H205 Cosmic Origins

Today: Finish Galaxy Evolution

Dark MatterEP 5

APOD

Two Public Lectures

Einstein’s BiggestBlunder: A Cosmic

Mystery Story

Lawrence KraussArizona State Univ.Saturday, April 18

12:30 PMJordan Hall 124

From the Big Bang to the Nobel Prize and

on to the James Webb Space Telescope

John MatherGoddard SFC

Tuesday, April 217:30 PM

Whittenberger, IMU

Exploring Galaxy

Evolution in Clusters

of Galaxies

Hercules

CentaurusComa

Perseus

gravity holds clusters together

galaxies aren’t the biggest structures in the Universe

Galaxy Clusters

DistanceNumber

of SpiralsNumber

of EllipticalsPercentageof Ellipticals

Nearby Clusters

Coma 99 Mpc 5 15 75Perseus 75 Mpc 7 13 65Centaurus 3.7 Mpc 9 17 60Distant Clusters

Abell 851 1700 Mpc 11 18 62Abell 1689 343 Mpc 2 18 90MS 1054 03 355 Mpc 10 10 50

DARK MATTER

The universe is NOT what it seems…

DARK MATTER• “Extraordinary claims require

extraordinary evidence.” (Carl Sagan) • “Extraordinary claims require

extraordinary proof.” (Marcello Truzzi) • “The weight of evidence for an

extraordinary claim must be proportioned to its strangeness” (Laplace)

• “A wise man, therefore, proportions his belief to the evidence” (David Hume)

Evidence for Dark Matter

Rotation of galaxies

Velocities of stars in dwarf

galaxies

Velocities of galaxies in clusters

Gravitational lensing

Collisions of galaxy clusters

Hot gas in galaxy

clusters

Galaxy interactions

Mass within Sun’s orbit:

~1011 MSun

Total mass:

~1012 MSun

Galaxy Rotation

What’s the PROBLEM???• The orbits of stars suggest that galaxies contain

several times more mass that we can find in stars, gas and dust

• MISSING MASS!

• Dark matter is the material believed to account for the discrepancy between the mass of a galaxy as found from the orbits of stars and the mass observed in the form of gas and dust

The visible portion of a galaxy lies deep in the heart of a large halo of dark matter

Rot Vel

Grav mass

lum Lum mass

Lum/grav

2 Kpc 100 4.6e9 5e8 1e9 .22

4 Kpc 120 1.3e10

1.7e9 3.4e9 .25

6 Kpc 130 2.4e10

2.8e9 5.6e9 0.23

8 Kpc 130 3.1e10

3.7e9 7.5e9 .23

10 Kpc

165 6.3e10

4.5e9 9e9 0.14

Velocity Dispersions in

Dwarf Galaxies Count the stars Add up the light Look for any gas Add up the mass

Velocity Dispersions in Dwarf Galaxies

• From spectra and the Doppler shift• Measure the velocity dispersion• Determine the total mass

Calculated for a sample of 194 stars with 32-33 stars per bin

astro-ph/0704126

M/L Ratios for MW DwarfsGalaxy MV L

Radius

Total mass

M/LGas

Fraction

(mag) (106 LSun) (pc)(106

MSun)(Sun=1)

Sculptor -11.1 2.15 110 6.4 3.0 0.004

Phoenix -10.1 0.90 310 33 37 0.006

Fornax -13.2 15.5 460 68 4.4 <0.001

Carina -9.3 0.43 210 13 31 <0.001

Leo I -11.9 4.79 215 22 4.6 <0.001

Sextans -9.5 0.50 335 19 39 <0.001

Leo II -9.6 0.58 160 9.7 17 <0.001

Ursa Minor

-8.9 0.29 200 23 79 <0.002

Draco -8.8 0.26 180 22 84 <0.001

Galaxy interactions require more mass than we can see

Antennae Galaxy (HST)Computersimulation

The real thing

Evidence for dark matter in clusters of galaxies

We can measure the velocities of galaxies in a cluster from their Doppler shiftsThe mass we find from galaxy motions in a cluster is about 50 times larger than the mass in stars!

Clusters contain large amounts of X-ray emitting hot gasTemperature of hot gas (particle motions) tells us cluster mass The mass is much more than gas and galaxies combined

85% dark matter 13% hot gas 2% stars

A view of the Coma Cluster in optical light (left) and at X-ray (right, from

Chandra) wavelengths

1E 0657-56 – The Bullet Cluster

Direct observation of Dark Matter

More Evidence for Dark Matter

• 1E 0657-56 – A collision of galaxy clusters

• A cluster of galaxies consists of three components

1. Galaxies

2. Hot Gas

3. Dark Matter

What’s going on with 1E 0657-56?

