Element Abundanc es We are stardustrisa.stanford.edu/compton/comptonspring05lec2.pdf · We are stardust!!! ¥All heavier element s are formed in the cores and explosions of s tars

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 #$$%

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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'/-

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made of?

)baryonic* matter: made of protons & neutrons

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Made of?Sta

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Element Abundances

• H and He

made a few

minutes

a.er the

big bang

• Where did

the rest

come from?

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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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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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vorbit =

√M/r


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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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!


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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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


The Coma Cluster

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)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


)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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)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*!

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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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Illustration by Martin Kornmesser &Lars Lindberg Christensen, ST-ECF

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a massive galaxy cluster

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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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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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What is Dark Matter?

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• Doesn6t emit or absorb light in any

wavelength

• Visible through gravitational e7ects

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• 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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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?


The Contents of Our

Universe


Next Time:

How did structure form in

our Universe?


• 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

Documents

Element Abundanc es We are stardustrisa.stanford.edu/compton/comptonspring05lec2.pdf · We are stardust!!! ¥All heavier element s are formed in the cores and explosions of s tars