The Universe of Galaxies

1. The Universe of Galaxies

2. A Brief History • 1610 - Galileo

3. 1610 - Galileo resolves the Milky Way into stars

4. 1784 – Hershel maps the Milky Way

5. 1784 – Hershel maps the Milky Way

6. 1912 – Leavitt discovers the period-luminosity relation for Cepheid variable stars.

7. This eventually became the key to measuring distances to galaxies

8. 1917 – Shapley measures the Galaxy using globular star clusters..

9. Using star clusters • Instead of 1000 ly across, it is 100,000 ly and the sun is not at the center.

10. The Galactic system

11. The First Extragalactic Nebula

12. The Distance to NGC 6822 • 1925 Hubble measures the distance to NGC 6822 and finds that it is a separate, distant galaxy, 500,000 ly away (the correct distance is 2 million ly)

13. M31

14. The Hubble Deep Field

15. Types of galaxies

16. Ellipticals • Elliptical in shape • Smooth light distribution – no structure • Mostly old stars (red) • Predominate in large clusters

17. Ellipticals • Elliptical in shape • Smooth light distribution – no structure • Mostly old stars (red) • Predominate in large clusters

18. A cluster of elliptical galaxies

19. Spiral galaxies • Sa galaxies • Large central bulge • Tightly wound arms • Arms smooth, dusty

20. Spiral galaxies • Sb galaxies • smaller central bulge • Less tightly wound arms • Arms blue, patchy

21. Spiral galaxies • Sc galaxies • small central bulge • Loosely wound arms • Arms blue, very patchy

22. Barred spiral galaxies • SB galaxies are divided into SBa, SBb, SBc, with similar characteristics to regular spirals, except for a centrally-oriented bar

23. Barred spiral galaxies • SB galaxies are divided into SBa, SBb, SBc, with similar characteristics to regular spirals, except for a centrally-oriented bar

24. Barred spiral galaxies • SB galaxies are divided into SBa, SBb, SBc, with similar characteristics to regular spirals, except for a centrally-oriented bar

25. Irregular galaxies • Irr galaxies • No spiral structure • Blue, many young stars • Patchy light distribution

26. Irregular galaxies

27. Dwarf galaxies • Dwarf galaxies • Low surface brightness • Less than 5000 ly across • Some are irregular, some are elliptical • Various star formation histories

28. NGC 4414

29. Interacting galaxies • Merging galaxies are found in the nearby universe and may have been very common in the early universe. • This is NGC 520

31. “The Mice”

32. Centaurus A

33. Cen A in radio

34. Cen A in x-rays

35. Milky Way mergers • The MW galaxy is tidally interacting with two small galaxies (the MCs) and recently has swallowed two dwarf galaxies. • This is a map of the Sagittarius dwarf, which is falling into the MW.

36. Milky Way mergers

37. Galaxies with active galactic nucleiAGNs

38. An AGN and a normal galaxy

39. Seyfert galaxies

40. The extreme case: quasars

41. 3C273, the first quasar

42. Identified by Schmidt in 1961

43. Quasars are at cosmological distances

44. Studied in detail by Burbidge and others

45. HST showed that quasars are the nuclei of galaxies

46. The Local Group MCs, Sculptor, Fornax, Ursa Minor, Draco, Sagittarius are too close to be identified here

47. Spiral Members • The Milky Way

48. MWG • Type: Sbc • Absolute magnitude: -21.0 • Diameter: 40 kpc • Disk Thickness: ~200 pc, depends on spectral type of stars; thin for massive stars, thick for old, low mass stars • Mass: 2 x 1011 solar masses

49. M31, the Andromeda Galaxy

50. M31 = NGC 224 • Type: Sb • Asolute magnitude: -21.8 • Diameter: 50 kpc + • Active star formation, especially at ~10 kpc • 2 close but separate companions: M32&NGC 205 • Evidence for past mergers • Mass: 4 x 1011 solar masses