Homework #9. Chapter 19 questions are for PRACTICE ONLY they will not factor into your Homework #9 grade, but will help you prepare for Exams #4/#5.

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1 Homework #9 Due Wednesday, April 25, 11:59PM Covers Chapters 18 and 17 Estimated time to complete: 40 minutes Read chapters, review notes before starting Last homework of the semester Chapter 19 questions are for PRACTICE ONLY they will not factor into your Homework #9 grade, but will help you prepare for Exams #4/#5.

2 If supermassive black holes did not exist, which of the following would be true? A) Stars/gas at the very centers of galaxies would not orbit so fast. B) AGN/quasars would not exist. C) Galaxies would become unglued since the decreased gravity wouldn t be able to hold the galaxy together. D) Both A) and B) are correct.

3 If supermassive black holes did not exist, which of the following would be true? A) Stars/gas at the very centers of galaxies would not orbit so fast. B) AGN/quasars would not exist. C) Galaxies would become unglued since the decreased gravity wouldn t be able to hold the galaxy together. D) Both A) and B) are correct. AGN/quasars are powered by black holes, but black holes are not responsible for the vast majority of the self-gravity of a galaxy.

4 Black Holes in Galaxies Many nearby galaxies perhaps all of them have supermassive black holes at their centers. These black holes seem to be dormant active galactic nuclei in most galaxies. All galaxies may have passed through an AGN/ quasar-like stage earlier in time. Most galaxies (including our Milky Way) are not currently in their active AGN/quasar phase (only ~1% are active now).

5 Chapter 16 Study Guide 1) Galaxies help us investigate cosmology, or the study of the structure and evolution of the Universe 2) Three main types of galaxies: spirals (ongoing star formation, young+old stars, gas/dust), ellipticals (no recent star formation, only old stars, no gas/dust), and irregulars (ongoing star formation, young+old stars, gas/dust) 3) Spirals: disk component (star formation) + spheroidal component (bulge+halo+globular clusters - no star formation) 4) Ellipticals: only a spheroidal component (no star formation) red and dead 5) Hubble s tuning fork categorizes galaxies by bulge-to-disk ratio, but no evolutionary sequence

6 Chapter 16 Study Guide 6) Spirals generally reside in sparse groups, ellipticals reside in rich clusters 7) We get the distance of all astronomical objects from the cosmic distance ladder: radar (to get AU) parallax star cluster main sequence fitting Cepheids (gets us out to other galaxies) white dwarf supernova, Hubble s Law (gets us to other side of Universe) 8) Each step of the cosmic distance ladder is dependent on the correct calibration of the step preceding it 9) Edwin Hubble discovered that nearly all galaxies are moving away from our Milky Way galaxy at a velocity that increases with how far away the galaxy is: Hubble s Law: velocity = H 0 distance (H 0 is Hubble s constant)

7 Chapter 16 Study Guide 10) If you can measure the recessional velocity of a galaxy from the redshift of its spectral features, you can get its distance from Hubble s Law. 11) The Universe is about 14 billion years old. 12) Space itself is expanding, taking the galaxies along for the ride, so it appears the galaxies are moving away from us (think of flat stickers on an expanding balloon 2-d example) 13) Cosmological Principle: (1) Matter (galaxies) are evenly distributed throughout the Universe; (2) Universe has no center or edge no special place in the Universe 14) Expansion of Universe caused photons to stretch light becomes redder as it travel through space

8 Chapter 16 Study Guide 15) Galaxies formed from very slight over-densities of gas in the very early Universe gravity magnified these overdensities, leading to collapse of protogalactic clouds 16) Spirals formed from more diffuse, faster spinning clouds, while ellipticals formed from dense, slower spinning clouds (hereditary) 17) Colliding spirals can form an elliptical (particularly in small, early Universe and in centers of rich galaxy clusters) environmental form central dominant elliptical galaxies in clusters by galactic cannibalism 18) Stars will not collide during a galaxy merger too sparsely spaced (two grapefruits separated by 4000 miles analogy).

