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Astronomy 1000
Lecture 25: Large Scale Structure
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Final Exam
Astro 1000
“I learned so much in this class … my mind is
reeling…” -J. Rand, Starship Academy grad.!
5th May (Tues.)
10:00 a.m. – noon.
•  final covers final 6-lectures plus!
•  two topics already covered in class!
- Planet Mars (Lect. #5)!
- Drake Equation (Lect. #12)!
“… if only I had studied for my ASTR 1000 final exam …
(oomph!)… maybe I wouldn’t be fighting some bozo!
in a cheap lizard suit … (gahh!)…” !
- J. T. Kirk, Starship Academy drop-out.!
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Today: The Large Scale Distribution of Galaxies
•  How are galaxies distributed in space: Randomly? Uniformly?!
(What is the “architecture” of space?)!
•  How can we find out?!
•  Basic properties of Galaxy Clusters!
•  The Dark Matter content of clusters!
•  Where is the Milky Way in all of this?!
Textbook: 24.2, 24.3, 25.1 & 25.5!
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Uncovering Large Scale Structure
C. Messier! • 
• 
• 
W. Herschel!
1768 - some 68 “nebulae” known (most by de Lacaille)
1771 - Messier publishes list of 103 objects to be avoided by comet hunters.
Most are in fact galaxies. A concentration of objects are in the constellation Virgo.
1864 - General Catalog published by (son) J. Herschel (4,630 “nebulae”,
all but 450 discovered by (father) William, Caroline & John!)
- W. Herschel initially believes “nebulae” to be Island Universes.
But he changes his mind: they must be young stars.
- J. Herschel notices Virgo concentration in plots (1/3rd of all
“nebulae” in 1/10th of sky).
John Herschel proposes that the nebulae form a spherical system
centered on Virgo, with branches & chains of other nebulae
extending from it. The Milky Way lies on a chain far from the
central distribution in Virgo (i.e., in the boon-docks!)
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2!
Distribution of Galaxies: 1878
Plot of Herschel’s General Catalog by R. Proctor (1878). This is
what John Herschel saw. The Virgo Cluster & hints of filaments
are visible. Even in 1878 there was strong evidence that galaxies
tend to cluster together in gigantic (millions of parsecs) structures.
John Herschel!
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Uncovering Large Scale Structure
•  An astonishingly accurate description of the local distribution
of galaxies existed in the 1870’s.
- J. Herschel never promoted it, not even in his own book!
•  1935 – “The Realm of the Nebulae” published by Hubble: galaxy
clusters are largest structures but also rare … atypical.
“… no evidence of conspicuous systematic variation in the distribution of
nebulae over the sky.”
“… the tendency to cluster appears to operate on a limited scale.”
•  1950s – Palomar Sky Survey: F. Zwicky & G. Abell map galaxy
clusters (2,700 by Abell alone).
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3!
Abell Galaxy Clusters
Abell 4038!
George Abell carefully!
examined the ~1,000!
Palomar Sky Survey plates!
and identified thousands!
of galaxy associations!
and clusters.!
The Abell Catalog was!
published in 1958 and is!
an essential research tool!
in this field.!
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2D Galaxy
Distribution
Map showing galaxy surface
density (galaxy/arcmin2) for
several 106 galaxies from Lick
Survey (Shane & Wirtanen).
Galaxy distribution is
clearly not random:
- many clusters of galaxies are
apparent, as well as arcs and
empty regions where galaxy
density appears lower.
We want the 3D
distribution of galaxies –
we need
their Distances.
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4!
Redshift
Hubble (and others) noticed that spectral features in more distant galaxies !
were progressively shifted to redder ".!
Lines from a distant galaxy!
!"
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(= Wavelength change)!
Definition of Redshift - “z”
Define “redshift” z to be:!
z = !"/"lab!
z = "obs - "lab!
"lab!
Lines from a distant galaxy!
!"
Example: z = 0.1 represents a 10% shift in wavelength to longer wavelengths.!
Hubble’s Law states that more distant galaxies have spectra with larger “z”.!
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5!
Redshift “z” and Distance
Redshift “z” is proportional to!
distance, which is why it is a handy!
quantity to use when talking about!
distances to galaxies & clusters.!
Above are three spectra of distant QSOs.!
The emission line has a laboratory!
wavelength (i.e., "lab) of 121.6 nm!
on this plot. Both have very redshifted!
spectra. How far away are they?!
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Observers watching a stationary
source of waves (water or light)
will measure the same wavelength.
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Doppler Effect: Wavelength “stretches” for
sources moving away
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Redshift and recessional speed
Vrecession!
Lines from a distant galaxy!
!"
Most astronomers thought that this “redshift” was due a Doppler Shift, i.e.,!
more distant galaxies were moving away from us at faster and faster speeds.!
