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Chapter 16
A Universe of Galaxies
Introduction
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At the beginning of the 20th century, the
nature of the faint, fuzzy “spiral nebulae”
was unknown.
In the mid-1920s, Edwin Hubble showed
that they are distant galaxies like our own
Milky Way Galaxy, and that the Universe is
far larger than previously thought.
Galaxies are the fundamental units of the
Universe, just as stars are the basic units
of galaxies.
Like stars, many galaxies are found in
clusters, and there are also superclusters
separated by enormous voids.
By looking back in time at very distant
galaxies and clusters, we can study how
they formed and evolved.
Surprisingly, we now know that all these
enormous structures consist largely of
“dark matter” that emits little or no
electromagnetic radiation.
16.1 The Discovery of Galaxies
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In the 1770s, the French astronomer Charles
Messier was interested in discovering comets.
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To do so, he had to be able to recognize whenever a
new fuzzy object (a candidate comet) appeared in
the sky.
To minimize possible confusion, he thus compiled a
list of about 100 diffuse objects that could always
be seen, as long as the appropriate constellation
was above the horizon.
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Some of them are nebulae, and others are star
clusters, which can appear fuzzy through a small
telescope such as that used by Messier.
16.1 The Discovery of Galaxies
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To this day, the objects in Messier’s list are commonly
known by their Messier numbers (see figures).
They are among the brightest and most beautiful objects
in the sky visible from mid-northern latitudes.
A set of photographs of all the Messier objects appears as
an Appendix.
16.1 The Discovery of Galaxies
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Other astronomers subsequently compiled additional
lists of nebulae and star clusters.
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By the early part of the 20th century, several thousand
nebulae and clusters were known.
The nebulae were especially intriguing: Although some
of them, such as the Orion Nebula (see figure, right),
seemed clearly associated with bright stars in our Milky
Way Galaxy, the nature of others was more
controversial.
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When examined with the largest telescopes
then available, many of them showed traces
of spiral structure, like pinwheels, but no
obvious stars (see figure, left).
16.1a The Shapley-Curtis Debate
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Some astronomers thought that these so-called spiral
nebulae were merely in our own Galaxy, while others
suggested that they were very far away—“island
universes” in their own right, so distant that the individual
stars appeared blurred together.
The distance and nature of the spiral nebulae was the
subject of the well-publicized “Shapley-Curtis debate,”
held in 1920 between the astronomers Harlow Shapley
and Heber Curtis.
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Shapley argued that the Milky Way Galaxy is larger than had
been thought, and could contain such spiral-shaped clouds
of gas.
Curtis, in contrast, believed that they are separate entities,
far beyond the outskirts of our Galaxy.
This famous debate is an interesting example of the
scientific process at work.
16.1a The Shapley-Curtis Debate
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Shapley’s wrong conclusion was based on
rather sound reasoning but erroneous
measurements and assumptions.
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For example, one distinguished astronomer
thought he had detected the slight angular
rotation of a spiral nebula, and Shapley
correctly argued that this change would
require a preposterously high physical rotation
speed if the nebula were very distant.
It turns out, however, that the measurement
was faulty.
16.1a The Shapley-Curtis Debate
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Shapley also argued that a bright
nova that had appeared in 1885 in
the Andromeda Nebula, M31, the
largest spiral nebula (see figures),
would be far more powerful than
any previously known nova if it
were very distant.
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Unfortunately, the existence of
supernovae (this object turned out
to be one), which are indeed more
powerful than any known nova,
was not yet known.
On the other hand, Curtis’s
conclusion that the spiral nebulae
were external to our Galaxy was
based largely on an incorrect
notion of our Galaxy’s size; his
preferred value was much too
small.
Moreover, he treated the nova in
Andromeda as an anomaly.
16.1b Galaxies: “Island Universes”
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The matter was dramatically settled in the mid1920s, when observations made by Edwin
Hubble (see figure) at the Mount Wilson
Observatory in California proved that the spiral
nebulae were indeed “island universes” (now
called galaxies) well outside the Milky Way Galaxy.
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As we saw in Chapter 11, Cepheids are named
after their prototype, the variable star d (the
Greek letter “delta”) Cephei.
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Using the 100-inch (2.5-m) telescope, Hubble
discovered very faint Cepheid variable stars in
several objects, including the Andromeda Nebula.
Their light curves (brightness vs. time) have a
distinctive, easily recognized shape.
Cepheids are intrinsically very luminous stars, 500
to 10,000 times as powerful as the Sun, so they
can be seen at large distances, out to a few
million light-years, with the 100-inch telescope
used by Hubble.
