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Chapter 16 Galaxies

Guidepost The preceding chapter was about our Milky Way Galaxy, an important object to us but only one of the many billions of galaxies visible in the sky. We can no more understand galaxies by understanding a single example, the Milky Way, than we could understand humanity by understanding a single person. This chapter expands our horizon to discuss the different kinds of galaxies and their complex histories. We take two lessons from this chapter. First, galaxies are not solitary beasts; they collide and interact with each other. Second, most of the matter in the universe is invisible. The galaxies we see are only the tip of a cosmic iceberg. We will carry the lessons of this chapter into the next, where we will discuss violently active galaxies, and on into Chapter 18, where we discuss the universe as a whole.

Outline I. The Family of Galaxies A. The Discovery of Galaxies B. The Shapes of Galaxies C. How Many Galaxies? II. Measuring the Properties of Galaxies A. Distance B. The Hubble Law C. Diameter and Luminosity D. Mass E. Supermassive Black Holes in Galaxies F. Dark Matter in Galaxies

Outline (continued) III. The Evolution of Galaxies A. Clusters of Galaxies B. Colliding Galaxies C. The Origin and Evolution of Galaxies D. The Farthest Galaxies

Galaxies Star systems like our Milky Way Contain a few thousand to tens of billions of stars. Large variety of shapes and sizes

Galaxy Diversity Even seemingly empty regions of the sky contain thousands of very faint, very distant galaxies Large variety of galaxy morphologies: Spirals Ellipticals Irregular (some interacting) The Hubble Deep Field: 10-day exposure on an apparently empty field in the sky

Galaxy Classification E0, …, E7 Sa Large nucleus; tightly wound arms E0 = Spherical E1 Sb Sc Small nucleus; loosely wound arms E7 = Highly elliptical E6

Gas and Dust in Galaxies Spirals are rich in gas and dust Ellipticals are almost devoid of gas and dust Galaxies with disk and bulge, but no dust are termed S0

Barred Spirals Some spirals show a pronounced bar structure in the center They are termed barred spiral galaxies Sequence: SBa, …, SBc, analogous to regular spirals

Irregular Galaxies Often: result of galaxy collisions / mergers Often: Very active star formation (“Starburst galaxies”) The Cocoon Galaxy NGC 4038/4039 Some: Small (“dwarf galaxies”) satellites of larger galaxies (e.g., Magellanic Clouds) Large Magellanic Cloud

Galaxy Types (SLIDESHOW MODE ONLY)

Distance Measurements to Other Galaxies (1) Cepheid Method: Using Period – Luminosity relation for classical Cepheids: Measure Cepheid’s Period  Find its luminosity  Compare to apparent magnitude  Find its distance b) Type Ia Supernovae (collapse of an accreting white dwarf in a binary system): Type Ia Supernovae have well known standard luminosities  Compare to apparent magnitudes  Find its distances Both are “Standard-candle” methods: Know absolute magnitude (luminosity)  compare to apparent magnitude  find distance.

Cepheid Distance Measurement Repeated brightness measurements of a Cepheid allow the determination of the period and thus the absolute magnitude.  Distance

The Most Distant Galaxies At very large distances, only the general characteristics of galaxies can be used to estimate their luminosities  distances. Cluster of galaxies at ~ 4 to 6 billion light years

Distance Measurements to Other Galaxies (2): The Hubble Law Distant galaxies are moving away from our Milky Way, with a recession velocity, vr, proportional to their distance d: vr = H0*d H0 ≈ 70 km/s/Mpc is the Hubble constant Measure vr through the Doppler effect  infer the distance

The Extragalactic Distance Scale Many galaxies are typically millions or billions of parsecs from our galaxy. Typical distance units: Mpc = Megaparsec = 1 million parsec Gpc = Gigaparsec = 1 billion parsec Distances of Mpc or even Gpc  The light we see left the galaxy millions or billions of years ago!! “Look-back times” of millions or billions of years

Galaxy Sizes and Luminosities Vastly different sizes and luminosities: From small, low-luminosity irregular galaxies (much smaller and less luminous than the Milky Way) to giant ellipticals and large spirals, a few times the Milky Way’s size and luminosity

Rotation Curves of Galaxies From blue / red shift of spectral lines across the galaxy  infer rotational velocity Plot of rotational velocity vs. distance from the center of the galaxy: Rotation Curve Observe frequency of spectral lines across a galaxy.

Determining the Masses of Galaxies Based on rotation curves, use Kepler’s 3rd law to infer masses of galaxies

Masses and Other Properties of Galaxies

Supermassive Black Holes From the measurement of stellar velocities near the center of a galaxy: Infer mass in the very center  central black holes! Several million, up to more than a billion solar masses!  Supermassive black holes

Dark Matter Adding “visible” mass in: stars, interstellar gas, dust, …etc., we find that most of the mass is “invisible”! The nature of this “dark matter” is not understood at this time. Some ideas: brown dwarfs, small black holes, exotic elementary particles.

