Chapter 15 Normal and Active Galaxies

1. Chapter 15Normal and Active Galaxies

2. Chapter 15Normal and Active Galaxies

3. Units of Chapter 15 Hubble’s Galaxy Classification The Distribution of Galaxies in Space Hubble’s Law Active Galactic Nuclei The Central Engine of an Active Galaxy

5. Question 1 a) giant globular clusters in the halo. b) small irregular galaxies that orbit the Milky Way. c) large molecular clouds in the disk of our Galaxy. d) the brightest ionized hydrogen regions in our Galaxy. e) spiral nebulae originally discovered by Herschel. The Magellanic Clouds are

6. Question 1 a) giant globular clusters in the halo. b) small irregular galaxies that orbit the Milky Way. c) large molecular clouds in the disk of our Galaxy. d) the brightest ionized hydrogen regions in our Galaxy. e) spiral nebulae originally discovered by Herschel. The Magellanic Clouds are

7. Hubble’s Galaxy Classification Spiral galaxies are classified according to the size of their central bulge.

8. Hubble’s Galaxy Classification Type Sa has the largest central bulge, Type Sb is smaller, and Type Sc is the smallest. Type Sa tends to have the most tightly bound spiral arms, with Types Sb and Sc progressively less tight, although the correlation is not perfect.

9. Question 2 a) ongoing star formation b) a disk, bulge, and halo c) globular clusters in the halo d) open clusters in the disk e) all of the above What property is shared by spiral galaxies?

10. Question 2 a) ongoing star formation b) a disk, bulge, and halo c) globular clusters in the halo d) open clusters in the disk e) all of the above What property is shared by spiral galaxies?

11. Hubble’s Galaxy Classification Similar to the spiral galaxies are the barred spirals.

12. Hubble’s Galaxy Classification Elliptical galaxies have no spiral arms and no disk. They come in many sizes, from giant ellipticals of trillions of stars, down to dwarf ellipticals of fewer than a million stars. Ellipticals also contain very little, if any, cool gas and dust, and show no evidence of ongoing star formation. Many do, however, have large clouds of hot gas, extending far beyond the visible boundaries of the galaxy.

13. Hubble’s Galaxy Classification Ellipticals are classified according to their shape, from E0 (almost spherical) to E7 (the most elongated).

14. Question 3 a) disk and spiral arms. b) halo. c) central bulge. d) open clusters. e) companion galaxies, the Magellanic Clouds. Based on their shapes and stars, elliptical galaxies are most like the Milky Way’s

15. Question 3 a) disk and spiral arms. b) halo. c) central bulge. d) open clusters. e) companion galaxies, the Magellanic Clouds. Based on their shapes and stars, elliptical galaxies are most like the Milky Way’s Like the stars and globular clusters in our halo, elliptical galaxies contain little or no gas and dust to make new stars.

16. Hubble’s Galaxy Classification S0 (lenticular) and SB0 galaxies have a disk and bulge, but no spiral arms and no interstellar gas.

17. Hubble’s Galaxy Classification The irregular galaxies have a wide variety of shapes. These galaxies appear to be undergoing interactions with other galaxies.

18. Hubble’s Galaxy Classification A summary of galaxy properties by type

19. Hubble’s Galaxy Classification Hubble’s “tuning fork” is a convenient way to remember the galaxy classifications, although it has no deeper meaning.

20. The Distribution of Galaxies in Space Cepheid variables allow measurement of galaxies to about 25 Mpc away. However, most galaxies are farther away then 25 Mpc. New distance measures are needed. Type I supernovae Tully-Fisher

21. Type I Supernovae Type I supernovae all have about the same luminosity, as the process by which they happen doesn’t allow for much variation.

22. Apparent Vs. Absolute

23. The Distribution of Galaxies in Space The rotation of a galaxy results in Doppler broadening of its spectral lines.

24. Tully-Fisher Tully-Fisher relation correlates a galaxy’s rotation speed (which can be measured using the Doppler effect) to its luminosity.

25. The Distribution of Galaxies in Space With these additions, the cosmic distance ladder has been extended to about 1 Gpc.

26. The Distribution of Galaxies in Space Here is the distribution of galaxies within about 1 Mpc of the Milky Way.

27. The Distribution of Galaxies in Space There are three spirals in this group – the Milky Way, Andromeda, and M33. These and their satellites – about 45 galaxies in all – form the Local Group. Such a group of galaxies, held together by its own gravity, is called a galaxy cluster.

28. The Distribution of Galaxies in Space A nearby galaxy cluster is the Virgo Cluster; it is much larger than the Local Group, containing about 3500 galaxies.

29. Question 4 a) Most galaxies showed redshifts. b) All galaxies showed blueshifts. c) Galaxies showed about half redshifts and half blueshifts. d) Galaxies showed no line shifts at all. e) Some galaxies showed a redshift that changed into a blueshift at other times. Hubble took spectra of galaxies in the 1930s. What did he find?