16.1b Galaxies: “Island Universes”
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Cepheids are very special to astronomers because
measuring the period of a Cepheid’s brightness variation
(using what we are calling Leavitt’s law, after Henrietta
Leavitt) gives you its average luminosity.
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And comparing its average luminosity with its average
apparent brightness tells you its distance, using the inversesquare law of light.
The Cepheids in the spiral nebulae observed by Hubble
turned out to be exceedingly distant.
The Andromeda Nebula, for example, was found to be
over 1 million light-years away (the value is now known to
be about 2.4 million light-years)—far beyond the
measured distance of any known stars in the Milky Way
Galaxy.
16.1b Galaxies: “Island Universes”
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From the distance and the measured angular size of the
Andromeda Nebula, its physical size was found to be enormous.
Clearly, the “spiral nebulae” were actually huge, gravitationally
bound stellar systems like our own Milky Way Galaxy, not
relatively small clouds of gas like the Orion Nebula (and so the
Andromeda Nebula was renamed the Andromeda Galaxy).
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The effective size of the Universe, as perceived by humans,
increased enormously with this realization.
In essence, Hubble brought the Copernican revolution to a new
level; not only is the Earth just one planet orbiting a typical star
among over 100 billion stars in the Milky Way Galaxy, but also
ours is just one galaxy among the myriads in the Universe!
Indeed, it is humbling to consider that the Milky Way is one of
roughly 50 to 100 billion galaxies within the grasp of the world’s
best telescopes such as the Keck twins and the Hubble Space
Telescope.
16.2 Types of Galaxies
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Galaxies come in a variety of shapes.
In 1925, Edwin Hubble set up a
system of classification of galaxies,
and we still use a modified form of it.
16.2a Spiral Galaxies
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There are two main “Hubble types” of galaxies.
We are already familiar with the first kind—the
spiral galaxies.
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Spiral galaxies consist of a bulge in the
center, a halo around it, and a thin
rotating disk with embedded spiral arms.
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The Milky Way Galaxy and its near-twin, the
Andromeda Galaxy (M31; see figure, left), are
relatively large examples containing several
hundred billion stars. (Most spiral galaxies
contain a billion to a trillion stars.)
Another near-twin is NGC 7331 (see figure,
below).
There are usually two main arms, with
considerable structure such as smaller
appendages.
Doppler shifts indicate that spiral
galaxies rotate in the sense that the
arms trail.
16.2a Spiral Galaxies
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In the figure, which is based on a very similar “tuning-fork diagram” drawn
by Hubble, shows different types of spiral galaxies.
Relative to the disk, the bulge is large in some spiral galaxies (known as
“Sa”), which also tend to have more tightly wound spiral arms.
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The bulge is progressively smaller (relative to the disk) in spirals known as
“Sb,” “Sc,” and “Sd,” which also tend to have more loosely wound spiral arms.
Moreover, spirals with smaller bulge-to-disk ratios generally have more gas
and dust, and larger amounts of active star formation within the arms at
the present time.
16.2a Spiral Galaxies
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Spiral galaxies viewed along, or nearly along, the plane of
the disk (that is, “edge-on”) often exhibit a dark dust lane
that appears to divide the disk into two halves (see figures).
16.2a Spiral Galaxies
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In nearly one half of all spirals, the
arms unwind not from the nucleus,
but rather from a relatively straight
bar of stars, gas, and dust that
extends to both sides of the nucleus
(see figure, top).
These “barred spirals” are similarly
classified in the Hubble scheme from
“SBa” to “SBd” (the “B” stands for
“barred”), in order of decreasing size
of the bulge and increasing openness
of the arms (see figure, bottom).
In many cases, the distinction between a barred and nonbarred spiral is
subtle.
Studies show that our own Milky Way Galaxy is a barred spiral, probably
of type SBbc (that is, intermediate between SBb and SBc).
16.2a Spiral Galaxies
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Because they contain many
massive young stars, the spiral
arms appear bluish in color
photographs.
Between the spiral arms, the
whitish-yellow disks of spiral
galaxies contain both old and
relatively young stars, but not
the hot, massive, blue mainsequence stars, which have
already died.
Very old stars dominate the
bulge, and especially the faint
halo (which is difficult to see),
and so the bulge is somewhat
yellow/orange or even reddish
in photographs (see figures).