Clusters of Galaxies Galaxies generally do not exist in isolation, but form larger clusters of galaxies. Rich clusters: 1,000 or more galaxies, diameter of ~ 3 Mpc, condensed around a large, central galaxy Poor clusters: Less than 1,000 galaxies (often just a few), diameter of a few Mpc, generally not condensed towards the center

Hot Gas in Clusters of Galaxies Space between galaxies is not empty, but filled with hot gas (observable in X-rays) That this gas remains gravitationally bound provides further evidence for dark matter. Visible light X-rays Coma Cluster of Galaxies

Our Galaxy Cluster: The Local Group Milky Way Andromeda galaxy Small Magellanic Cloud Large Magellanic Cloud

Neighboring Galaxies Some galaxies of our local group are difficult to observe because they are located behind the center of our Milky Way, from our view point. Spiral Galaxy Dwingeloo 1

Interacting Galaxies Cartwheel Galaxy Particularly in rich clusters, galaxies can collide and interact. Galaxy collisions can produce ring galaxies and tidal tails. NGC 4038/4039 Often triggering active star formation: starburst galaxies

Tidal Tails Computer simulations produce similar structures. Example for galaxy interaction with tidal tails: The Mice Computer simulations produce similar structures.

Simulations of Galaxy Interactions Numerical simulations of galaxy interactions have been very successful in reproducing tidal interactions like bridges, tidal tails, and rings.

Radio image of M 64: Central regions rotating backward! Mergers of Galaxies Radio image of M 64: Central regions rotating backward! NGC 7252: Probably result of merger of two galaxies, ~ a billion years ago: Small galaxy remnant in the center is rotating backward! Multiple nuclei in giant elliptical galaxies

Galactic Cannibalism Collisions of large with small galaxies often result in complete disruption of the smaller galaxy. Small galaxy is “swallowed” by the larger one. NGC 5194 This process is called “galactic cannibalism”

ultraluminous infrared galaxies Starburst Galaxies Starburst galaxies are often very rich in gas and dust; bright in infrared: ultraluminous infrared galaxies M 82 Cocoon Galaxy

Large Scale Structure Superclusters = clusters of clusters of galaxies Superclusters appear aligned along walls and filaments. Vast regions of space are completely empty: “voids”

The Farthest Galaxies The most distant galaxies visible by HST are seen at a time when the universe was only ~ 1 billion years old.

New Terms spiral nebula island universe Shapley–Curtis Debate elliptical galaxy spiral galaxy barred spiral galaxy irregular galaxy megaparsec (Mpc) distance indicator standard candle distance scale look-back time Hubble law Hubble constant (H) rotation curve rotation curve method cluster method velocity dispersion method rich cluster poor cluster ring galaxy galactic cannibalism ultraluminous infrared galaxy starburst galaxy

Discussion Questions 1. From what you know about star formation and the evolution of galaxies, do you think the Infrared Astronomy Satellite should have found irregular galaxies to be bright or faint in the infrared? Why or why not? What about starburst galaxies? What about elliptical galaxies? 2. Imagine that we could observe a gas cloud at such a high look-back time that it is just beginning to form one of the first galaxies. Further, suppose we discovered that the gas was metal rich. Would that support or contradict our understanding of galaxy formation?

Quiz Questions 1. How was William Parsons (Lord Rosse) in 1845 able to see spiral structure in some nebulae, whereas others had not noticed this spiral structure before? a. He had incredibly large pupils and keen eyesight. b. His observatory was located on a high, dry mountain peak. c. His telescope, with a diameter of 72 inches, was the largest in the world. d. No nebulae had been observed with a telescope before his time. e. His long time exposure photographs that revealed the spiral structure.

Quiz Questions 2. What did William Parsons (Lord Rosse) think the spiral nebulae were? a. Spiral clusters of low luminosity stars located nearby. b. New planetary systems in the process of formation. c. Spiral star clusters located in the Milky Way. d. Dying high-mass stars. e. Island universes.

Quiz Questions 3. What was the topic of the Shapley-Curtis Debate of 1920? a. The location of the spiral nebulae. b. The size of the Milky Way Galaxy. c. The period-luminosity relationship of Cepheid variable stars. d. The period-luminosity relationship of RR Lyrae variable stars. e. The time-sharing schedule of the new 100 inch diameter telescope.

Quiz Questions 4. Edwin Hubble resolved the Shapley-Curtis debate in 1924 by measuring the distance to large, bright spiral nebulae. What distance method did Hubble employ? a. The parallax method. b. The Hubble Law method. c. The Cepheid variable star method. d. The spectroscopic parallax method. e. The RR Lyrae variable star method.

Quiz Questions 5. Galaxies with active star formation also have which of the following? a. Plenty of gas and dust. b. O and B associations. c. Emission nebulae d. A bluish tint. e. All of the above.

