Galaxy Classification

1. Galaxy Classification The Categories • Stellar systems • Stellar Clusters • Galaxies • Galaxy Groups and Clusters • Galaxy Superclusters • The Universe • Galaxies are classified by 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 or Barred Spirals • Classified by amount of arms, and how tight or loose they are • S0 - no distinguishable spiral arms • Sa, Sb, Sc, Sd - more spiral arms, and looser

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 • Classified by amount of arms, and how tight or loose they are • SB0 - no distinguishable spiral arms • SBa, SBb, SBc, SBd - more spiral arms, and looser • Milky Way is probably barred spiral SBb

9. SB0 Central Bar Disk • No spiral arms

10. SBa Central Bulge Disk • Tight spiral arms

11. SBb Central Bar Disk • Spiral arms Milky Way?

12. SBc Central Bar Disk • 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 • Elliptical galaxies have the largest range of masses • Dwarf Spheroidal are the smallest • Giant Ellipticals are the largest Q. 93: Only Old Stars in Elliptical Galaxies

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. Spiral and Barred Spiral Galaxy Structure • The disk • The bulge • The nucleus • The halo • Globular clusters Q. 94: Looking For Dark Matter in Spiral Galaxies

27. Dark Matter in Spiral Galaxies • Spirals Rotate • Rotation measured by Doppler shift • Mass, again, not concentrated in the center • 85% of mass is dark matter • Flat rotation curves  dark matter

28. Elliptical Galaxy Structure • The visible part • The nucleus • The halo • Globular clusters

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 85% of the mass of the galaxy is dark matter in the halo

30. Differences - Spirals vs. Ellipticals • 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. Hot Gas Vs. Cool Gas • Hot gas has low density • Low density gas cools slowly • Will not cool off in age of universe • Hot gas has pressure: • Gravity vs. pressure = sphere • An elliptical galaxy • Cool gas has high density • High density gas cools more quickly • Can cool off further in short time • Cool gas has little pressure, but still has rotation • Gravity vs. rotation = disk • A spiral (or barred spiral) galaxy

32. What Determines Galaxy Type? • If we have a source of cool gas: • Gas will form a disk • Disk will form stars • There will be young stars • If all gas is hot: • Gas remains in a giant halo, no disk • No star formation • No young stars • Conclusion: If you have a source of cool gas, you get a spiral or barred spiral, otherwise get elliptical

33. Galaxy Collisions Why They Matter When two galaxies collide or nearly collide, they can affect each other • The stars themselves do not collide – they are too far apart • Clouds of gas can collide • Bursts of star formation • Gas can get fed to the center (nucleus) of the galaxy • Gas can be heated • The gravity from each galaxy affects the other • May distort the shape of the two galaxies • Galaxies may merge Q. 95: The Most Damaging Galaxy Collisions

34. Near Misses • As they pass near each other,the gravity of each distorts theother • The slower they pass, the bigger the effect • If unequal in size, smaller galaxy is affected most • There is “tidal friction” which slows down the motion of the two galaxies • Over time, the two galaxies will move closer and closer with each pass • Eventually, a true collision will occur

35. Galaxy Collisions What happens depends on relative size of the two galaxies Big + Small: • Small galaxy is completely disrupted • Stars slowly absorbed to larger galaxy • This is currently happening to our own galaxy • Sagittarius Dwarf and Canis Major Dwarf - currently getting eaten • Virgo Stellar Stream – Former galaxy being digested Two Equal sized galaxies: • Resultant galaxy will be irregular, initially • Gas will get heated by collision – depending on speed • Galaxies will settle into elliptical or spiral

36. Colliding Galaxies

37. Colliding Galaxies

38. Colliding Galaxies MW - Andromeda Collision

39. Colliding Galaxies