16.2a Spiral Galaxies
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Since young, massive stars heat the dusty clouds from
which they formed, resulting in the emission of much
infrared radiation, the current rate of star formation in a
galaxy can be estimated by measuring its infrared power.
Space telescopes such as the Infrared Astronomical
Satellite (IRAS, in the mid-1980s) and the Infrared Space
Observatory (ISO, in the mid-1990s) were very useful for
this kind of work, and it is being continued with the
infrared camera on the Hubble Space Telescope and, at
even longer infrared wavelengths, with the Spitzer Space
Telescope.
16.2a Spiral Galaxies
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About half of the energy emitted by
our own Milky Way Galaxy is in the
infrared, indicating that a lot of stars
are being formed.
But we don’t know why the
Andromeda Galaxy, which in optical
radiation resembles the Milky Way,
emits only 3 per cent of its energy in
the infrared.
This galaxy and the Sombrero
Galaxy emit infrared mostly in a ring
rather than in spiral arms (see
figure).
16.2b Elliptical Galaxies
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Hubble recognized a second major galactic
category: elliptical galaxies (see figures).
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These objects have no disk and no arms, and
generally very little gas and dust.
Unlike spiral galaxies, they do not rotate very
much.
At the present time, nearly all of them
consist almost entirely of old stars, so they
appear yellow/orange or even reddish in
true-color photographs.
The dearth of gas and dust is consistent
with this composition: There is insufficient
raw material from which new stars can
form.
In many ways, then, an elliptical galaxy
resembles the bulge of a spiral galaxy.
16.2b Elliptical Galaxies
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Elliptical galaxies can be roughly circular in
shape (which Hubble called type E0), but are
usually elongated (from E1 to E7, in order of
increasing elongation).
Since the classification depends on the
observed appearance, rather than on the
intrinsic shape, some E0 galaxies must
actually be elongated, but are seen end-on
(like a cigar viewed from one end).
16.2b Elliptical Galaxies
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Most ellipticals are dwarfs, like the two main
companions of the Andromeda Galaxy (see figure
below and (b)), containing only a few million solar
masses—a few per cent of the mass of our Milky
Way Galaxy.
Some, however, are enormous, consisting of a few
trillion stars in a volume several hundred thousand
light-years in diameter (see figure (a)).
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Many ellipticals may have resulted from two or more
spiral galaxies colliding and merging, as we will
discuss later.
16.2c Other Galaxy Types
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“Lenticular” galaxies (also known as “S0” [pronounced
“ess-zero”] galaxies) have a shape resembling an optical
lens; they combine some of the features of spiral and
elliptical galaxies.
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They have a disk, like spiral galaxies.
On the other hand, they lack spiral arms, and generally
contain very little gas and dust, like elliptical galaxies.
Hubble put them at the intersection between spiral and
elliptical galaxies in his classification diagram (see figure
on the next slide).
Though sometimes called “transition galaxies,” this
designation should not be taken literally: The diagram is
not meant to imply that spiral galaxies evolve with time
into ellipticals (or vice versa) in a simple manner, contrary
to the belief of some astronomers decades ago.
16.2c Other Galaxy Types
16.2c Other Galaxy Types
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A few per cent of galaxies at the present time
in the Universe show no clear regularity.
Examples of these “irregular galaxies” include
the Small and Large Magellanic Clouds, small
satellite galaxies that orbit the much larger
Milky Way Galaxy (see figures).
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Sometimes traces of regularity—perhaps a
bar—can be seen.
Irregular galaxies generally have lots of gas
and dust, and are rapidly forming stars.
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Indeed, some of them emit 10 to 100 times as
much infrared as optical energy, probably
because the rate of star formation is greatly
elevated.
16.2c Other Galaxy Types
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Some galaxies are called “peculiar.”
These often look roughly like spiral or elliptical
galaxies, but have one or more abnormalities.
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For example, some peculiar galaxies look like
interacting spirals (see figure, above), or like
spirals without a nucleus (that is, like rings, (see
figure, left)), or like ellipticals with a dark lane of
dust and gas.
The ring galaxies are the result of collisions of
galaxies.
16.3 Habitats of Galaxies
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Most galaxies are not solitary;
instead, they are generally found
in gravitationally bound binary
pairs, small groups, or larger
clusters of galaxies.
Binary and multiple galaxies
consist of several members.
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An example is the Milky Way
Galaxy with its two main
companions (the Magellanic
Clouds (see figure, top)), or
Andromeda and its two main
satellites (see figure, bottom).
Both Andromeda and the Milky
Way have several even smaller
companions.