• TWO clusters of galaxies collide

The gas interacts, the dark matter and galaxies don’t

• The galaxies and dark matter pass through unimpeded, but the hot gas is separated from the clusters

Gravitational Lensing

• Light from a distant, bright source is bends around a massive object (such as a massive galaxy or cluster of galaxies) between the source object and the observer

• Gravitational lensing is predicted by Einstein's theory of general relativity (Einstein 1936)

General Relativity

• The lens phenomenon exists because gravity bends the paths of light rays

• In general relativity, gravity acts by producing curvature in space-time

• The paths of all objects, whether or not they have mass, are curved if they pass near a massive body

• Prediction confirmed in the 1919 solar eclipse

Discovering Gravitationa

l Lenses

• Mysterious arcs discovered in 1986 (a) Cluster Abell 370 (left)

• cluster redshift z=0.37• arc redshift z=0.735

(b) Cluster C12244 (right)• cluster redshift z=0.31 • arc redshift of 2.24

• Bright knots on the arcs show the structure of of the galaxies, whose images are strongly distorted

• The influence of individual lensing cluster galaxies the arc can also be seen

Three Classes of Gravitational Lenses

• Microlensing - no distortion in shape can be seen but the amount of light received from a background object changes with time– Microlensing occurs with

stars and extrasolar planets

• Strong lensing - easily visible distortions – Einstein rings, arcs, and multiple images

• Weak lensing - distortions are much smaller– Detected by analyzing large numbers of objects to find

distortions of only a few percent. – The lensing shows up statistically as a preferred

stretching of the background objects perpendicular to the direction to the center of the lens

The Einstein Cross

The Double Quasar –the first gravitational lens

Unlike optical lenses, gravitational lenses produce multiple images

• In an optical lens, maximum bending occurs furthest from the central axis

• In a gravitational lens, maximum bending occurs closest to the central axis

• A gravitational lens has no single focal point • If the source, the lens, and the observer lie in a

straight line, the source will appear as a ring around the lens

• If the lens is off-center, multiple images will appear. The lensed image will always be distorted

Simulating Gravitational Lenses

• HST MDS WFPC2 HST Gravitational Lens Simulation (mds.phys.cmu.edu/ego_cgi.html)

• A galaxy having a mass of over 100 billion solar masses will produce multiple images separated by only a few arcseconds

• Galaxy clusters can produce separations of several arcminutes

source andlens aligned

source andlens not aligned

Arcs in the Galaxy Cluster

Abell 2218 (z=0.175)

• Several arcs surround the cluster center – Arc A0 has a redshift of 2.515; – Near A2 is another image of the same galaxy

• More arcs surround a second mass concentration (upper right)• Multiple images of the same distant galaxies allows detailed

model of the mass of the lensing cluster

cluster center

Cluster of Galaxies Cl0024+16• The reddish objects

are galaxies in the lensing cluster at z=0.39

• The bluish objects are multiple images of a distant galaxy at z=1.63 lensed by the cluster

• Reconstruct the distant galaxy individual pieces of the arc

Galaxy Cluster

Cl1358+62

• The reddish arc is a lensed image of a background galaxy with z=4.92 – upper right - an enlarged version of the lensed galaxy – lower right - a reconstruction of the unlensed source

Abell 2390

• A thick arc with z=0.913• Two more arc systems are also seen

(indicated by the letters A and B) – system A has redshift z=4.04 – system B has redshift z=4.05

The Bottom Line…• The visible matter

does not provide enough gravity to produce the gravitational lenses we see from galaxies and galaxy clusters

• Dark matter must be present to account for what we observe

cluster center

Arcs let us map the distribution of dark matter in clusters of galaxies

All methods of measuring cluster mass indicate similar amounts of dark matter

Dark Matter The universe contains matter we cannot

see Dark matter interacts with normal

matter through gravity Dark matter does NOT interact with light

the way the normal matter does The Universe contains 5 or 6 times

MORE dark matter than normal matterAll galaxies are embedded in clouds of

dark matter

Alternative to Dark Matter: MOND - Modified Newtonian Dynamics

MOND can‘t explain DM in clusters and far out in halos

For accelerations a less than a0, reduce gravity acceleration by the factor a/a0

a(a/a0) = GM/r2

This gives flat rotation curves

A single value of a0 works for galaxy rotation curves

But MOND is untested experimentally

MOND can’t explain it all

• While MOND can reproduce galaxy rotation curves, it is harder to explain– Galaxy cluster velocity dispersions– Observations of gravitational lenses– The Bullet Cluster and the DM ring

• MOND still requires DM to account for all the observations

• Which is a simpler explanation, DM or MOND+DM?