30. Question 4 a) Most galaxies showed redshifts. b) All galaxies showed blueshifts. c) Galaxies showed about half redshifts and half blueshifts. d) Galaxies showed no line shifts at all. e) Some galaxies showed a redshift that changed into a blueshift at other times. Hubble took spectra of galaxies in the 1930s. What did he find? Redshifts of galaxies indicate they are moving away from us.

31. Hubble’s Law Universal recession: All galaxies (with a couple of nearby exceptions) seem to be moving away from us, with the redshift of their motion correlated with their distance.

32. Hubble’s Law These plots show the relation between distance and recessional velocity for the five galaxies in the previous figure, and then for a larger sample.

33. Hubble’s Law The relationship (slope of the line) is characterized by Hubble’s constant H0: recessional velocity = H0 distance The value of Hubble’s constant is currently uncertain, with most estimates ranging from 50 to 80 km/s/Mpc. Measuring distances using Hubble’s law actually works better the farther away the object is; random motions are overwhelmed by the recessional velocity.

34. Question 5 a) more distant galaxies showing greater blueshifts. b) distant quasars appearing proportionally dimmer. c) more distant galaxies showing greater redshifts. d) slowly varying Cepheid variables appearing brighter. e) more distant galaxies appearing younger. Hubble’s law is based on

35. Question 5 a) more distant galaxies showing greater blueshifts. b) distant quasars appearing proportionally dimmer. c) more distant galaxies showing greater redshifts. d) slowly varying Cepheid variables appearing brighter. e) more distant galaxies appearing younger. Hubble’s law is based on

36. Question 6 a) the density of galaxies in the universe. b) the luminosity of distant galaxies. c) the reddening of light from dust clouds. d) the speed of a galaxy. e) the rate of expansion of the universe. Hubble’s constant measures

37. Question 6 a) the density of galaxies in the universe. b) the luminosity of distant galaxies. c) the reddening of light from dust clouds. d) the speed of a galaxy. e) the rate of expansion of the universe. Hubble’s constant measures Hubble’s law relates how fast galaxies are moving away from us at different distances. A larger value for H0 implies a faster expansion rate. Velocity = H0 x Distance

38. Question 7 a) the size of the universe. b) distances to galaxies. c) the speed of recession of galaxies. d) the density of matter in the universe. e) the temperature of the Big Bang. To calibrate Hubble’s constant, astronomers must determine

39. Question 7 a) the size of the universe. b) distances to galaxies. c) the speed of recession of galaxies. d) the density of matter in the universe. e) the temperature of the Big Bang. To calibrate Hubble’s constant, astronomers must determine Distances to galaxies are determined using a variety of “standard candles,” including Cepheid variables, supernova explosions, model galaxies, and model clusters.

40. Hubble’s Law This puts the final step on our distance ladder.

41. Active Galactic Nuclei About 20–25 percent of galaxies don’t fit well into the Hubble scheme – they are far too luminous. Such galaxies are called active galaxies. They differ from normal galaxies in both the luminosity and type of radiation they emit.

42. Active Galactic Nuclei The radiation from these galaxies is called nonstellar radiation. Many luminous galaxies are experiencing an outburst of star formation, probably due to interactions with a neighbor. These galaxies are called starburst galaxies, and we will discuss them later. The galaxies we will discuss now are those whose activity is due to events occurring in and around the galactic center.

43. Active Galactic Nuclei Active galaxies are classified into three types: Seyfert galaxies, radio galaxies, and quasars. Seyfert galaxies resemble normal spiral galaxies, but their cores are thousands of times more luminous.

44. Question 8 a) supermassive black holes at their cores. b) dark matter. c) self-sustaining star formation. d) spiral density waves. e) hypernova explosions. Seyfert and radio galaxies could be powered by

45. Question 8 a) supermassive black holes at their cores. b) dark matter. c) self-sustaining star formation. d) spiral density waves. e) hypernova explosions. Seyfert and radio galaxies could be powered by The Circinus Galaxy, a Seyfert galaxy about 4 Mpc away

46. Active Galactic Nuclei The rapid variations in the luminosity of Seyfert galaxies indicate that the core must be extremely compact.

47. Active Galactic Nuclei Radio galaxies emit very strongly in the radio portion of the spectrum. They may have enormous lobes, invisible to optical telescopes, perpendicular to the plane of the galaxy.

48. Active Galactic Nuclei Radio galaxies may also be core dominated.

49. Active Galactic Nuclei Core-dominated and radio-lobe galaxies are probably the same phenomenon viewed from different angles.

50. Active Galactic Nuclei Many active galaxies have jets, and most show signs of interactions with other galaxies.