16.3 Habitats of Galaxies
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Galaxies and clusters of galaxies all over the
Universe are studied with the Hubble Space
Telescope in the ultraviolet, visible, and
near-infrared, the Spitzer Space Telescope
in the infrared, and the Chandra X-ray
Observatory in x-rays.
NASA’s Galaxy Evolution Explorer (GALEX),
launched in 2003, is a small satellite that is
studying galaxies and surveying the sky in
the ultraviolet.
16.3a Clusters of Galaxies
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The Local Group is a small cluster of about 30
galaxies, some of which are binary or multiple
galaxies.
Its two dominant members are the Andromeda
(M31) and Milky Way Galaxies.
M33, the Triangulum Galaxy (see figures, top), is
a smaller spiral.
M31 and M33, at respective distances of 2.4 and
2.6 million light-years, are the farthest objects
you can see with your unaided eye.
The Local Group also contains four irregular
galaxies, at least a dozen dwarf irregulars (see
figure, bottom), four regular ellipticals, and the
rest are dwarf ellipticals or the related “dwarf
spheroidals.”
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The diameter of the Local Group is about 3 million
light-years.
16.3a Clusters of Galaxies
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The Virgo Cluster (in the direction of the
constellation Virgo, but far beyond the
stars that make up the constellation), at
a distance of about 50 million light-years,
is the largest relatively nearby cluster
(see figure, top).
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It consists of at least 2000 galaxies
spanning the full range of Hubble types,
covering a region in the sky over 15° in
diameter—about 15 million light-years.
The Coma Cluster of galaxies (in the
direction of the constellation Coma
Berenices) is very rich, consisting of over
10,000 galaxies at a distance of about
300 million light-years (see figure,
bottom).
16.3a Clusters of Galaxies
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A majority of the galaxies in rich clusters are ellipticals,
not spirals.
There is often a single, very large central elliptical galaxy
(sometimes two) that is cannibalizing other galaxies in
its vicinity, growing bigger with time (see figure, top).
X-ray observations of rich clusters reveal a hot
intergalactic gas (10 million to 100 million K) within
them, containing as much (or more) mass as the
galaxies themselves (see figure, bottom).
The gas is clumped in some clusters, while in others it is
spread out more smoothly with a concentration near the
center.
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This may be an evolutionary effect; the clumps occur in
clusters that only recently formed from the gravitational
attraction of their constituent galaxies and groups of
galaxies.
As clusters age, the gas within them becomes more
smoothly distributed and partly settles toward the
center.
16.3b Superclusters of Galaxies
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Clusters are seen to vast distances, in a few cases up
to 8 billion light-years away.
When we survey their spatial distribution, we find
that they form clusters of clusters of galaxies,
appropriately called superclusters.
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These vary in size, but a typical diameter is about 100
million light-years.
The Local Group, dozens of similar groupings nearby,
and the Virgo Cluster form the Local Supercluster.
16.3b Superclusters of Galaxies
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Superclusters often appear to be
elongated and flattened.
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The thickness of the Local Supercluster,
for example, is only about 10 million
light-years, or one tenth of its diameter.
Superclusters tend to form a network of
bubbles, like the suds in a kitchen sink
(see figures).
Large concentrations of galaxies (that is,
several adjacent superclusters) surround
relatively empty regions of the Universe,
called voids, that have typical diameters
of about 100 million light-years (but
sometimes up to 300 million light-years).
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They formed as a consequence of matter
gravitationally accumulating into
superclusters; the regions surrounding
the superclusters were left with little
matter.
16.3b Superclusters of Galaxies
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Does the clustering continue in scope?
Are there clusters of clusters of clusters,
and so on?
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The present evidence suggests that this is
not so.
Surveys of the Universe to very large
distances do not reveal many obvious
super-superclusters.
There are, however, a few giant structures
such as the “Great Wall” that crosses the
center of the slices shown in the figures.
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We will discuss in Chapter 19 how the
“seeds” from which these objects formed
were visible within 400,000 years after the
birth of the Universe.
16.4 The Dark Side of Matter
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There are now strong indications that much of
the matter in the Universe does not emit any
detectable electromagnetic radiation, but
nevertheless has a gravitational influence on its
surroundings.
One of the first clues was provided by the flat
(nearly constant) rotation curves of spiral
galaxies.
16.4a The Rotation Curve of
the Milky Way Galaxy
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The rotation curve of any spinning galaxy is a plot of its orbital speed
as a function of distance from its center.
For example, the rotation curve of the Milky Way Galaxy has been
determined through studies of the motions of stars and clouds of gas
(see figure).