Summary: Dark Matter EvidenceMany dynamical phenomena cannot be

explained with the observed mass content of the universe

Problem can be solved with one radical assumption

85% of all matter is dark matter initially distributed as ordinary matterinteracts with normal matter only through

gravity

Stars, gas are now more concentrated than dark matter

Why is DARK MATTERDARK MATTER important?

The formation of structure and of galaxies requires the extra mass provided by dark matter.

Without dark matter, the Universe would

not exist as we know it

Dark Matter Dominates the

Structure of the Universe

Center for Cosmological Physics,University of Chicago

http://cosmicweb.uchicago.edu/index.html

• The formation of clusters and filaments in a universe filled with cold dark matter

• The box is 43 million parsecs (or 140 million light years) • Simulation begins at z=30 - the Universe is less than 1% of

its current age and matter is uniformly distributed• Small fluctuations grow to large structures• Structures formed by z=0.5

The Evolution of Dark Matter

Observed with Hubble

• Dark matter filaments form under the pull of gravity, and clump

• Dark matter filaments provide the structure for the formation of stars and galaxies from ordinary matter

• Gravity from dark matter needed to form structures and galaxies

• Formation proceeds hierarchically

• Small-mass objects form at z>5, grow and merge, to form larger and larger systems

• Galactic "cannibalism" ongoing

• The two objects approaching at z~0 will merge in about a billion years

• Many of the small systems become satellites orbiting larger systems

4.3 Mpc or 14 million LY

The formation and evolution of these groups, which are very common in the Universe, are dominated by the gravitational pull of dark matter

Forming Galaxy Groups (like ours!)

Galaxy Formation

• A disk galaxy forming when the virtual universe was "only" one and a half billion years old

• The galaxy forms where several large-scale filaments of dark matter intersect

• These filaments provide gas and dark matter to the galaxy

• The gas fuels star formation, while the galaxy grows by accreting dark matter and smaller galaxies

36 kpc

288 kpc

72 kpc

144 kpc

Dark matter provides the Dark matter provides the gravitational mass necessary for gravitational mass necessary for

galaxy formation to proceedgalaxy formation to proceed

The cosmic web of dark matter, gas, and galaxies in a young universe

Intergalacticgas

Clumpsconcentratedby darkmatter lead to galaxies

Galaxies Grow through Mergers

Galaxy building blocks

observed withHubble

Simulation

The real thing

What is DARK MATTER?

Can’t see it, taste it, touch it, smell it…We can only detect it by gravityWe don’t know!

Detecting Dark Matter is one of the most active areas of high energy physics, and a reason to build large accelerators.

So, What Is Dark Matter?• Non-baryonic, to reconcile with

primordial nucleosynthesis and large-scale structure growth

• Slow Moving: must not escape from potential wells (slow moving = cold)

• Dark Matter Candidates:– Black holes– Low-mass objects (“MACHO”s, free-floating

planets) (but this stuff is baryonic)– Elementary particles

Can Dark Matter Be Black

Holes??

Plausible mass range: ~106 Msun Such massive black holes cannot be

the dark matter in dwarf galaxies That many BH’s in Draco would disrupt

the galaxy!

What about Big, Dark Rocks?Or Loose Planets?

MACHO’s: Massive Compact Halo ObjectsMass range: 0.08 MSun (stellar limit) to MEarth

Observational test: gravitational microlensingif all the dark matter in the Milky Way’s

halo was MACHOS one in 106 chance that a star has a MACHO

exactly along the line of sightfocussing brightening of the star’s imageas star moves brightness changes

Searching for Microlenses

Large Magellanic Cloud

Micro-Lensing Cartoon

Lensing Lightcurve

Are MACHOs the Dark Matter?

•NO – Not enough lensing events are detected

•MACHO’s make up (at most) 15% of the Milky Ways halo mass

•Inferred mass range for MACHOs:

0.4MSun (Faint MW or LMC stars)

MACHOs are not the solution to the dark matter problem

What about WIMPS??

• “Weakly Interacting Massive Particles”– As yet undiscovered elementary particles

• High energy particle theories suggest such elementary particles exist

WIMPS are a plausible, but not firm, consequence of several theories in particle physics

• Cold, collisionless, dark matter explains a wide range of phenomena (not only rotation curves)

• Nature of dark matter unknown

•We only know what it is NOT!

Dark Matter

For WednesdayChapter 22 – Dark Energy

Complete EP5