1. Masses of Galaxies Measure mass by: motion within a galaxy
motion of different galaxies about each other
gravitational lensing
Gives
- most mass isn’t in stars and “normal” matter (gas, dust, neutron stars, black holes)  DARK MATTER
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2. Visible and Dark Matter
“visible” matter - star, gas, dust, neutron star, black hole
“dark” matter - not understood -
- cooled down white dwarves or black holes leftover from early universe (MACHO study) NO
- new physics - neutrinos having enough mass, new particles (WIMPS) YES??
more than 75% of mass is not understood - Dark Matter mystery
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3. Mass of Galaxy
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4. Differential Rotation of Galaxy
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5. Mass of Milky Way
Sun is 30,000 light years from center (2,000,000,000 AU); period of 200,000,000 years
same as for planets:
Dist3 / Period2 = M (inside) giving 200 billion mass(Sun) inside the Sun’s radius
repeat for 150,000 LY  >1000 billion Mass(Sun) for Galaxy mass inside that distance
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6. Differential Rotation of Galaxy
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7. Mass of Galaxies mass doesn’t match observed amount of matter
unseen “dark matter” unknown composition
extends beyond visible part of Milky Way
observed in other Galaxies ( ). Vera Rubin pioneered though no Nobel Prize as (partially) she wasn’t a male (she was first female allowed to use some telescopes)
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8. Other Dark Matter Observations
look at velocities of individual galaxies in a cluster about each other  missing mass first observed by Fritz Zwicky in the 1930s (Caltech; he also introduced name “supernova”)
look at gravitation lensing by a nearby galaxy of a more distant galaxy (many including NIU students Donna Kubik and Matt Weisner. see their theses at
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9. Gravitational Lensing by Galaxies
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10. Composition of the Universe
95% not understood
Graphics courtesy: NASA
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11. Galaxy Formation Rotating gas cloud about 13 billion years ago
- local concentrations give first stars
Cloud collapses due to gravity
large rotation  spiral
small rotation  elliptical
near other big galaxy  irregular
Interacting with other galaxies  rotation
Gas/Dust/Star formation persist in spiral and irregular
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12. Milky Way Formation old stars in halo give shape early in formation
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13. Elliptical vs Spiral Galaxy Formation
if less initial rotation easier for early star formation prior to collapse into disk
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14. Elliptical vs Spiral Galaxy Formation
elliptical galaxies tend to have older stars
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15. Colliding and Merging Galaxies
galaxies pull on each other by gravity
 orbits
 interact
 can merge
happens over billions of years
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16. Hubble Law v=Hd
measuring recessional velocity vs distance
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17. Hubble Law  Measure Age of Universe
1/H gives approximate age of Universe
need to convert 71 km/sec/Mpc to inverse years (DON’T NEED TO KNOW. On exercise)
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18. Extra Slides
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19. Colliding and Merging Galaxies
smaller galaxies often consumed by the larger galaxy
nicadd.niu.edu/~hedin/162/andromeda.mov M31-M33
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20. MACHO search
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21. MACHOs vs WIMPS Massive Astrophysical Compact Halo Objects - MACHOs
- not new physics – look for by gravitational lensing
Weakly Interacting Massive Particles – WIMPs new subatomic particles look for in high energy experiments at CERN and Fermilab or in ultra cold experiments in deep underground mines (will be in movie)
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22. Donna Kubik Thesis
NIU students work with Fermilab astrophysicists. Use Sloan Digital Sky Survey data to find “Einstein” ring candidates. Then use better telescope to improve image. Size of ring tells mass in closer galaxy  amount of dark matter
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