1. Galaxy Classification
Classifying galaxies is hard, and different people do it different ways
Except for nearby, we can only see bright galaxies
I will ignore dwarf spheroidal (dSph) and dwarf elliptical (dE) classifications
Galaxies come in four broad categories, based on their appearance
Spiral
Central roundish bulge plus disk
Barred Spiral
Central elongated bulge plus disk
Elliptical
Elongated bulge, no disk
Irregular
No discernible shape

2. Spiral galaxies Pinwheel-like Central Bulge, spiral arms
Spiral arms, etc., signs of rotation
Young and old stars, gas, dust
80% of large galaxies are Spirals
Classified by amount of arms
S0 - no spiral arms
Sa, Sb, Sc, Sd - more spiral arms

3. S0
Central Bulge
Disk
No Spiral Arms

4. Sa
Central Bulge
Disk
Tight spiral arms

5. Sb
Central Bulge
Disk
Spiral arms

6. Sc
Central Bulge
Disk
Loose spiral arms

7. Sd
Central Bulge
Disk
Very loose spiral arms

8. Barred Spirals Like Spirals, but bulge is oblong
Central Bulge, spiral arms
Spiral arms, etc., signs of rotation
Young and old stars, gas, dust
Milky Way is probably barred spiral SBb
Classified by amount of arms
SB0 - no spiral arms
SBa, SBb, SBc, SBd - more spiral arms

9. SB0
Central Bar
Disk
No spiral arms

10. SBa
Central Bar
Disk
Tight spiral arms

11. SBb
Central Bar
Disk
Spiral arms
Milky Way?

12. SBc
Disk
Central Bar
Looser spiral arms

13. SBd
Central Bar
Disk
Very loose spiral arms

14. Elliptical Galaxies Look like a sphere or a flattened sphere
Little gas and dust
Mostly old stars
Classified by how round they look
E0 looks circular
E7 is very elongated

15. E0

16. E1

17. E2

18. E3

19. E4

20. E5

21. E6

22. E7

23. Elliptical Galaxy Shapes
Appearance may depend on angle of view
Amount of flattening probably has to do with rotation

24. Irregular Galaxies Gas, dust, young and old stars
Like a galactic disk, no spirals, a mess
Classified as Irr

25. Hubble’s Tuning Fork
Spirals, Barred Spirals, and Ellipticals fit together in a pattern called the Tuning fork Diagram
S0 - Sa - Sb - Sc - Sd
E0-E1-E2-E3-E4-E5-E6-E7
SB0-SBa-SBb-SBc-SBd

26. What determines a galaxy’s classification?
It’s not size or mass
Rotation!
Rotation measured by Doppler shift
Most mass is dark matter, again
To Observer

27. Spiral Galaxy Structure
Disk
Bulge
Nucleus
Halo
Globular Clusters

28. Elliptical Galaxy Structure
Visible Part
Nucleus
Halo

29. Elliptical Halos
Elliptical galaxies don’t have thick clouds, but they do have diffuse, hot gas
These gasses emit X-rays
Gravity vs. pressure – they expand to make a giant sphere
Amount of gravity tells us 90% of the mass of the galaxy is dark matter in the halo

30. Can we explain these differences?
Differences - Spirals vs. Ellipticals
Spirals appear to have more spin
Spirals have disks and spiral structure
Spirals have dust/gas/young stars in the disk
Ellipticals have hot gas spread out through a large halo
Can we explain these differences?

31. The Effects of Spin Spin causes flattening – formation of a disk
And disks naturally form spiral structure!
Rotation prevents the gas and dust from all falling to the center
In an elliptical galaxy, any loose gas and dust falls into the dense center
Star formation gets very fast
Lots of supernovas
Loose gas gets swept out to halo
No more star formation

32. Near Miss Collisions Galaxy Collisions
When two galaxies collide or nearly collide, they can affect each other
Mostly through gravitational interactions
Near Miss Collisions
As they pass near each other, the gravity of each distorts the other
The slower they pass, the bigger the effect
If unequal in size, smaller galaxy is affected most
Tidal heating – energy is transferred from net motion to internal motion of stars
Star motions get more randomized
Energy comes from kinetic energy of orbit – orbit loses energy and galaxies move closer together
Over time, the two galaxies will move closer and closer with each pass
Eventually, a true collision will occur

33. Actual Galaxy Collisions
What happens depends on relative size of the two galaxies
Big + Small:
Small galaxy is completely disrupted
Stars enter large galaxy
Over time, they get absorbed
This is currently happening to our own galaxy
Sagittarius Dwarf and Canis Major Dwarf – currently being disrupted
Virgo Stellar Stream – a dead galaxy whose stars are being absorbed
Two Equal sized galaxies:
Resultant galaxy will be irregular, initially
Based on total spin, resultant galaxy eventually settles down to spiral or elliptical

34. Colliding Galaxies

39. Giant Elliptical Galaxies
Sometimes many galaxies collide and merge
If large numbers collide, spin cancels out
Giant galaxy becomes an elliptical
Giant Elliptical

40. Looking Out = Looking Back
Light travels at about 0.3 pc per year
The farther away you are looking, the longer ago you are seeing
1 kpc  3.3 ky
1 Mpc  3.3 My
1 Gpc  3.3 Gyr
You can see back almost to the beginning of the Universe!
Galaxies in the past: how do they differ?
Galaxies long ago
Smaller than modern galaxies
Irregulars are more common
Why?
Galaxies collided a lot in the past
Many irregulars from recent collisions
Galaxies got bigger from mergers

41. Galaxies Long Ago

42. Long, Long Ago