24. Normal Galaxies

1. 24. Normal Galaxies • The discovery of other galaxies • Edwin Hubble proved galaxies are very distant • Edwin Hubble classified galaxies by shape • Methods for determining distances to galaxies • The Hubble Law & the Hubble constant • Clusters & superclusters of galaxies • Colliding galaxies produce spectacular features • Most matter in the Universe is undiscovered • Theories of galaxy formation

2. The Discovery of Other Galaxies • An historical perspective • Immanuel Kant suggested “island universes” 1755 • Vast collections of stars beyond the Milky Way • Milky Way was thought to be the only galaxy • No clear evidence of anything at really great distances • William Herschel discovered & cataloged “nebulae” • Thought to be distant gas & dust clouds • Lord Rosse built the largest telescope to date 1845 • Mirror was 6 feet in diameter with a 60 foot focal length • Discovered that some “nebulae” had a spiral structure • Made many sketches of celestial objects he saw

3. Lord Rosse’s “Leviathan” (1845)

4. The National Academy of Sciences • The “Great Debate” of 26 April 1920 • Harlow Shapley Measured the Milky Way’s size • Thought that “spiral nebulae” were forming solar systems • Heber Curtis Studied solar eclipses • Thought that “spiral nebulae” were separate galaxies • The resolution of the debate • Edwin Hubble photographs the “Andromeda nebula” • He discovered Cepheid variables • Average luminosities ~ 2.0 . 104.L☉ • This revealed the ~ 2 million ly distance to Andromeda • Implication: Universe contains billions of galaxies

5. Table 24-1: Properties of Galaxies

6. Hubble Classified Galactic Shapes • Spiral galaxies • Grand design Spiral arms are narrow & distinct • Flocculent Spiral arms are broad & hazy • Barred spiral galaxies • Nucleus varies in relative size • Bars may be short & wide or long & narrow • Elliptical galaxies • Complete absence of spiral arms • Some appear spherical but may be seen end-on • Depends entirely on our perspective from Earth • Irregular galaxies • No obvious structure

7. Some Interesting Galaxy Facts • Spiral galaxies • Barred are ~ 2 x as common as classical spirals • Distinctly different amounts of gas & dust • Sa ~ 4% of mass is in the form of gas & dust • Sb ~ 8% of mass is in the form of gas & dust • Sc ~ 25% of mass is in the form of gas & dust • Elliptical galaxies • Universe’s smallest & largest galaxies are elliptical • Dwarf elliptical galaxies Quite common • Giant elliptical galaxies Quite rare • Stellar motion in elliptical galaxies • Isotropic Typical in flattened ellipticals • Anisotropic Typical in nearly round ellipticals • These are virtually devoid of gas & dust • Minimal new-star formation

8. Three Kinds of Spiral Galaxies Sa Sb Sc

9. Modern View of Spiral Galaxy M51

10. 3 Kinds of Barred Spiral Galaxies Sba SBb SBc

11. Three Kinds of Elliptical Galaxies E0 E3 E6

12. Giant Ellipticals: The Virgo Cluster

13. Hubble’s Tuning Fork Diagram

14. http://skyserver.fnal.gov/en/proj/advanced/galaxies/images/TuningFork.jpghttp://skyserver.fnal.gov/en/proj/advanced/galaxies/images/TuningFork.jpg Hubble’s Diagram: Another Look

15. Large & Small Magellanic Clouds Large Magellanic Cloud Small Magellanic Cloud

16. Bright Stars as Standard Candles • Distance < 60 Mpc • Cepheid variables are reliable standard candles • Visible as far as ~ 60 Mpc (~ 200 Mly) • Average luminosities ~ 2.0 . 104.L☉ • Luminosity is correlated to their period • 60 Mpc < Distance < 150 to 250 Mpc • Supergiant stars are reliable standard candles • Red supergiants Maximum luminosities ~ 1.0 . 105.L☉ • Visible to distances of ~ 150 Mpc • Blue supergiants Maximum luminosities ~ 2.0 . 105.L☉ • Visible to distances of ~ 250 Mpc

17. Clusters & Nebulae as Standard Candles • 250 Mpc < Distance < 400 Mpc • Globular clusters • Brightest are about ~ 1.0 . 106.L☉ • Visible as far as ~ 400 Mpc (~ 1.3 billion ly) • 400 Mpc < Distance < 900 Mpc • H II nebulae • Brightest are about ~ 6.0 . 106.L☉ • Visible as far as ~ 900 Mpc (~ 3.0 billion ly)

