The Story of Galaxy
Formation in Our Universe:
What is the Universe
Made of?Dr! Risa H! Wechsler
Hubble Fellow" Enrico Fermi Fellow
University of ChicagoKavli Inst! for Cosmological Physics"
Enrico Fermi Institute &
Dept of Astronomy & Astrophysics
Arthur H! Compton Lectures Spring #$$%
Le
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Ap
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http:''kicp!uchicago!edu'(risa'compton'
Risa H! Wechsler" Spring #$$% Compton Lectures
• Evidence for the Expanding Universe +/'01-
• What is the Universe made of? +Today-
• Seeds of Galaxy Formation and the
Development of Structure +/'&$-
• The Age" Shape and Expansion History of
the Universe +%'2-
• A Basic Picture of Galaxy Formation +%'0/-
• Galaxy Interactions and the Hubble
Sequence +%'#0-
• Frontiers in Galaxy Formation +%'#3-
• The Future of the Universe; Summary +1'/-
Le
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Risa H! Wechsler" Spring #$$% Compton Lectures
What is the Universe
made of?
)baryonic* matter: made of protons & neutrons
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Risa H! Wechsler" Spring #$$% Compton Lectures
What is the Universe
Made of?Sta
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Risa H! Wechsler" Spring #$$% Compton Lectures
Element Abundances
• H and He
made a few
minutes
a.er the
big bang
• Where did
the rest
come from?
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Risa H! Wechsler" Spring #$$% Compton Lectures
We are stardust!!!
• All heavier elements are formed in the
cores and explosions of stars
• Main sequence +sun,like- stars: fusion
occurs in the cores
• Abundances of various elements set by
the temperatures of stellar cores and
supernovae +and nuclear physics-
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Risa H! Wechsler" Spring #$$% Compton Lectures
Escape Velocity
• Escape velocity is the
velocity required to
escape a gravitational
4eld
• Larger for more massive
things +vesc Earth ( 00k's;
vesc Jupiter (1$ km's-
• Smaller for smaller
distances +escape velocity
from sun at earth radius (
/# km's" at pluto (2 km's-
•
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De
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vorbit =
√M/r
Risa H! Wechsler" Spring #$$% Compton Lectures
The Critical Density
• The concept of an escape velocity is relevant on
various scales!
• For a given mass and distance" there is a critical
amount of kinetic energy +velocity- required to
escape!
• In the case of the big bang" given the initial amount
of energy" there is a critical density of mass that is
required to overcome this!
• Above this mass" the Universe will contract under
the weight of its own gravity +like the ball returning
to earth-; below this it will expand forever! +More
on this in the next two lectures!-
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Risa H! Wechsler" Spring #$$% Compton Lectures
Are the stars it?
• Adding up all the stars: we
get ($!%5 of the critical
density!
• With this small amount"
gravity6s e7ect on the
expansion would be very
weak!
• Stars form from the
primordial gas" much of this
gas must still exist!
Risa H! Wechsler" Spring #$$% Compton Lectures
Total )baryon* fraction• )Baryons*: matter made up
from protons and neutrons
• Not all )baryonic* material is
in stars! Some remains in
primordial gas +mostly
hydrogen" both cold and hot-
that has not yet formed stars!
• BBN prediction is
!
• In good agreement with
several other constraints
• Still a very small fraction of
the critical density!
Total baryon fraction
predicted by BBN
Ωb = 0.019h−2
! 0.04
Vis
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Ma
tte
r: T
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n F
ract
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Risa H! Wechsler" Spring #$$% Compton Lectures
Is what we can see all
there is?
• Light does not
necessarily trace mass
• The amount of mass in
gravitationally bound
objects can be
determined by looking
at their rotation
velocities
• v =
√M/r
Risa H! Wechsler" Spring #$$% Compton Lectures
The Coma Cluster
Da
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Risa H! Wechsler" Spring #$$% Compton Lectures
)Missing Mass* in
Galaxy Clusters• Fritz Zwicky measured velocities for
(0$$$ galaxies in the Coma Cluster
• Expect
• Zwicky found +ApJ" 08&2-:
More mass
than light" by
a large factor!
1937ApJ....86..217Z
1937ApJ....86..217Z
v =
√M/r
Risa H! Wechsler" Spring #$$% Compton Lectures
)Missing Mass* in
Spiral Galaxies
• Based on known
stars in rotating disk
galaxies" expect the
rotation curves to
fall o7 quickly
• Similar to planet
rotation: stars on
the outskirts should
move more slowlyDa
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in S
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Ga
lax
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Risa H! Wechsler" Spring #$$% Compton Lectures
)Missing Mass* in
Spiral Galaxies
• In mid 081$6s" Vera Rubin
measured the rotation curve of
the Andromeda galaxy
• It was close to 9at" not decreasing
at large radii
• Subsequent measurements on
other galaxies showed the same
feature!
