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Lecture 6: Weighing Galaxies --Dark Matter

Lecture 6: Weighing Galaxies --Dark Matter. Astronomy 5: The Formation and Evolution of the Universe Sandra M. Faber Spring Quarter 2007 UC Santa Cruz. The Doppler effect. The Doppler effect only exists along the line of motion. Motion crosswise to line of sight has no effect.

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Lecture 6: Weighing Galaxies --Dark Matter

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  1. Lecture 6: Weighing Galaxies --Dark Matter Astronomy 5: The Formation and Evolution of the Universe Sandra M. Faber Spring Quarter 2007 UC Santa Cruz

  2. The Doppler effect

  3. The Doppler effect only exists along the line of motion. Motion crosswise to line of sight has no effect. No effect this way

  4. Fritz Zwicky: cluster mass measurements, 1932 Fritz Zwicky, Caltech astronomer, started making mass estimates of clusters of galaxies in 1932. He was the first to notice that galaxies inside the clusters were moving too fast. How could the clusters hold together? They should fly apart! This was the beginning of the discovery of dark matter.

  5. Interstellar gas Stars How a spiral galaxy rotates: differential rotation Movie by TJ Cox, UCSC Physics Dept.

  6. Stars form from dense clouds of gas Giant H II region in Messier 33 Messier 33 galaxy, a nearby member of the Local Group

  7. Galaxy spectrum showing spectral “features”: H II regions plus starlight O O H H Stars O H S N 400 nm 450 nm 500 nm 550 nm 600 nm 650 nm 700 nm Wavelength (nm)

  8. How rotation translates into a “rotation curve” This plot of orbital speed versus radius is called a “rotation curve.”

  9. Typical observed galaxy “rotation curve” OH emission from Earth’s atmosphere Red end Bright central streak is starlight from middle, where galaxy is brightest Which side is going away? Emission lines from galaxy Blue end Raw spectrum Sky Subtracted

  10. Unexpected: rotation curves are flat in the outer parts of galaxies! from stars alone

  11. A Milky Way-like external galaxy seen edge on bulge disk NGC 891

  12. Schematic model of a visible galaxy surrounded by its dark matter halo 100,000 lyr 1,000,000 lyr

  13. A binary galaxy

  14. Another binary galaxy M51, the Whirlpool galaxy, and its companion.

  15. A small group: Hickson 87

  16. Another small group: Seyfert’s Sextet

  17. A giant cluster: Abell 1689

  18. Another giant cluster: Cl0024+1654

  19. Another giant cluster: Cl0024+1654 Gravitationally lensed background galaxies

  20. Galaxy cluster Abell 2218: note grav lenses!

  21. The simplest nuclear reaction that makes stars shine

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