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Galaxies

0. Note that the following lectures include animations and PowerPoint effects such as fly-ins and transitions that require you to be in PowerPoint's Slide Show mode (presentation mode). 0. Galaxies. Chapter 16. 0. Guidepost.

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Galaxies

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  1. 0 Note that the following lectures include animations and PowerPoint effects such as fly-ins and transitions that require you to be in PowerPoint's Slide Show mode (presentation mode).

  2. 0 Galaxies Chapter 16

  3. 0 Guidepost Our Milky Way Galaxy is only one of the many billions of galaxies visible in the sky. This chapter will expand your horizon to discuss the different kinds of galaxies and their complex histories. Here you can expect answers to five important questions: • What are the different types of galaxies? • How do astronomers measure the distances to galaxies? • How do galaxies differ in size, luminosity, and mass? • Do other galaxies contain supermassive black holes and dark matter, as does our galaxy? • Why are there different kinds of galaxies?

  4. 0 Guidepost (continued) As you begin studying galaxies, you will discover they are classified into different types, and that will lead you to insights into how galaxies form and evolve. In the next chapter, you will discover that some galaxies are violently active, and that will give you more clues to the evolution of galaxies.

  5. 0 Outline I. The Family of Galaxies A. The Discovery of Galaxies B. How Many Galaxies are there? C. The Shapes of 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 G. Gravitational Lensing and Dark Matter

  6. 0 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

  7. 0 Galaxies • Star systems like our Milky Way • Contain a few thousand to tens of billions of stars • Large variety of shapes and sizes

  8. 0 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

  9. 0 Galaxy Classification Ellipticals: Spirals: Large nucleus; tightly wound arms Sa E0, …, E7 E0 = Spherical E1 Sb Sc Small nucleus; loosely wound arms E7 = Highly elliptical E6

  10. 0 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

  11. 0 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

  12. 0 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

  13. 0 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

  14. 0 Cepheid Distance Measurement Repeated brightness measurements of a Cepheid allow the determination of the period and thus the absolute magnitude. Distance

  15. 0 Distance MeasurementUsing Type Ia Supernovae Remember: Type Ia supernovae (collapse of an accreting white dwarf) have almost uniform luminosity → Absolute magnitude Observe apparent magnitude Distance

  16. 0 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

  17. 0 Distance Measurements to Other Galaxies (2): The Hubble Law E. Hubble (1913): 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 theHubble constant • Measure vr through the Doppler effect infer the distance

  18. 0 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

  19. 0 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

  20. 0 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.

  21. 0 Determining the Masses of Galaxies Based on rotation curves, use Kepler’s 3rd law to infer masses of galaxies

  22. 0 Masses and Other Properties of Galaxies

  23. 0 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

  24. 0 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

  25. 0 Gravitational Lensing According to General Relativity, light will be bent towards a massive object when passing it. This phenomenon is called Gravitational Lensing. It can be used to detect otherwise invisible Dark Matter and measure its mass.

  26. 0 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

  27. 0 Our Galaxy Cluster: The Local Group Milky Way Andromeda galaxy Small Magellanic Cloud Large Magellanic Cloud

  28. 0 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.

  29. 0 The Canis Major Galaxy The Canis Major Dwarf Galaxy has orbited around the Milky Way a number of times, and tidal forces have ripped away stars and gas.

  30. 0 Interacting Galaxies Particularly in rich clusters, galaxies can collide and interact. Galaxy collisions can produce ring galaxies and Cartwheel Galaxy tidal tails. NGC 4038/4039 Often triggering active star formation: starburst galaxies

  31. 0 Tidal Tails Example for galaxy interaction with tidal tails: The Mice Computer simulations produce similar structures

  32. 0 Simulations of Galaxy Interactions Numerical simulations of galaxy interactions have been very successful in reproducing tidal interactions like bridges, tidal tails, and rings.

  33. 0 Mergers of Galaxies Radio image of M 64: Central regions rotating backward! NGC 7252: Probably result of merger of two galaxies, about one billion years ago: Multiple nuclei in giant elliptical galaxies Small galaxy remnant in the center is rotating backward!

  34. 0 Starburst Galaxies Starburst galaxies: Galaxies in which stars are currently being born at a very high rate Starburst galaxies contain many young stars and recent supernovae, and are often very rich in gas and dust; bright in infrared: ultraluminous infrared galaxies

  35. 0 Interactions of Galaxies with Clusters When galaxies pass through the thin gas of their home cluster, their own gas can be almost completely stripped away.

  36. 0 The Farthest Galaxies The most distant galaxies visible by HST are seen at a time when the universe was only ~ 1 billion years old

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