9 Chapter 16 Study Guide 19) Active galactic nuclei (AGN) and quasars generate a tremendous amount of energy at the centers of a small percentage of galaxies. 20) 1 trillion Suns energy generated in a region smaller than a solar system in a quasar must be from accretion onto a supermassive black hole 21) Accretion disks around black holes: grav. energy kinetic energy heat (from friction) tremendous energy 22) All galaxies have central black holes, but only 1% are active AGN/quasars needs a trigger

10 Chapter 18 Dark Matter, Dark Energy, and the Fate of the Universe

11 What do we mean by dark matter and dark energy?

12 Unseen Influences Dark matter: An undetected form of mass that emits little or no light but whose existence we infer from its gravitational influence key point, without gravitational influence, no evidence of dark matter Dark energy: An unknown form of energy that seems to be the source of a repulsive force causing the expansion of the Universe to accelerate

13 Contents of Universe Normal matter: ~5% also called baryonic matter Normal matter of stars: ~0.5% Other normal matter (e.g., gas): ~4.5% Dark matter: ~27% Dark energy: ~68% 95% of the Universe is stuff we have little clue about!

14 What is the evidence for dark matter in galaxies?

15 We measure the mass of stars using the orbits of planets or other stars. Orbital period Average distance We measure the mass of galaxies using the orbits of stars and/or gas in the galaxy. Orbital velocity Orbital radius Both methods rely on gravity, and Newton s law of gravitation.

16 Rotation curve A plot of orbital speed versus orbital radius Solar system s rotation curve declines because Sun has almost all the mass generic feature when mass is concentrated Rotation Curve of the Solar System Farther away from Sun slower orbital speed

17 Rotation curve of a galaxy disk rises with radius in the center, since you are enclosing more and more mass. Rotation Curve for a Solid Disk But, velocity should decline once you are outside the disk (but it doesn t)!

18 The rotation curve of the Milky Way stays flat at larger distances. Rotation Curve of a Spiral Galaxy There must be substantial mass beyond the stellar disk to keep the velocity from declining. What is it???

19 The mass in the Milky Way is spread out over a much larger region than the stars, but we don t see this mass. Most of the Milky Way s mass seems to be dark matter!

20 If there were no dark matter, how would the velocity of gas change with increasing radius in the outskirts of the galaxy beyond most of the stars? A) It would decrease with increasing radius. B) It would increase with increasing radius. C) It would remain the same with increasing radius. D) It would first decline, and then rise with increasing radius.

21 If there were no dark matter, how would the velocity of gas change with increasing radius in the outskirts of the galaxy beyond most of the stars? A) It would decrease with increasing radius. B) It would increase with increasing radius. C) It would remain the same with increasing radius. D) It would first decline, and then rise with increasing radius. Much like a solar system, the velocity would decrease with increasing radius, once you have enclosed (nearly) all the mass dark matter must be there to keep velocity constant with increasing distance from center of galaxy.

22 The visible portion of a galaxy lies deep in the heart of a large halo of dark matter. Sun Measure velocity and radius of trace amounts of hydrogen gas in outskirts of galaxy (beyond stars) to get mass of entire galaxy.

23 Mass within Sun s orbit: 100 billion M Sun Total mass: 1 trillion M Sun ~90% of Milky Way s mass is dark matter!!

24 We can measure orbital velocities in other spiral galaxies using the Doppler shift of a radio line of cold hydrogen gas at a wavelength of 21 centimeters in the radio.

25 Spiral galaxies all tend to have orbital velocities that remain constant at large radii, indicating large amounts of dark matter (the velocity should decrease with increasing distance if there were no dark matter) key point

26 What is the evidence for dark matter in clusters of galaxies? Three methods to determine the total mass of galaxy clusters

27 Method #1 (Galaxy velocity method) We can measure the velocities of galaxies in a cluster from their Doppler shifts. Once again, use velocities and distances to get mass of cluster.

28 Too Much Mass! This galaxy velocity method (Method #1) was first feasible in the 1930s. Fritz Zwicky first determined the mass of the Coma Cluster of galaxies. The mass he found from galaxy motions in a cluster was about 50 times larger than the mass in stars/galaxies! Fritz Zwicky Result was largely ignored at the time because it was not understood.

