up together