Milky Way Galaxy: Keeper of the Darkest Secrets of the Universe

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Milky Way Galaxy Keeper of the Darkest Secrets
of the Universe

• Oleg Gnedin
• Saturday Morning Physics 4/4/09

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What is a Galaxy to you?
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To all of us, Galaxy is the Milky Way
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What is the Milky Way Galaxy?What is it hiding?
What can it tell us about the Universe?
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Milky Way became a galaxy only in 1610 when
Galileo used his telescope and resolved the
white river into individual stars
Still, it was difficult to understand what the
Milky Way is just by counting
stars. William Herschels 1785 map of the
Galaxy
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Whenever we observe the Milky Way in a new range
of wavelengths of electromagnetic radiation, it
reveals new objects and new structures
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Some of the darkest secrets can only be revealed
indirectly, by measuring the motion of luminous
objects
Astronomers can measure rotation velocity in the
disk of the Milky Way using the Doppler shift of
emission lines in the spectrum of stars and gas
clouds
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Contemporary of Galileo, Kepler finds a law that
describes the orbits of planets in Solar System
period2 distance3NB distance from Sun, not
Earth
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Newton later reformulates Keplers law using his
new theory of gravity V2 G M(r) / r V
rotation velocity G Newtons constant r
average distance from center M(r) mass
within sphere of radius r grows
with r if matter is spread out In Solar System
rotation curve declines because Sun has almost
all the mass But in Milky Way rotation curve
stays approximately constant with distance
Matter in the Galaxy must be more spread out
than in Solar System
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but the rotation curve should still fall at
distances beyond the disk
our neighbor galaxy M33
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The observed rotation curve can be explained if
the visible part of the Galaxy is embedded into
a large halo of dark (not emitting any light)
matter
The first model of the Galaxy with a dark matter
halo was calculated by Jerry Ostriker and Jim
Peebles in 1973
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Dark matter Bright future Jerry Ostriker and
Jim Peebles
1973
2005 Crafoord Prize
National Medal 2000
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Another dark secret Newtons law reveals that
there is a black hole at the Galactic center with
a mass of 4 million solar masses
Motion of stars at the Galactic Center in
near-infrared light
Visible light is highly obscured
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To understand dark matter in the Milky Waywe
must consider the evolution of the whole Universe
Cosmic Structure grows under the influence of its
own gravity from primordial density fluctuations
seen in
the Cosmic Microwave Background (radio waves)
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Dave Wilkinson UM 57 62
Wilkinson Microwave Anisotropy Probe (WMAP) shows
the seeds of cosmic structure fluctuations of 1
part in 100,000 in the intensity of the CMB signal
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These fluctuations gradually evolve into the
galaxies we observe
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Our modern understanding of the formation of
galaxies is based on numerical modeling ?
supercomputer simulations
Numerical techniques are borrowed from aerospace
engineering they are well tested
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However, mistakes are known to happen in the
history of science and technology so we are
constantly checking our models against
observations
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Simulations of the whole universe require big
computers MareNostrum in Barcelona 10240
processors
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Time in billions of years
0.5
2.2
5.9
8.6
13.7
13
35
70
93
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Size of expanding box in millions of light years
Models confirm that gravity of dark matter pulls
mass into denser regions Universe grows lumpier
with time (perhaps even too lumpy)
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Formation of the Milky Way halo
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Note the abundance of structure in the Milky Way
halo
Do we observe these satellite galaxies?
Predicted satellite galaxies are missing in
observations Missing Satellites Problem
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But wait, there is more!

• Recently many new satellites were discovered by
  the Sloan Digital Sky Survey

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However even after counting the new dwarfs the
discrepancy with the models is still large most
satellites must remain completely dark
Number of dwarfs vs. velocity (proxy for mass)
How do we test whether such dark halos exist?
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Can we actually see dark matter? Maybe!
Theory of Super-Symmetry all particles have
counterparts (anti-particles) Einstein
grandfather of supersymmetry E mc2
energy can be converted into matter Dark matter
particles (WIMPs) are their own anti-particles
and should annihilate when two meet.
This process of annihilation (conversion of mass
into matter) creates very high energy radiation
gamma-rays. Where to find gamma rays? Look again
at the Galactic Center
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H.E.S.S. telescopes, Namibia
Look for a fall-off in the spectrum of very
energetic particles at 1-10 TeV (1 TeV
1,000,000,000,000 electron-Volt, visible light ?
2 eV)
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Fermi Gamma Ray Telescope launched June 11, 2008
will survey the whole sky and look for sources of
gamma-rays, including dark matter annihilation
Enrico Fermi
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Gamma-Ray sky as seen by Fermi Telescopefirst
data release
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Current models using competing numerical
simulations disagree on where the strongest
signal should be
No detection results yet But they are expected
later this year! Watch science news and go to
public talks
Milky Way may finally reveal the dark matter
particle, the best kept secret of the
Universe since Galileo.