Vrecession ~ c z!
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7!
Redshift and recessional speed
Vrecession!
Lines from a distant galaxy!
!"
Example: How fast does the above galaxy appear to be moving away from us? !
Vrecession = c z = c (0.1) = 30,000 km/s!
!
!
!
(= 70,000,000 mph!!!) !
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Galaxy Distances From “Hubble Law”
Hubble found that the distance and Doppler Shift velocity (“Recessional Velocity”)
are strongly correlated.
As a result:
If you can measure the
Recessional Velocity of
a galaxy (i.e., Doppler
Shift), you can use this
to determine its distance.
To get distances for a whole
lot of galaxies, you have to
get spectra for a whole lot of
galaxies and look for Doppler
Shift velocities.
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8!
The Third-Dimension: Distances
The 3D distribution of galaxies on large scales requires distances, i.e., Vrec
D = Vrec/Ho
Number of Vrec has grown from ~10 (1914) to 100,000’s. Will be
1,000,000’s soon thanks to a number of large surveys.
CfA2 survey
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The Third-Dimension: Distances
“The distribution of galaxies in the redshift survey slice looks like a slice through the!
suds in the kitchen sink; it appears that galaxies are on the surfaces of bubble-like!
structures with a diameter of 25-50 Mpc.”!
“This topolgy poses serious challenges for current models of the formation of large-!
scale structure.”!
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9!
Spectroscopic Surveys - get thousands
of redshifts - get thousands of distances!
Completed Surveys:
•  Las Campanas Redshift Survey
~26,000 (1.5ox80o slices)
•  CfA Redshift Survey
~30,000 redshifts
•  AAT 2dF
~100,000s
SDSS: 640 fibers per mask
Ongoing Surveys:
•  Sloan Digital Sky Survey
>106 redshifts, ! of sky
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Large Scale Distribution of Galaxies
210,000 galaxies!
Galaxies distribution is “frothy” - large regions with few galaxies (“Voids”),
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surrounded by “Sheets” of galaxies.
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The Large Scale Fabric of Local Universe
1) Most galaxies are distributed along vast filamentary networks.
2) Galaxy clusters and super-clusters superposed on filaments.
- roughly 10% of galaxies are in obvious clusters.
- chains of clusters (“walls”) are visible forming superclusters.
3) Large relatively empty regions (“voids”) also exist.
- not really empty
- Szomoru et al. 1994 found that void galaxies are no
different (apparently) from filament galaxies.
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Redshift and “Look-back Time”
It is an interesting fact that when we observe more distant objects we see them!
as they were in the past. The more distant they are, the further we “look back”!
in time.!
This follows from the finite speed of light: c = 3.0 x 108 m/s.!
We’re already seen several examples (e.g., if the sun stopped shining, we wouldn’t!
know on Earth for ~8-minutes. When you look at the sun in the sky you see it as!
it was ~8-minutes ago, not at the “current” time).!
This is because it took ~8-min for the light you see to travel to from the sun.!
Another example: Your Aunt Gladys in Sydney, Australia sends you a post-card!
every day telling you about the weather there. Each one takes a week to reach you.!
So when you read about the sunny weather on Bondi Beach you’re actually!
reading about conditions a week earlier, not at that instant.!
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11!
Redshift and “Look-back” Time
Since “z” is a measure of distance!
it is also a measure of look-back!
time.!
As “z” increases, we are actually!
examining the universe when it!
was younger.!
At z = 2 we are looking at the!
universe only ~3-billion years!
after it was created 13.7-billion !
years ago.!
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QSOs & Quasars
The most luminous Active Galaxies in the Universe are Quasars and QSOs
QSO stands for “Quasi Stellar Object”,
that is, objects that sort-of look like stars
but are most definitely not stars - they are
distant galaxies whose nuclei are emitting
much more light than the rest of the host
galaxy.
Here are some examples. Compare their
nuclei with the image of a star (to the same
scale) at far upper-left.
QSOs are some of the most luminous objects
in the Universe - most are thousands of
times more luminous than the Milky Way
Galaxy!
Quasars are “Quasi-stellar Radio Sources”
i.e., QSOs that are also radio sources.24
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Redshift and “Look-back” Time
Astronomers can take advantage of this fact to see the universe evolve. One!
example: the changing numbers of quasars over time.!
Quasars are among the most!
luminous objects in the universe,!
so we can see them over vast!
distances, i.e., we can see them!
far into the universe’s past!!
Plot at left shows the number of!
Quasars per unit volume as a!
function of the universe’s age!
(i.e., a function of “z”).!
Quasars were 35x more common!
at z~2.5 than today, ~1010 years!
ago. (They are rare today).!