18. Reliability of Standard Candles • Reasonably reliable standard candles • Cepheid variables & supergiant stars • Globular clusters & H II nebulae • Type Ia supernovae • Reasons for their reliability • Very bright Visible at great distances • Well-known luminosities Inverse square intensity • Easily identified Unlike all other objects • Relatively common Distance to many galaxies

19. One More Distance Measure • The Tully-Fisher relation • Basic observation • The 21 cm hydrogen line width depends on galaxy mass • High-mass spiral galaxies have wide 21 cm lines • Low-mass spiral galaxies have narrow 21 cm lines • Basic physical process • The greater the mass, the faster the rotation speed • This results in greater Doppler shifts of the 21 cm line • The greater the mass, the greater the compression • This results in more & brighter stars • The basic strategy • Estimate the mass of distant spiral galaxies • The 21 cm line width is directly proportional to brightness • The basic problem • Poor correlation with standard candle distances

20. A Supernova In Galaxy NGC 4526

21. Six Steps on the Distance Ladder

22. Masers: Microwave Lasers • Basic physical process • Nearby stars cause H2O to emit microwave l’s • Microwave amplification by stimulated emission of radiation • Basic observations • Use of the VLBA 1990s • Ten 25-meter radio antennas between Hawaii & Caribbean • Extreme detail possible because of synthetic aperture • Observed masers in the spiral galaxy M106 • Some approaching, some receding, some tangential • Measure blue & red Doppler shifts to determine orbital speed • Measure proper motion of tangentially moving masers • Calculated a distance of ~ 7.2 Mpc (~ 23 Mly) • Basic problem • The technique is still in its infancy • Method is completely independent of other approaches

23. Masers As Standard Candles

24. The Hubble Law & Hubble Constant • An historical perspective • Slipher starts spectral study of “spiral nebulae” 1914 • 11 of 15 “spiral nebulae” had substantial redshifts • Noted by Curtis during the 1920 Shapley-Curtis debate • Evidence that these features were far beyond the Milky Way • Hubble & Humason extend spectral analyses 1920s • Also analyzed Cepheid variables in these features • The farther away they are, the faster they are moving away • The Hubble flow • Redshift is directly proportional to recessional speed • Definition of redshift • z = (l – l0) / l0 = Dl / l0 • Definition of the Hubble law • v = H0. d where H0 = the Hubble constant

25. The Hubble Constant • Current understanding • H0 = 69.32 ± 0.80 km . sec–1.Mpc–1 • At 100 Mpc, galaxies recede at 6,932 km . sec–1 • At 1,000 Mpc, galaxies recede at 69,320 km . sec–1 • Current problems • Different distance methods yield different H0 values • Supernova H0 values from 40 to 65 km . sec–1. Mpc–1 • Tully-Fisher H0 values from 80 to 100 km . sec–1.Mpc–1 • HST Cepheid H0 values ~ 73 km . sec–1.Mpc–1 • Precision is + 20%

26. Redshifts In Spectra of 5 Galaxies

27. Hubble Law: Redshift & Distance

28. Galaxy Clusters & Superclusters • Basic observations • Galaxies are not uniformly distributed in space • Some regions of space have few galaxies • Other regions of space have many galaxies • High-density regions of space • Poor clusters • One example is our Local Group of galaxies • Rich clusters • One example is the nearby Virgo cluster of galaxies • Superclusters • Great Wall is one example of a supercluster • Southern Wall is another example of a supercluster

29. The Local Group of Galaxies • Contains ~ 40 galaxies • Uncertainty due to sparseness of many galaxies • Sagittarius Dwarf Discovered in 1994 • Antilla Discovered in 1997 • Dust in the Milky Way plane obscures some galaxies • Most Local Group galaxies are dwarf ellipticals • Two large spiral galaxies ~ 2.2 Mly apart • Andromeda galaxy M31 • Largest galaxy in the Local Group • Milky Way galaxy • Second largest galaxy in the Local Group

