1. COLLECTIONS OF STARS, GAS (and DARK MATTER): HUGE VARIETY OF TYPES
NORMAL GALAXIES
COLLECTIONS OF STARS, GAS (and DARK MATTER): HUGE VARIETY OF TYPES

2. What are the three major types of galaxies?

3. Hubble Ultra Deep Field

4. Hubble Ultra Deep Field

5. Hubble Ultra Deep Field
Spiral Galaxy

6. Hubble Ultra Deep Field
Spiral Galaxy

7. Hubble Ultra Deep Field
Elliptical Galaxy
Elliptical Galaxy
Spiral Galaxy

8. Hubble Ultra Deep Field
Elliptical Galaxy
Elliptical Galaxy
Spiral Galaxy

9. Hubble Ultra Deep Field
Elliptical Galaxy
Elliptical Galaxy
Irregular Galaxies
Spiral Galaxy

10. Basic Galactic Facts
SIZES kpc kpc across M M dwarf galaxies down to 107 M
BASIC STRUCTURES SPIRAL -- Milky Way & most overall
ELLIPTICAL -- most dwarfs and giants
IRREGULARS -- often ``satellites'’
LOCATIONS Isolated (Field) Groups (Local group includes ~50) Clusters (100s to 1000s of galaxies) Superclusters (clusters of clusters!)
Voids (galaxies not there)

11. ELLIPTICAL GALAXIES
Ellipticity: E0 -- E7 (round to flatest) Projected on the sky:
# = (1 - b/a) x 10
Can be more elliptical than they are seen to be (Projection Effect)
Often really tri-axial
Some (e.g. Centaurus A) include dust disk -- big elliptical swallowed a spiral!

12. Elliptical Galaxy Shapes
M49 is close to a circle: E2
M84 is an “average” E3
M110 is a dwarf E (Andromeda satellite): E5

13. SPIRAL GALAXIES
Classify by Hubble Type: S0; Sa--Sc; SBa--SBc (tuning fork diagram)
S0: disk seen, but no spiral arms
Sa: prominent nucleus, tightly wound arms
Sb: significant nucleus, moderate arms
Sc: small nucleus, patchy loose arms
SBa, SBb, SBc: central bar from which arms emerge
Milky Way is a SBb (weak bar though, and between SBb and SBc)

14. “Regular” Spiral Types

15. M101-pinwheel galaxy 51 HST images

16. Barred Spiral Types

17. S0 (lenticular) Galaxies
S0 and SB0 have disk and bulge but no visible spiral arms

18. Irregular Galaxies: Magellanic Clouds

19. Irregular Galaxies: Interactions and Starbursts

20. Key Info on Galaxy Types
Colors: E's red, S's various, Irr's, blue
Populations: E: Pop II Irr: Pop I
S: disk, Pops I+II; halo, Pop II
Sizes: E's kpc, S's: 3-50 kpc; Irr's 1-15 kpc
# of Stars: Ellipicals: 107 (dwarfs) to 1013 (cD central dominant) Spirals: Irregulars:

21. Gas Content and Star Formation
Gas Content: E: up to 30% of ordinary matter (baryonic) mass, but very hot (T > 107 K) S: typically 5-15% of baryonic mass, in many ISM phases (10 K < T < 106 K) Irr: typically 20-50% of baryonic mass, in many ISM phases
Star Formation: E: very little, if any, currently (so RED) S: moderate amount in disk (some BLUE with many YELLOW and RED) Irr: often lots currently (so BLUE)

22. Thought Question
Why does ongoing star formation lead to a blue-white appearance?
A. There aren’t any red or yellow stars
B. Short-lived blue stars outshine others
C. Gas in the disk scatters blue light

23. Thought Question
Why does ongoing star formation lead to a blue-white appearance?
A. There aren’t any red or yellow stars
B. Short-lived blue stars outshine others
C. Gas in the disk scatters blue light

24. What Determines Galactic Shapes?
The quasi-spherical shapes of Ellipticals as well as halo and bulge stars in spirals arise from their stars' original RANDOM VELOCITIES.
The orbits of stars in spiral galaxies come from stars mainly forming in a flattened disk, supported by ROTATION.
The odd shapes of Irregulars are not well understood but many probably arise from tidal distortion by bigger galaxies.

25. Formation of a Spiral Galaxy

26. Summary: The Hubble Sequence
Like stars on the Main Sequence galaxies are born into a type in the Hubble Sequence and they DO NOT usually move along the Hubble Sequence.