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The universe ~10 years ago was very different! The number of Quasars evolves.!
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Extremely Distant QSOs
The three QSOs shown below have redshifts (left to right): z = 4.75, 4.93 & 5.10!
These massive galaxies (and their super-massive black holes) had formed before the universe
was 1-billion years old!!
A z = 7.1 QSO has recently been discovered. It formed ~300-million years after the creation of
the universe!!
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13!
Clusters Of Galaxies
Galaxy clusters are gravitationally bound condensations of galaxies that stand
out against the frothy backdrop of walls & voids making up the
3-D distribution of luminous matter.
They are the largest and most
massive objects that can be
observed at cosmological
distances to study the evolution
of Large Scale Structure.
The gravitational field of their!
collective mass distribution binds!
galaxy clusters together. In that !
sense they resemble the solar system,!
solar systems, and individual galaxies.!
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Rich Galaxy Clusters
Abell 1689!
•  Rough Spherical Symmetry
•  Galaxies highly concentrated
in cluster center.
•  Ellipticals outnumber Spirals
E : S0 : S = 3 : 4 : 2
•  Galaxy segregation: one
finds Ellipticals & S0’s in the
centers; Spirals in the outer
parts of the cluster.
•  Richest galaxy clusters tend to be
this type (e.g., Coma Cluster has
1000’s of E at R<1.5 Mpc)
•  Appear bound & permanent
Tcross= R/<V>
= 2 Mpc/103 km/s (Coma)
= 2 Gyrs << Hubble time
Coma cluster dissipated long ago
if not gravitationally bound.
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14!
Why so many
E’s & S0’s?
Another way of asking !
the same question:!
What happened to all the gas!
rich star forming spirals???!
Clue #1: galaxies in clusters!
are moving … some up to!
1,000,000 mph!
Clue #2: galaxy clusters are!
not empty. They are filled!
with a low density & hot gas!
Intra-Cluster Medium!
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Why so many
E’s & S0’s?
Another way of asking !
the same question:!
What happened to all the gas!
rich star forming spirals???!
Clue #1: galaxies in clusters!
are moving … some up to!
1,000,000 mph!
Clue #2: galaxy clusters are!
not empty. They are filled!
with a low density & hot gas!
Intra-Cluster Medium!
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15!
The Intra-Cluster Medium
The space between galaxies in clusters is not empty! The UHURU X-ray telescope discovered
intense X-ray emission from rich clusters in the 1970’s. The X-ray emission comes from
ultra-hot (107 – 108 K) and dense (n ~ 3x10-3 cm-3) gas.
! The X-ray gas is very extended and fills volume occupied by galaxies.
Coma Galaxy Cluster
(optical light)!
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The Intra-Cluster Medium
The colored image
shows the distribution
of 100-million K X-ray!
emitting gas that fills
the space between
galaxies in the Coma
cluster.!
All the galaxies in the
cluster (and there are
103’s of them) move at
high speed through
this!
cluster “atmosphere”.!
Some galaxy orbits take
them through the
cluster core at speeds
of a few million mph!!
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16!
Ram Pressure Stripping
Spiral galaxies passing at high speeds through Intra-Cluster Medium feel a strong “wind”!
that exherts pressure on its ISM: P = nICM V2. If strong enough it can blow out the ISM!!
Jachym et al. (2007)!
V!
after several trips through the cluster center!
a spiral can lose most of its ISM.!
no ISM no star formation ! red color ! S0 galaxy!
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You can actually see gas stripped out of galaxies.
ESO 137-001!
NGC 4522!
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another example … IC 3148
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The “Local Group” of Galaxies
The Milky Way Galaxy is not a loner. We “live” in a small group of galaxies called
The “Local Group”, comprised of about 45-galaxies, most of which are dwarfs.
This figure shows all the
known galaxies within
500,000 light years of the
MWG.
There are no galaxies as
large as us within this
volume.
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18!
The “Local Group” of Galaxies
The Milky Way Galaxy is not a loner. We “live” in a small group of galaxies called
The “Local Group”, comprised of about 45-galaxies, most of which are dwarfs.
This figure shows all the
known galaxies within
5-million light years of
the MWG.
Over this volume there are
only two comparably sized
galaxies:
M 31 (Andromeda galaxy)
M 33 (Triangulum galaxy)
Everything else: dwarfs!
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The “Local Group” of Galaxies
On larger scales, the Local Group is part of a long stream of galaxies projecting out
of the massive Virgo Cluster (~20 Mpc away), which is comprised of ~2,500 galaxies.
This figure shows all the
known galaxies within
50-million light years of
the MWG.
In addition to the Virgo
Cluster, which is full of
massive galaxies, other
galaxy clusters, such as
the Format Cluster.
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19!
