Classifying galaxies is hard, and different people do it different ways

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. 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. 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 gravityof 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