Galaxies

1. Galaxies • Read Your Textbook: Foundations of Astronomy • Chapter 16, 17 • Homework Problems Chapter 16 • Review Questions: 1, 2, 5-7, 10 • Review Problems: 1, 5, 9, 10 • Web Inquiries: • Homework Problems Chapter 17 • Review Questions: 2, 4, 7, 8-10 • Review Problems: 1, 5, 9, 10 • Web Inquiries:

2. Galaxy Types • Ellipticals (Triaxial ellipsoids) • Lacking significant star formation

3. Ellipticals E0 through E9 E0 more spherical, E9 more elliptical (cigar)

4. Galaxy Types • Spirals (Disks) • Significant star formation, gas, dust

5. Spiral Galaxies S0 through S9 S0 spiral arms not well defined S9 spiral arms very well defined

6. Spiral Galaxies Sa, Sb, Sc Sa tightly wound spiral arms Sb less so Sc barely wrapped spiral arms

7. Barred Galaxies • Barred Spirals

8. Barred Galaxies SBa, SBb, SBc (Spiral Arm winding)

9. Barred Galaxies • Barred Spirals

10. Galaxy Types • Irregulars (includes those that are interacting) • Lots of star formation, gas and dust

11. Interacting • Interacting (Irregular) • Mergers

12. Hubble “Tuning Fork” Classification

13. Milky Way Map Our view from within our own galaxy home.

14. Hydrogen 21-cm Radio Image

15. The Galactic Center Picture (c): Home of a supermassive black hole?

16. You Are Here

17. Disk and Halo Stars

18. Population I and II Stars • Population II Stars (Halo objects): • Older stars • “First or Second” generation • Metal poor chemical composition (Anything else but H, He) • Large space velocities relative to the sun • High inclination orbits • Population I Stars (Disk objects): • Younger stars (solar-type) • “> Second” generation • Metal rich chemical composition (Formed from enriched ISM) • Small space velocities relative to the sun • Low inclination orbits

19. Galaxy Formation Population II stars: formed first spherically distributed globular clusters Population I stars: formed later disk distribution open clusters

20. Galaxy Schematic

21. Solar Galactic Orbit

22. Spiral Arm Structure Spiral structure viewed as the precession of elliptical galactic orbits creating density waves.

23. Spiral Density Waves

24. Galaxy Rotation Invariably, it is found that the stellar rotational velocities remain constant, or "flat", with increasing distance away from the galactic center. This result is highly counterintuitive since, based on Newton's law of gravity, the rotational velocity would steadily decrease for stars further away from the galactic center. Analogously, inner planets within the Solar System travel more quickly about the Sun than do the outer planets (e.g. the Earth travels around the sun at about 100,000 km/hr while Saturn, which is further out, travels at only one third this speed). Kepler’s Third Law: P2 ~ a3 One way to speed up the outer planets would be to add more mass to the solar system, between the planets.

25. Galactic Rotation Curve

26. Galaxy Rotation Curves

27. Dark Matter • There is a LOT of non-luminous matter. • Gravitationally, our observations show that the universe is almost 90% non-luminous matter! • 90% of the universe is made up of stuff we can not see! • Solar System Mass to Light Ratio ~ 1 • The Sun has 99.85% of the mass and 100% of the light • This changes to ~100 on galactic and extra-galactic scales • There is a LOT of mass inferred by gravity, but not much light

28. Dark Matter • What is this stuff? Normal Stuff (protons, neutrons) • very small faint objects • brown dwarfs (large “jupiter” planets, star duds) • gas and dust • stellar remnants • white dwarfs • neutron stars • black holes • known or exotic yet undiscovered particles with mass • nutrinos

29. Galaxy Red Shifts

30. “Redshift” • Hubble wanted to know the distance to the faint nebulosities • He took galaxy spectra to obtain their radial velocities. • Armed with distance determinations to local galaxies utilizing pulsating Cepheid stars, Hubble discovered that galaxies were in general moving away from the Milky Way. • The recessional velocity of galaxies increases with increasing distance. (Redshift-Distance relation, a.k.a. the Hubble Law)

31. Hubble’s Redshift-Distance Relation

32. Hubble Law Vr = H0D Vr = radial velocity (a.k.a. redshift,doppler shift) D = Distance H0 = Constant (a.k.a. Hubble constant) Vr D

33. Hubble Flow • Galaxies A, B, and C separated by distance d • After some time (Dt) this distance has doubled (2d) • Distance between A,B and B,C is now 2d, and A,C is 4d • Recession velocity of B from A is v = d/Dt, and B from C from A is v = 2d/Dt • The farther away you are, the fasteryou appear to recede!

34. Local Group Galaxies

35. Hubble Law

36. Galaxy Clusters

37. Hubble Constant Vr = H0D H0 = Hubble constant = 60-80 km/s/Mpc Units for 1/H0 = seconds This is an age estimate for our universe. The true age should be less than this maximum age estimate because matter has surely caused some amount of deceleration.

38. Age Estimation H0 = 60-80 km/s/Mpc 1/H0 ago, all matter was piled up onto each other if there has been no acceleration or deceleration. 1/H0 ~ 10-20 Billion years (Gigayears)

39. H = H(t) Has expansion remained constant throughout time? Has expansion been slowing down due to the matter in the universe gravitationally attracting? OR Is the universe accelerating? Therefore, H0 = H(tnow) The Hubble constant is not a fundamental constant, it is variable with time.

40. How Far?