14 Dark Matter Join me on the Dark Side

14 Goals Why do we think there is dark matter? Where do we think it is? How much is there?

14 Mass and Gravity All mass has gravity. Gravity attracts all things with mass. Kepler’s Third Law tells us how mass moves due to gravity. Use Kepler’s Third Law to find out how much mass is where.

14 Mass and Luminosity Most mass gives off light. Amount of light indicates how much mass is present. Where there’s more light, there is more mass. –More light from galaxy centers vs. edges. –Conclude more mass in center vs. edges. Copyright – Kelly & Henden

14 Dark Matter Look at: –Our galaxy. –Other galaxies. –Pairs of galaxies. –Clusters of galaxies. Mass due to gravity. Mass indicated by luminosity. Same? No!  Dark Matter.

14 Galaxy Rotation Objects in the disk, orbit in the disk. Kepler’s Third Law gives the total mass in orbits.

14 Distributed Mass In Kepler’s Law, the total mass is the mass “inside” the orbit.

14 What should we expect? Solar System: PlanetSeparation (AU) Velocity (km/s) Mercury0.548 Earth130 Saturn10 Pluto405 + V D

14 What do we see? From variable stars we know distances. From Doppler shift we know rotation velocity. Edges of Milky Way goes too fast. Must be extra mass near edges of galaxy.

14 + V D

14 Other Galaxies Observe HI in other galaxies.

14 Rotation Curves If HI gas is rotating or moving, the 21cm radiation will be Doppler Shifted.

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14 Dark Matter Look at the stars and gas giving off light at the very edge of the galaxy. Still going just as fast as inner part of galaxy. Must mean there is still more mass affecting the edges of galaxies. How much? –Mass/Luminosity = 90! –Conclude 90% of matter is “dark.”

14 More Galaxy Masses Apply Kepler’s Laws to galaxy pairs. Get mass due to gravity. Look at total light from both galaxies. Estimate mass from luminous objects.

14 M/L ~ 20 – 130!

14 Even More Galaxy Masses Look for gravitational lenses near galaxy clusters. More lensing means more mass. Compare mass from lensing to luminosity.

14 Gravitational Lensing

14 What is it? Dark Matter: Ordinary or Extraordinary? What kind of ordinary matter is dark? –Black holes –Black dwarfs (cool white dwarfs) –Brown Dwarfs (failed stars) –Planets –Bowling Balls

14 Brown dwarfs and planets, Copyright – G. Schneider et al. (U of A)

14 More Gravity Lensing MACHO project. Look for lensed stars in our galaxy. Not enough MACHOs to account for all Dark Matter. WIMPS?

14 Conclusions We see the effects of gravity. More gravity than the luminosity would suggest. See this on all scales: –Individual galaxies. –Pairs of galaxies. –Groups of galaxies. –Clusters of galaxies. Maybe 90% mass of Universe is “dark.”

14 Homework #12 Read: Bennett Ch 19.4, 21.5 – 21.6 Do Ch19: –Review Questions: 12 –Problems: 6, 7, 8 Do Ch21: –Review Questions: 16 –Problems: 6