30. Some Galaxies in the Local Group

31. The Andromeda Galaxy • The largest galaxy in the Local Group • Diameter of ~ 125,000 ly • Covers ~ 3° of area in the night sky • We see only the small core as a fuzzy patch of light • Apparent magnitude of ~ 3.0 spread out over a large area • Most distant object visible to the unaided eye • An Sb spiral inclined ~ 15° to our line of sight • Rather tightly wound spiral arms • Contains ~ 2 times as many stars as the Milky Way • Unusual properties • HST images suggest Andromeda may have 2 cores • Possibly the result of ancient collision with another galaxy • Andromeda is approaching us at ~ 68 mi . sec–1 • This may portend a future collision with the Milky Way

32. The Andromeda Galaxy (M31)

33. Clusters of Galaxies • The Virgo cluster • Covers an area ~ 10° by 12° in the sky • Located ~ 15 Mpc (~ 50 Mly) away • A moderately rich irregular cluster • Dominated by 3 giant ellipticals ~ 750 Kpc across • Diameters are ~ 5% their distance from the Local Group • The Coma cluster • Located ~ 90 Mpc (~ 300 Mly) away • A rich regular cluster • Dominated by 2 giant ellipticals • Telescopic images show ~ 1,000 galaxies • Probably 10 times that many dwarf ellipticals

34. The Hercules Cluster of Galaxies

35. Superclusters of Galaxies • Usually include dozens of clusters • Spread over ~ 30 Mpc (~ 100 Mly) of space • Delicate filamentary patterns at the largest scale • The Great Wall in the northern sky • The Southern Wall in the southern sky

36. Large-Scale Distribution of Galaxies

37. Spectacular Colliding Galaxies • Galaxy collisions are relatively common • Galaxies orbit one another like planets orbit stars • Occasionally they actually hit each other • Possible interactions • Most of the volume of galaxies is empty space • Stars seldom hit one another • Gas & dust are much more widespread than stars • Interactions are far more common • Substantial compression occurs • Substantial star formation ensues • Cores may orbit each other or even merge • Possible double core of the Andromeda galaxy • Possible future for the Andromeda & Milky Way galaxies • Discovery of 2 supermassive black holes in 1 galaxy

38. Simulated Collision of Two Galaxies

39. Colliding Galaxies With “Antennae”

40. Most Matter Remains Undiscovered • Dark matter is evident only from gravity effects • Coma cluster should have disintegrated long ago • Abundant “dark matter” keeps the cluster bound together • A partial solution • X-ray emitting gas 1930s • Comparable to the mass of stars in typical rich clusters • Can only account for ~ 10% of the dark matter • The distribution of dark matter • May be deduced from galactic rotation curves • Orbital speeds nearly constant to visible edge of galaxy • Gravitational lensing by massive galaxies • Dark matter constitutes ~ 90% the mass of galaxies • Dark matter is distributed much like visible matter

41. Rotation Curves of 4 Spiral Galaxies

42. “Double” Quasar: Gravitational Lensing

43. Theories of Galaxy Formation • Basic observations • The greater the distance, the farther back in time • The most distant galaxies are seen in their earliest stages • Galaxies were bluer long ago than they are now • Vigorous formation of OB associations • HST work by Dressler, Oemler, Gunn & Butcher • Two rich clusters with z = 0.4 ~ 4 billion years old • About 30% of galaxies in distantrich clusters are spirals • About 5% of galaxies in nearby rich clusters are spirals • Galactic collisions probably consume spirals • Ellipticals were probably the end product of these collisions • Basic theories • Gravitational collapse of huge nebulae 1960s • Gravitational merging of smaller nebulae 1977 • Gravitational merging of many tiny nebulae

44. Stellar Birthrates In Galaxies

45. The problem of “spiral nebulae” The Great Debate of 1920 They are forming solar systems They are distant separate galaxies Cepheid variables solve the debate The classification of galaxies Ellipticals From giants to dwarfs Normal spirals Grand design & flocculent spirals Barred spirals Irregular Determining distance to galaxies Normal & spectroscopic parallax Standard candles Cepheids & supergiants Globular clusters, & H II nebulae Additional techniques Tully-Fisher relationship & masers The Hubble Law & Hubble constant Totally consistent galactic redshift Proportional to distance H0 = 65 km . sec–1. Mpc –1 Estimates of H0 vary by factor of 2 Uneven distribution of galaxies Groups Irregular & regular Clusters Poor & rich Superclusters Colliding galaxies Spirals more common in old clusters Blue color of many OB associations Collisions form ellipticals Formation of galaxies Three common hypotheses Collapsing and/or merging nebulae Important Concepts