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Galaxies – Classifications, Evolution

Galaxies – Classifications, Evolution. Here’s the Story We’ll Unfold for You… Hubble’s galaxy classifications: Spirals, ellipticals , irregulars. Structural parts of galaxies: disk, halo, nucleus, bulge Spiral arms – tracers of star formation Globular clusters, relics of galaxy collisions

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Galaxies – Classifications, Evolution

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  1. Galaxies – Classifications, Evolution Here’s the Story We’ll Unfold for You… Hubble’s galaxy classifications: Spirals, ellipticals, irregulars. Structural parts of galaxies: disk, halo, nucleus, bulge Spiral arms – tracers of star formation Globular clusters, relics of galaxy collisions Elliptical Galaxies, clues to their structure and formation Galaxy mergers Giant Black Holes in Galaxies: Quasars, Seyferts, Radio Galaxies Dark Matter – the dominant mass in the universe

  2. The Hubble Sequence of Galaxy Types.From left to right, the gas+dust fraction goes up, the star formation rate goes up, and the disk/bulge ratio goes up

  3. Hubble Classification Ellipticals: like amped-up globular clusters, and looking a bit squashed as projected onto the sky. S0’s: Bulge + disk, but the disk is faint and has no spiral arms or star formation going on. Spirals: Bulge + disk. Disk has spiral arms and is where the new star formation is happening. Barred Spirals: Same, but additionally have a straight “bar” of stars centered on the center of the galaxy. Milky Way is a barred spiral Irregular: Messy looking galaxies, now known mostly to be galaxies undergoing mergers

  4. A Classic Spiral Galaxy - Andromeda

  5. Spiral gallery

  6. The closest Elliptical Galaxy NGC 205 (lower left); Andromeda’s neighbor

  7. N4013EdgeOnSpiral

  8. EdgeOn Spiral Gallery

  9. HST Sombrero

  10. Sab, n4622

  11. M33 gendler

  12. N3949LateSpiral

  13. N253 late-type spiral

  14. Galaxy Colors: Clue to Stellar Populations • The bulge is reddish – made up of Pop II stars: old stars who’s hot massive members have already died, leaving the cooler red giants or lower mass main sequence stars. • The disk is bluish – made up of Pop I stars and lit up especially by those few, young stellar “superstars” that shine hot and bright and blue.

  15. Spiral arms are bluish from hot young stars, reddish central bulge from old Pop II red giants

  16. Whirlpool

  17. 3 spirals. UltraViolet light from hot young massive stars in the spiral arms

  18. M64 blackeye galaxy

  19. N2787 hst

  20. N4414FloculentSpiral

  21. A Barred Spiral; type SBc

  22. Barred latetype

  23. n4319Barred + seyfert back

  24. Milky Way companion – the Large Magellanic Cloud; a tidally striped dwarf barred spiral?

  25. The Small Magellanic Cloud (and a globular cluster at right).

  26. Dwarf Spheroidal galaxy companions to the Milky Way • Dwarf spheroidals are small, low mass dark-matter dominated galaxies. • The escape velocity from these low-mass systems is low enough to be a problem… • They have few stars because the initial star formation produced supernovae which blew the gas right out of the galaxy, shutting off further star formation. • Dwarf galaxies, whether they are spheroidals or irregulars are the most common type of galaxy in the universe

  27. SagittariusDwarfIrr

  28. Galaxies are fragile! Especially those floppy disks! • And it’s a dense environment out there. Smash-ups do happen. It’s not uncommon. • Near – misses or encounters with dwarfs may just warp your disk… • See lots of slightly disturbed, to badly mangled spiral galaxies in collision in the following slides….

  29. warpedSpiral

  30. Warped disk

  31. CenA

  32. CenAunsharp

  33. CenA Chandra Xray

  34. Cen A core

  35. HicksonGroup

  36. 2SpiralCollisionHST

  37. SuperposedSpirals

  38. Spirals Tidal Tail

  39. N6745 spiral messy collision

  40. NGC 4676 tidal tails

  41. M82 wiyn

  42. StephenQuintet

  43. Ring Galaxies • If a small galaxy has a central collision with a larger spiral galaxy, the gravitational pulse can compress gas/dust and make a star formation burst in a ring. • Collision energy added to the central bulge can stretch it out into a very non-spherica shape.

  44. Ring galaxy collisio

  45. HoagsGal

  46. N4650 polar ring

  47. The Formation of Elliptical Galaxies

  48. After these Messy Collisions Gravitationally Settle Down… • …all the detailed delicate structure of the former galaxies has been blender’ed away • It is a blob of stars, more concentrated in the center, but otherwise pretty featureless

  49. They Can be Non-Round. Can you Guess What Astronomers 40 years Ago Thought as to Why?

  50. They figured they were just rotationally flattened objects, like spiral disks but “fatter” in thickness. • But there was an interesting test to see if this were true. • First, consider that we only see the full 3-dimensional shape of these galaxies as it projects onto the 2-dimensional sky. • What are the 3 possible types of shapes that E galaxies can have and look like they do on photographs?

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