Quiz Questions 6. The Hubble deep field image reveals more than 1500 distant galaxies in a region about 1 arc minute in diameter. If this density of galaxies is typical over the whole sky, how many distant galaxies are hidden from view at one instant by the Moon, with an angular diameter of about 30 arc minutes? a. Approximately 150 distant galaxies are hidden behind the Moon at any given time. b. Approximately 1500 distant galaxies are hidden behind the Moon at any given time. c. Approximately 15,000 distant galaxies are hidden behind the Moon at any given time. d. Approximately 150,000 distant galaxies are hidden behind the Moon at any given time. e. More than 1,000,000 distant galaxies are hidden behind the Moon at any given time.

Quiz Questions 7. How does a Sa galaxy differ from a Sc galaxy? a. The Sa galaxy has a smaller nuclear bulge. b. The Sa galaxy has more loosely wound spiral arms. c. The Sa galaxy has less gas and dust. d. Both a and c above. e. Both b and c above.

Quiz Questions 8. What gives elliptical galaxies a redder color than spiral galaxies? a. Elliptical galaxies are more massive, and thus have a larger gravitational red shift. b. Elliptical galaxies are more distant, and thus have more interstellar reddening. c. Elliptical galaxies are more distant, and thus have larger red shifts. d. Elliptical galaxies have a higher percentage of iron. e. Elliptical galaxies have less gas and dust.

Quiz Questions 9. What must we know about an object to use it as a distance indicator? a. The object’s luminosity. b. The object’s linear size. c. The object’s age. d. Either a or b above. e. Either a, b, or c above.

Quiz Questions 10. Which of the following is NOT a distance indicator used in galactic astronomy? a. White dwarfs. b. Cepheid variable stars. c. Planetary nebulae. d. Type Ia supernovae. e. Brightest globular cluster.

Quiz Questions 11. What observable property of a standard candle must be measured to determine its distance? a. Age. b. Mass. c. Luminosity. d. Angular size. e. Apparent magnitude.

Quiz Questions 12. Why is a supernova type Ia standard candle better to use in measuring very long distances than either the brightest globular cluster or Cepheid variable star standard candles? a. Type Ia supernovae are more luminous. b. Globular star clusters and Cepheid variables exist only in the Milky Way Galaxy. c. Type Ia supernovae are very common. d. The calibration of Type Ia supernovae is more precise. e. Both c and d above.

Quiz Questions 13. When viewing a distant galaxy, the amount of look-back time in years is equal to the a. distance to the galaxy in light years. b. round-trip distance to the galaxy in light years. c. time that has passed since the galaxy was first discovered. d. exposure time of a photograph that is taken of that galaxy. e. time since you last looked at the galaxy.

Quiz Questions 14. What is the distance to a galaxy that has a recessional speed of 3500 km/s if the Hubble constant is 70 km/s/Mpc? a. 10 Mpc. b. 20 Mpc. c. 30 Mpc. d. 40 Mpc. e. 50 Mpc.

Quiz Questions 15. What parameter must we know to determine the linear diameter and luminosity of a particular galaxy? a. The type of galaxy. b. The age of the galaxy. c. The mass of the galaxy. d. The distance to the galaxy. e. The amount of gas and dust in the galaxy.

Quiz Questions 16. Which type of galaxy has the largest range in both size and mass? a. Spiral. b. Elliptical c. Irregular. d. Both a and b have the same range. e. All of the above have the same range.

Quiz Questions 17. What assumption is made in determining mass by the rotation curve method, the cluster method, and the velocity dispersion method? a. That Newton's law of gravity is valid. b. That the system being measured is holding itself together. c. That spiral galaxies rotate faster than elliptical galaxies. d. Both a and b above. e. All of the above.

Quiz Questions 18. Which feature of a galaxy seems to be related to the mass of the galaxy's central supermassive black hole? a. The size of the galaxy's central bulge. b. The rotation curve of the galaxy. c. The overall color of the galaxy. d. The mass of the galaxy. e. The age of the galaxy.

Quiz Questions 19. Which of the following is the least important factor in galactic evolution? a. Collision of galaxies. b. The merger of larger galaxies. c. The harassment by galaxies that pass nearby. d. Small galaxies are cannibalized by larger galaxies. e. The elemental composition of the material from which galaxies form.

Quiz Questions 20. In a few billion years our own Milky Way may merge with the Andromeda galaxy. Which type of galaxy is most likely to result from such a merger? a. An irregular galaxy. b. A larger spiral galaxy. c. An elliptical galaxy. d. Either a or c above. e. Either b or c above.

Answers 1. c 2. e 3. a 4. c 5. e 6. e 7. c 8. e 9. d 10. a 11. e 12. a 13. a 14. e 15. d 16. b 17. d 18. a 19. e 20. c

Hubble (SLIDESHOW MODE ONLY)