29 urbandictionary.com

30 Method #2 (Hot gas method) Clusters contain large amounts of X ray emitting hot gas (this gas component was unknown to Zwicky). The temperature of hot gas (particle motions) tells us the total cluster mass, too. Hot gas only accounts for small part of missing mass still need dark matter

31 Clusters contain large amounts of X ray emitting hot gas (this gas component was unknown to Zwicky). The temperature of hot gas (particle motions) tells us cluster mass: 85% dark matter 13% hot gas 2% stars/galaxies

32 Method #3 (Gravitational lensing method) Gravitational lensing, the bending of light rays by gravity, can also tell us a cluster s mass also requires dark matter

33 A gravitational lens distorts our view of things behind it. Gravitational Lensing Illustrated

34 Lensing Also Works for Galaxy Groups and Individual Galaxies Cheshire Cat Lens Group Horseshoe Lens - Galaxy

35 All three methods of measuring cluster mass indicate similar amounts of dark matter very strong evidence that dark matter exists!

36 What kind of measurement does not tell us the mass of a cluster of galaxies, or give us evidence for dark matter? A) Measuring velocities of a cluster s galaxies B) Measuring the total mass of all the stars of the galaxies in the cluster C) Measuring the temperature of the cluster s hot gas D) Measuring distorted images of background galaxies by gravitational lensing

37 What kind of measurement does not tell us the mass of a cluster of galaxies, or give us evidence for dark matter? A) Measuring velocities of a cluster s galaxies B) Measuring the total mass of all the stars of the galaxies in the cluster C) Measuring the temperature of the cluster s hot gas D) Measuring distorted images of background galaxies by gravitational lensing Stars only represent a few % of the cluster it would be useless to count up all the stars since they represent such a small fraction of the total cluster mass

38 Does dark matter really exist?

39 Our Options 1. Dark matter really exists, and we are observing the effects of its gravitational attraction. 2. Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter.

40 Our Options 1. Dark matter really exists, and we are observing the effects of its gravitational attraction. 2. Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter. Because gravity is so well-tested, most astronomers prefer option #1 (although there is a small, vocal minority who oppose dark matter).

41 What might dark matter be made of?

42 Two Basic Options Ordinary Matter (MACHOs) - macroscopic Massive Compact Halo Objects: brown dwarfs or large Jupiters in halos of galaxies Give off too little light to see individually Must be A LOT of them Exotic Particles (WIMPs) - microscopic Weakly Interacting Massive Particles: mysterious neutrino-like particles (but more massive than neutrinos) Not discovered in the laboratory yet

43 Two Basic Options Ordinary Matter (MACHOs) - macroscopic Massive Compact Halo Objects: brown dwarfs or large Jupiters in halos of galaxies Give off too little light to see individually Must be A LOT of them Exotic Particles (WIMPs) - microscopic Weakly Interacting Massive Particles: mysterious neutrino-like particles (but more massive than neutrinos) Not discovered in the laboratory yet The best bet, but not 100% certain

44 MACHOs occasionally make other stars appear brighter through lensing but not enough lensing events to explain all the dark matter. MACHOs therefore no longer favored as a viable dark matter candidate.

45 Why WIMPs? There s not enough ordinary matter. WIMPs could be left over from the Big Bang. Models involving WIMPs explain how galaxy formation works (make galaxy collapse easier to accomplish) key Physicists are actively looking for WIMPs in the laboratory, but they must be very difficult to detect, and don t interact much with normal matter.

46 Why have WIMPs not been detected yet? A) They have too small a mass. B) They are very rare. C) The don t interact very much with normal matter. D) They probably don t exist.

47 Why have WIMPs not been detected yet? A) They have too small a mass. B) They are very rare. C) The don t interact very much with normal matter. D) They probably don t exist. Weakly Interacting Massive Particles they probably do exist, they just don t interact much with matter so our detectors can t detect them (yet).

48 What is the role of dark matter in galaxy formation?

49 Models show gravity of dark matter is what caused protogalactic clouds to contract early in time (helped process along). Without dark matter, galaxies have a hard time collapsing in the simulations.

50 Time in billions of years Size of expanding box in millions of light-years Models show that the gravity of dark matter pulls mass into denser regions the universe grows lumpier with time.

51 WIMPs can t contract to the center because they don t radiate away their orbital energy or angular momentum (WIMPs are noninteractive). Explains why dark matter halos are so large and spherical.