• Indicates substantially more
matter around galaxies than is
seen!Risa H! Wechsler" Spring #$$% Compton Lectures
Stability of Spiral Disks
• In 082&" Ostriker and Peebles did one of the 4rst
gravitational )N,body* simulations
• Basic idea: N identical test particles" under the
in9uence of a gravitational force! Evolve and see
what happens! +Much more on this in next lecture-
• They found that a disk galaxy was unstable ,, hard
to keep it a disk for very long! The addition of a
spherical )halo* around the disk was able to stabilize
the disk! )The halo masses of our Galaxy and of
other spiral galaxies exterior to the observed disks
may be extremely large*!
Da
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Risa H! Wechsler" Spring #$$% Compton Lectures
Gravitational Lensing
• Zwicky predicted that this additional mass should be
observable by the de9ection of light around the
galaxies'clusters
• Einstein6s General Relativity theory makes precise
predictions for this de9ection
• Massive objects actually distort the spacetime
• Light passing through a strong gravitional 4eld
follows the curved paths of this distorted space
• By measuring the de9ection" we can measure mass
Illustration by Martin Kornmesser &Lars Lindberg Christensen, ST-ECF
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Risa H! Wechsler" Spring #$$% Compton Lectures
Gravitational Lensing
Illustration by Martin Kornmesser &Lars Lindberg Christensen, ST-ECF
Da
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Risa H! Wechsler" Spring #$$% Compton Lectures
Gravitational Lensing
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Risa H! Wechsler" Spring #$$% Compton Lectures
a massive galaxy cluster
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Risa H! Wechsler" Spring #$$% Compton Lectures
Abell 0138 Image with Hubble ACSRisa H! Wechsler" Spring #$$% Compton Lectures
Measuring the Total
Mass of the Universe
• The expansion rate of the Universe depends on
the total amount of mass
• More mass : more gravity : slowing
expansion rate" faster evolution of structure
• We will see in the next # lectures that the
amount of mass required for structure
formation" and to explain the expansion rate
+Hubble- is ;; /5 +at least (0'/ of the critical
density-
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Risa H! Wechsler" Spring #$$% Compton Lectures
Evidence for Dark
Matter
• Fast moving galaxies in clusters
• Flat rotation curves of spiral galaxies
• Stability of disk galaxies
• Gravitational Lensing
• Baryon fraction +(/5- vs! Total Mass in
Universe +;#%5-
• Large,scale structure +next lecture-
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Risa H! Wechsler" Spring #$$% Compton Lectures
What is Dark Matter?
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Risa H! Wechsler" Spring #$$% Compton Lectures
What is Dark Matter?
• Doesn6t emit or absorb light in any
wavelength
• Visible through gravitational e7ects
Da
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Risa H! Wechsler" Spring #$$% Compton Lectures
What is Dark Matter?
• Normal stu7 that doesn6t emit light?
Brown Dwarfs or planets +)MACHOs*-"
black holes" dark galaxies and clusters?
• Exotic particles +*non,baryonic*-?
• )Hot* Dark Matter: neutrinos?
• )Cold* Dark Matter: lightest super
symetric partner particle?
• Incorrect law of gravity?
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Risa H! Wechsler" Spring #$$% Compton Lectures
Supersymmetry
• Two types of particles: bosons and fermions
• Particles decay" but only into lighter things
• Basic idea: each known particle has a supersymmetric
partner particle" with almost idential properties" except
that bosons +photons" quarks- have fermionic partners
and fermions +electrons" neutrinos- have boson partners
• Supersymmetry is broken at very high energies +in the
early universe-: supersymmetric particles are much more
massive than their partners
• The lightest supersymmetric particle +neutralino"
gravitino-will have nothing to decay into" so there will be
a lot of them le.: the dark matter?
Risa H! Wechsler" Spring #$$% Compton Lectures
The Contents of Our
Universe
Risa H! Wechsler" Spring #$$% Compton Lectures
Next Time:
How did structure form in
our Universe?
Risa H! Wechsler" Spring #$$% Compton Lectures
• General: http://www.tufts.edu/as/wright_center/cosmic_evolution/
good introduction to the origin of the heavy elements
• WMAP Cosmology 0$0" on dark matter: http://map.gsfc.nasa.gov/m_uni/uni_101matter.html
• Alpha & Omega: The Search for the Beginning and End of the Universe
Charles Seife +#$$&- see Chapter 1" The Dark Universe
• Dark Matter in the Universe" Vera Rubin
Scienti4c American" December 0883
• Supersymmetry" Gordon Kane +#$$$-
further reading
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