It rises from zero in the center, to a value of about 200 km /sec at a
radial distance of about 5000 light-years.
The rotation curve farther out is rather “flat”; the orbital speed stays
constant, all the way out to distances well beyond that of the Sun.
16.4a The Rotation Curve of
the Milky Way Galaxy
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The speed of a star at any given distance from the center
is determined by the gravitational field of matter enclosed
within the orbit of that star—that is, by the matter closer
to the center. (It can be shown that matter at larger
distances, outside the star’s orbit, does not affect the
star’s speed as long as the galaxy’s disk has a smooth,
symmetric distribution of matter.)
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So, we can use the rotation curve to map out the
distribution of mass within our Galaxy.
The speeds and distances of stars near our Galaxy’s edge,
for example, are used to measure the mass in the entire
Galaxy.
16.4a The Rotation Curve of
the Milky Way Galaxy
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Specifically, Kepler’s third law can be
manipulated to give an expression for the mass
(M) enclosed within an orbit of radius R from the
center.
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A similar method is used to find the amount of
mass in the Sun by studying the orbits of the
planets, or the amount of a planet’s mass by
observing the orbits of its moons.
In the 17th century, Newton developed this
technique as part of his derivation and
elaboration of Kepler’s third law of orbital motion
(see Chapter 5).
16.4a The Rotation Curve of
the Milky Way Galaxy
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If we insert the Sun’s distance from the Galactic center
(26,000 light-years) and the Sun’s orbital speed (200 km
/sec) into the formula, we find that the matter within the
Sun’s orbit has a mass of about 100 billion (1011) solar
masses!
But the mass of a typical star is about half that of the Sun.
Thus, if most of the matter in our Galaxy is in the form of
stars (rather than interstellar gas and dust, black holes,
etc.), we conclude that there are about 200 billion stars
within the Sun’s orbit, closer to the Galaxy’s center than
the Sun is.
16.4a The Rotation Curve of
the Milky Way Galaxy
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The next thing to notice is that
the flat rotation curve of the
Milky Way Galaxy (see figure)
is quite different from the
rotation curve of the Solar
System.
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As discussed in Chapter 5, the orbital speeds of distant planets are slower
than those of planets near the Sun, instead of being roughly independent of
distance.
In the Solar System, the Sun’s mass greatly dominates all other masses;
the masses of the planets are essentially negligible in comparison with
the Sun.
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But in the Milky Way Galaxy, the flat rotation curve implies that except in the
central region (where the rotation curve isn’t flat), the mass grows with
increasing distance from the center.
16.4a The Rotation Curve of
the Milky Way Galaxy
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The growth in mass of our Galaxy continues to large distances
beyond the Sun’s orbit. (The rotation curve way out from the
center is usually determined from the measured speeds of
clouds of hydrogen gas, which can be easily seen at radio
wavelengths.)
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This is very puzzling because few stars are found in those regions:
The number of stars falls far short of accounting for the derived
mass.
For example, at a distance of 130,000 light-years from the
center, the enclosed mass is about 5 X 1011 solar masses, and
the corresponding number of typical stars (each having half a
solar mass) would be about a trillion—yet there are too few
stars visible, by a large margin.
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Indeed, studies of the outer parts of the Milky Way Galaxy
throughout the electromagnetic spectrum do not reveal sufficient
quantities of material to account for the derived mass.
16.4b Dark Matter Everywhere
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We conclude that the Milky Way Galaxy contains large
quantities of “dark matter”—material that exerts a
gravitational force, but is invisible or at least very
difficult to see!
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This material has sometimes been called the “missing
mass,” especially in older texts, but the term is not
appropriate because the mass is present.
Instead, it is the light that’s missing.
Many other spiral galaxies also have flat (speed roughly constant) rotation
curves, as was first shown by Vera Rubin (see figure).
Estimates suggest that 80 to 90 per cent of the mass of a typical spiral
galaxy consists of dark matter.
However, it has been shown that the amount of matter in the disk cannot
exceed what is visible by more than a factor of 2.
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Instead, the dark matter is probably concentrated in an extended, spherical,
outer halo of material that extends to perhaps 200,000 light-years from the
galactic center.
16.4b Dark Matter Everywhere
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Similar studies show that elliptical galaxies also contain large
amounts of dark matter.
Gas and stars are moving so quickly that they would escape if
the visible matter alone produced the gravitational field.
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There must be another, more dominant, contribution to gravity in
these galaxies.