27. Mergers and Cannabalism
The biggest E and S galaxies have almost certainly MERGED WITH comparable sized galaxies, or CANNABALIZED several smaller galaxies over billions of years.
Typically S+S  E, S+E  E, E+E  E So as the universe ages the fractions of: S's goes down, E's up.
BUT sometimes mergers induce more star formation and spiral disks
Cen A: new dust disk from
S swallowed by big E

28. Where are they found?
Most Es (except dwarfs) near CENTERS OF CLUSTERS.
Most S's: in the FIELD or toward EDGES OF CLUSTERS.
Irr's locations are less well known; probably like Spirals.
Coma Cluster about 100 Mpc away

29. DISTRIBUTION OF GALAXIES
Most galaxies are in clusters;
Most clusters are part of superclusters.
Our Local Group has about 50 members. MW + LMC, SMC, Draco, Fornax, Sculptor, Leo etc is one sub-group; Andromeda (M31) + M32, M33, NGC and more is another sub-group
Total extent about 1 Mpc (M31 is 700 kpc from MW)

30. The Local Group

31. Some Properties of CLUSTERS
The nearest CLUSTER is the Virgo Cluster, about 15 Mpc away;
Clusters vary in size and richness, from 100 up to over 5000 galaxies.
Within clusters, E's and S0's dominate the central parts (90% or so) but S's and SB's dominate the outskirts of clusters.
Start here on 4/9

32. Cluster Merger Movie
Many clusters grow through mergers of smaller clusters
Some clusters are still growing today
Collisions can heat gas in clusters (intracluster medium) to ~108K, giving off X-rays
START HERE ON 11/28

33. THE COSMIC DISTANCE LADDER
Review of VARIABLE STARS
Giants and supergiants will PULSATE in the INSTABILITY STRIP; above the MS for A and F stars, where variations in He opacity drive increases and decreases in R and T.
Some variable stars calibrate distances All RR LYRAE stars are nearly the same luminosity, some 70 times the Sun's. Periods between 2 and 24 hours.
CEPHEID VARIABLES have luminosites proportional to their periods from ~200 L (for 1 day) to ~10000 L (for 50 days)
Both are "STANDARD CANDLES" that allow DISTANCE DETERMINATIONS to NEARBY CLUSTERS and many, relatively nearby GALAXIES.
Start here on 11/27

34. Next Step: Tully-Fisher Relation
There is a very strong correlation between rotational speeds and luminosities for Sc galaxies. Why?
Roughly: rotation speed ~ mass ~ luminosity
Measure brightness and estimate luminosity, then distance.
The 21 cm H I line is broader in the faster rotating galaxies; IR magnitudes give better estimates of total brightness
This Tully-Fisher relation is good out to 200 Mpc!

35. Cosmic Distance Ladder, Illustrated Type 1a SNe take us out beyond 1 Gpc
Start here on 11/10/09

36. White-dwarf supernovae can also be used as standard candles
Type Ia SNe as Standard Candle

37. Apparent brightness of white-dwarf supernova tells us the distance to its galaxy
(up to 10 billion light-years)

38. HUBBLE's LAW
Back in 1920's Edwin Hubble found that nearly all galaxies showed REDSHIFTS!
Even more interesting, the fainter the galaxy, therefore, probably the more distant the galaxy, the greater the redshift.
When distances (r) were calibrated using Cepheid variables, Hubble found:
v = H0 r where v = c (/) is the “expansion velocity” and z = / is the redshift. (So v = cz)

39. Galaxy Spectra and Hubble’s Law
Discover Hubble's Law

40. Using Hubble’s Law
If one knows enough galaxy distances from independent measurements and has z's for all of those galaxies then one gets a value for
H0 = average of all (v/r) measurements.
This yields: 50 < H0 < 100 km/s/Mpc and most likely, H0 = 72 km/s/Mpc (with an error of 3 km/s/Mpc)
An example: say a line of 5000 Å is seen at 5500 Å
z = / = (5500 Å Å)/5000 Å = So v = cz = 0.10 x 3.00 x 105 km/s = 3.00 x 104 km/s If H0 = 75 km/s/Mpc, then
r = v/ H0 = (30,000 km/s)/(75 km/s/Mpc) = 400 Mpc

41. Distances between faraway galaxies change while light travels
Cause of Hubble's Law
Distances between faraway galaxies change while light travels
Astronomers think in terms of lookback time rather than distance
distance?

42. Copernican Principle, Expanded
VERY IMPORTANT POINT: The expansion of the Universe shown by the Hubble Law should be independent of location in the Universe. EVERYONE WOULD SEE AN EQUIVALENT EXPANSION AWAY FROM THEM.
In other words, we do not believe we are at a “special” place in the universe.