Download Time From the Perspective of a Particle Physicist

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
PHYS 162
1
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
PHYS 162
2
Mass of Galaxy
PHYS 162
3
Differential Rotation of Galaxy
PHYS 162
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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
PHYS 162
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Differential Rotation of Galaxy
PHYS 162
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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 (1960-1970). Vera Rubin
pioneered though no Nobel Prize as (partially) she
wasn’t a male (she was first female allowed to use
some telescopes)
PHYS 162
7
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
www.physics.niu.edu/physics/academic/grad/theses1.shtml)
PHYS 162
8
Gravitational Lensing by Galaxies
PHYS 162
9
Composition of the Universe
95% not
understood
Graphics courtesy: NASA
PHYS 162
10
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
PHYS 162
11
Milky Way
Formation
old stars in
halo give
shape early in
formation
PHYS 162
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Elliptical vs Spiral Galaxy Formation
if less initial
rotation easier for
early star formation
prior to collapse
into disk
PHYS 162
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Elliptical vs Spiral Galaxy Formation
elliptical
galaxies tend to
have older stars
PHYS 162
14
Colliding and Merging Galaxies
galaxies pull on
each other by
gravity
 orbits
 interact
 can merge
happens over
billions of years
PHYS 162
15
Hubble Law v=Hd
• measuring recessional velocity vs distance
PHYS 162
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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)
1 Mpc 3.3MLY
1LY  300,000 km / sec1 year
1
Mpc
3.3  106  3  105 km / sec year



H 71km / sec
71km / sec
1

 14 billion years
H
PHYS 162
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Extra Slides
PHYS 162
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Colliding and Merging Galaxies
smaller galaxies often consumed by the larger galaxy
nicadd.niu.edu/~hedin/162/andromeda.mov M31-M33
PHYS 162
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MACHO search
PHYS 162
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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)
PHYS 162
21
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
PHYS 162
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