Galaxy Cluster Masses
For typical Rich clusters: M(R<1.5 Mpc) = 1014 –1015 M"
For Groups/Poor clusters: M(R<1.5 Mpc) = 1012 – 1014 M"
In 1937 F. Zwicky used distribution & velocities of Coma cluster galaxies
to estimate the total cluster mass (using Kepler’s Third Law):
Mtotal = 3/5 (Rv2/G) = (0.6) x {(0.8x106)(3.16x1018) (5x1015)}/6.67x10-8
= 5 x 1013 M"
There are about 103 galaxies in this volume, so on average you expect that:
# M ~ 5 x 1010 M" per galaxy
But typical Coma galaxy has a mass of M ~ 109 M"!
#  Only ~2% of the total mass in the Coma Cluster is in the form of
Galaxies! Something provides sufficient mass & gravity to hold
this cluster together … but it doesn’t emit any photons!
This was actually the first indication of Dark Matter in astronomy.
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The Intra-Cluster Medium
Q: Why doesn’t the hot (i.e., high-speed) X-ray gas just blow away?
A: The cluster’s total mass is so great that the hot gas is gravitationally bound.
Note that the distribution of X-ray gas and galaxies is very similar. What is
doing the gravitational “binding”? The stars? The cold gas? X-ray gas???
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X-ray Gas & Galaxies
The X-ray gas is primarily hydrogen & helium with trace amounts of
heavier elements (typically ~1/3 solar abundances).!
In general:
MX-Ray gas ~ 2! (M* + Mgas) in the galaxies.!
The X-ray emitting gas is not the Dark Matter. There isn’t enough of it.!
The distributions of galaxies in a cluster and the diffuse
X-ray emission are remarkably similar.
The Dark Matter (whatever it is), which defines the gravitational!
field of the entire cluster and traced by the distribution of hot gas,!
is distributed like the visible matter! !
Somehow Luminous Matter & Dark Matter know about each other!
in some deep fundamental way. But how??? !
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Gravitational “Lensing” by the Sun
Light feels the effects of Gravity!!
Light passing close by a massive!
object - like the Sun - is slightly!
deflected (i.e., changes direction)!
by the force of its Gravity.!
The photograph at right was taken in 1919 during a!
total solar eclipse to try and measure the deflection of!
stars near the Sun’s position. On this plate four stars!
are indicated (they are fairly bright stars, but hard to!
see even during an eclipse).!
Comparisons of the four star’s positions at a later date!
(when the Sun wasn’t around!) show that they indeed!
changed positions in 100% agreement with Einstein’s!
Theory of Relativity (Albert was sweating bullets!).!
If the sun didn’t shine you could!
figure out that it was there by the
simple fact that it slightly
changes!
the positions of stars near it!!
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Gravitational “Lensing” by the Sun
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Gravitational Lensing by Galaxy Clusters
In this HST image the light!
from distant galaxies is!
distorted in passing through!
the gravitational field of a!
massive foreground cluster.!
The blue & red “arcs” and!
“smears” are the distant!
lensed galaxies.!
The big tan galaxies mark!
the center of the big!
foreground cluster doing!
the distortion.!
Some of them are very very!
messed up looking.! 44
22!
Gravitational Lensing
by Galaxy Clusters
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Gravitational Lensing by
Galaxy Clusters
The luminous arcs are “lensed” background galaxies. We
can “invert” the problem and determine the Dark Matter
distribution needed to produce the observed arcs.!
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Weak Lensing & Mass Distributions
Matter (dark or luminous) changes the path light travels - acts like a “lens”.
Distant galaxies will be distorted by the “Gravitational Lens” of a massive
foreground cluster. By studying the distortion of these background galaxies
one can work out the distribution of matter in the foreground cluster.
Abell 3266
Abell 3667
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Joffre, U. of Chicago
Summary
1) Large scale 3-Dimensional distribution of galaxies requires we determine their
distances from us.
2) Telescopic surveys try to measure as many spectra as possible, and derive
distances using “Hubble Law”.
3) Galaxies are found along vast “sheets” surrounding ~Mpc sized “voids” where few
galaxies are found. Galaxy clusters appear as large (~2-5 Mpc) concentrations on
these sheets.
4) “Regular” galaxy clusters are ~spherical, centrally concentrated, very massive, and
are dominated by large ellipticals/S0’s in their cores. Spirals are found in the
outskirts. Large amounts of ~107 K X-ray emitting gas is present.
5) “Irregular” clusters are smaller, looser, and dominated by spirals.
6) Galaxy clusters are dominated by Dark Matter. Can’t be brown dwarf stars!
7) Analysis of Gravitational “Lensing” reveals Dark Matter distribution. Still don’t
know what it is (but we can rule out what it isn’t in most cases).
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24!