Transcript A105 Stars and Galaxies

H205
Cosmic Origins
APOD
Today: Finish Galaxy Evolution
Dark Matter
EP 5
Two Public Lectures
Einstein’s Biggest
Blunder: A Cosmic
Mystery Story
Lawrence Krauss
Arizona State Univ.
Saturday, April 18
12:30 PM
Jordan Hall 124
From the Big Bang to the
Nobel Prize and on to
the James Webb
Space Telescope
John Mather
Goddard SFC
Tuesday, April 21
7:30 PM
Whittenberger, IMU
Coma
Centaurus
Exploring
Galaxy
Evolution in
Galaxy Clusters
Clusters of
galaxies aren’t the
Galaxies
biggest
structures
in the Universe
gravity holds
clusters together
Perseus
Hercules
Distance
Number
of Spirals
Number
of Ellipticals
Percentage
of Ellipticals
Nearby Clusters
Coma
99 Mpc
Perseus
75 Mpc
Centaurus
3.7 Mpc
5
7
9
15
13
17
75
65
60
11
2
10
18
18
10
62
90
50
Distant Clusters
Abell 851
1700 Mpc
Abell 1689
343 Mpc
MS 1054 03
355 Mpc
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
Galaxy
interactions
Velocities
of galaxies
in clusters
Hot gas in
galaxy
clusters
Collisions of
galaxy
clusters
Gravitational lensing
Galaxy Rotation
Mass within
Sun’s orbit:
~1011 MSun
Total mass:
~1012 MSun
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
astro-ph/0704126
Calculated for a sample of 194
stars with 32-33 stars per bin
M/L Ratios for MW Dwarfs
Galaxy
MV
L
Radius
Total
mass
M/L
(mag)
(106 LSun)
(pc)
(106 MSun)
(Sun=1)
Gas Fraction
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
Computer
simulation
Antennae Galaxy (HST)
The real
thing
Evidence for dark matter in
clusters of galaxies
We can measure the
velocities of galaxies
in a cluster from
their Doppler shifts
The mass we find
from galaxy motions
in a cluster is about
50 times larger than
the mass in stars!
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
Clusters contain large amounts of X-ray emitting hot gas
Temperature of hot gas (particle motions) tells us cluster
mass
The mass is much more than gas and galaxies combined
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 065756?
• 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
Gravitational
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
• Strong lensing - easily visible distortions
– Einstein rings, arcs, and multiple images
The Einstein Cross
• 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
• 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
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)
source and
lens aligned
source and
lens not
aligned
• 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
Arcs in the
Galaxy
Cluster
Abell 2218
(z=0.175)
cluster center
• 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 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 matter
All galaxies are embedded in clouds of dark
matter
Alternative to Dark Matter: MOND Modified Newtonian Dynamics
 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 DM
in clusters and far out in
halos
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 Evidence
Many 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 matter
interacts with normal matter only through gravity
Stars, gas are now more concentrated than
dark matter
Why is DARK 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
Forming Galaxy
Groups (like ours!)
The formation and evolution of these
groups, which are very common in
the Universe, are dominated by the
gravitational pull of dark matter
4.3 Mpc or 14 million LY
• 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
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
Dark matter provides the
gravitational mass necessary for
galaxy formation to proceed
36 kpc
72 kpc
144 kpc
288 kpc
Galaxies Grow through Mergers
Intergalactic
gas
Galaxy
building blocks
observed with
Hubble
Clumps
concentrated
by dark
matter
lead to
galaxies
Simulation
The cosmic web of dark matter, gas,
and galaxies in a young universe
The real thing
What is DARK MATTER?
Can’t see it, taste it, touch it, smell it…
We can only detect it by gravity
We 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:
6
~10
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 Objects
Mass range: 0.08 MSun (stellar limit) to MEarth
Observational test: gravitational microlensing
if 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 sight
focussing  brightening of the star’s image
as 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
Dark Matter
• 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!
For Wednesday
Chapter 22 – Dark Energy
Complete EP5