52 Maps of galaxy positions reveal extremely large structures: superclusters and voids.

53 Real data Simulation with WIMPs Models with dark matter in the form of WIMPs look more like the real, observed galaxy distribution more evidence for WIMPs.

54 Will the universe continue expanding forever?

55 Does the Universe have enough kinetic energy to escape its own gravitational pull? Fate of a Launched Cannonball Or is there too little mass for gravity to halt the initial expansion?

56 Fate of universe depends on the amount of dark matter Lots of dark matter Critical density of matter Not enough dark matter Is there enough of it to stop the expansion?

57 If density > critical density Universe collapses ( Big Crunch ) If density < critical density Universe expands forever Critical density = grams per cm 3 (a few hydrogen atoms in a region the size of a closet - air has 200,000,000,000,000,000,000,000,000 particles in the same closet at sea level). Amount of matter (including dark matter) is ~27% of the critical density, suggesting fate is eternal expansion. Universe should coast forever (we thought)! Not enough dark matter

58 In fact, expansion appears to be speeding up, not slowing down! We believe we live in an accelerating Universe. (akin to a ball thrown upwards and continuing to accelerate upwards rather than slowing down) Dark energy? This is our current view of the Universe. Not enough dark matter

59 What is Dark Energy? We don t know exactly! Whatever it is, it acts as a repulsive force that is causing the Universe to expand at a faster rate, rather than slowing down. In other words, dark energy is winning out over dark matter. Biggest single mass-energy component of our Universe 68%

60 Is the expansion of the universe accelerating?

61 The brightness of distant white dwarf supernovae tells us how much the Universe has expanded since they exploded main evidence that Universe is accelerating

62 If dark energy did not exist, what would be the fate of our Universe? A) It will still be accelerating. B) It would coast forever. C) It would eventually collapse. D) It would come to a stop at infinity.

63 If dark energy did not exist, what would be the fate of our Universe? A) It will still be accelerating. B) It would coast forever. C) It would eventually collapse. D) It would come to a stop at infinity. The density of the Universe is only 27% of what is needed to stop expansion, so the Universe would continue to expand, but slowing down as it did (no Big Crunch ), if there is no dark energy. (But there is dark energy! Universe is accelerating)

64 Chapter 18 Study Guide 1) Dark matter and dark energy are mysterious and poorly-understood, but major components of our Universe (95% of all mass-energy in the Universe) 2) Main evidence for dark matter halos in galaxies is from flat rotation curves of spiral galaxies (velocity should decline with radius outside where there are few stars/ gas if dark matter did not exist) derived from 21-cm observations of hydrogen clouds in galaxies. 3) Dark matter halos extend to much larger radii than stellar disk comprises 90% of the mass of the Milky Way 4) Clusters of galaxies have even larger scale, very massive dark matter halos

65 Chapter 18 Study Guide 5) Three methods for finding mass of clusters (1) cluster galaxy velocities, (2) temperature of hot gas within cluster, and (3) gravitational lensing of a background galaxy all lead to consistent results 6) In clusters, massive amounts of dark matter are required (hardly any of the mass of a cluster is in the form of stars/ galaxies, only modest amount in the form of hot gas) 7) Either dark matter exists, or our understanding of gravity is greatly flawed (most astronomers believe the former) 8) Dark matter might be either MACHOs (large numbers of dim dead stars/planets), or WIMPs (new atomic particle)

66 Chapter 18 Study Guide 9) More lensing events should have been seen if dark matter is from MACHOs, so WIMPs are now the preferred explanation for dark matter. 10) Dark matter likely helped in the process of galaxy formation, its gravity pulling matter together into protogalactic clouds to form mass structures 11) Since dark matter does not interact much, it remained at rather large radii during galaxy formation process dark matter halo much bigger than stellar disk 12) Universe is clumpy now, with regions with clusters of galaxy clusters (superclusters), and regions with few galaxies (voids)

67 Chapter 18 Study Guide 13) WIMPs make galaxy formation models match observed data very well more evidence for WIMPs 14) There is not quite enough matter (including dark matter) in the Universe to keep it from expanding forever 15) In fact, white dwarf supernova data show that the Universe is accelerating, and not slowing down 16) Dark energy (68% of total Universe content) is believed to be responsible for this acceleration..but what is it?

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