1414

Dark MatterDark Matter

Join me on the Dark Side

1414

GoalsGoals• Why do we think there is dark matter?• Where do we think it is?• How much is there?

1414

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.

1414

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

1414

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.

1414

Galaxy Rotation• Objects in the disk,

orbit in the disk.• Kepler’s Third Law

gives the total mass in orbits.

Mass Total)(Separation)(Period

32 AUyrs

1414

What should we expect?• Solar System:

Planet Separation (AU)

Velocity (km/s)

Mercury

0.5 48

Earth 1 30

Saturn 10 10

Pluto 40 5

+

V

D

1414

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.

1414

+

V

D

1414

Other Galaxies• Observe HI in other galaxies.

1414

Rotation Curves• If HI gas is rotating or moving, the

21cm radiation will be Doppler Shifted.

1414

1414

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.”

1414

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.

1414M/L ~ 20 – 130!

1414

Even More Galaxy Masses• Look for gravitational lenses near galaxy

clusters.• More lensing means more mass.

• Compare mass from lensing to luminosity.

1414

Gravitational

Lensing

1414

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

1414Brown dwarfs and planets, Copyright – G. Schneider et al. (U of A)

1414

More Gravity Lensing• MACHO

project.• Look for

lensed stars in our galaxy.

• Not enough MACHOs to account for all Dark Matter.

• WIMPS?

1414

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.”

1414

Homework #12Homework #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