The orbital speeds of galaxies in binary pairs, groups, and
clusters can be used to determine the masses of these systems.
Astronomers find that in essentially all cases, the amount of
mass required to produce the observed orbital speeds is larger
than that estimated from the visible light (which is assumed to
come from stars and gas).
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A related technique is to measure the typical speeds of particles of
gas bound to a cluster of galaxies—and once again, the particles
could not be gravitationally bound to the cluster if its mass
consisted only of that provided by the visible matter.
16.4b Dark Matter Everywhere
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Decades ago, the Caltech astronomer
Fritz Zwicky was the first to point out
that clusters of galaxies could not
remain gravitationally bound if they
contain only visible matter.
He postulated the existence of some
form of dark matter.
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However, this idea was largely ignored
or dismissed—it was too far ahead of its
time.
16.4c What Is Dark Matter?
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What is the physical nature of the dark matter in single and
binary galaxies, groups, and clusters?
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A tremendous number of very faint normal stars (or even brown
dwarfs) is a possibility, though it seems unlikely, extrapolating
from the numbers of the faintest stars that we can study.
There is some evidence (see Section 16.5 of the following
slides) that part of the dark matter consists of old white dwarfs.
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We just don’t know—this is one of the outstanding unsolved
problems in astrophysics.
If these and other corpses of dead stars (neutron stars, black
holes) accounted for most of the dark matter, however, then where
is the chemically enriched gas that the stars must have ejected
near the ends of their lives?
Other candidates for the dark matter are small black holes,
massive planets (“Jupiters”), and neutrinos.
16.4c What Is Dark Matter?
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We will see in Chapter 19 that certain kinds of
measurements indicate that only a small fraction of the
dark matter can consist of “normal” particles such as
protons, neutrons, and electrons; the rest must be exotic
particles.
Most of the normal dark matter consists of tenuous,
million-degree gas in galactic halos.
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This gas was recently detected by the absorption spectra it
produced in the radiation from background objects, and also
from its emission at relatively long x-ray wavelengths.
Though no longer technically “dark” (because we have seen
it!), such matter is still generally considered to be part of the
“dark matter” that pervades the Universe; it is difficult to
detect.
16.4c What Is Dark Matter?
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Probably the most likely candidate for a majority of the dark
matter (the “abnormal” part) is undiscovered subatomic
particles with unusual properties, left over from the big bang,
such as WIMPs—“weakly interacting massive particles.”
Physicists studying the fundamental forces of nature suggest
that many WIMPs exist, though it is disconcerting that none has
been unambiguously detected in a laboratory experiment.



If it is unsatisfactory to you that most of the mass in our Galaxy
(and indeed, most of the mass in the Universe!) is in some
unknown form, you may feel better by knowing that astronomers
also find the situation unsatisfactory.
But all we can do is go out and conduct our research, and try to
find out more.
Clever new techniques, such as one described on the next
slides, may provide the crucial clues that we seek.
16.5 Gravitational Lensing

The phenomenon of gravitational lensing of light
provides a powerful probe of the amount (and in some
cases the nature) of dark matter.


If the light from a distant object passes through a
gravitational field, the light is bent—that is, it follows a
curved path through the warped space–time.


About one hundred cases of gravitational lensing have been
found thus far.
This is analogous to (but differs in detail from) the effect
that a simple glass lens has on light.
It is, perhaps, more akin to the warping of images we get
from a fun-house mirror, but the terms “lens” and
“lensing” have caught on.
16.5 Gravitational Lensing

We have already encountered lensing in Chapter 10: Recall that
Einstein’s general theory of relativity predicts that the Sun should bend
the light of stars beyond it by an amount twice that predicted with
Newtonian theory, and that this effect was first measured in 1919 by
Arthur Eddington and others during a total solar eclipse.

If an observer, a galaxy acting as a gravitational lens, and a more distant,
very compact object are nearly perfectly aligned (colinear), the distant
object will look like a circle (known as an “Einstein ring”) centered on the
lens (see figures).
16.5 Gravitational Lensing

More usually, deviations from symmetry (for
example, slight misalignment of the lens) lead to
the formation of several discrete, well-separated
images (see figures).



When a massive cluster of galaxies lenses many
distant galaxies, a collection of arcs tends to be
seen (see figures next slide).
The number of arcs, their magnification factors,
and their distorted pattern depend on the mass
of the cluster, as well as on the distribution of
mass within the cluster.


The apparent brightness of the object is
magnified, in some cases by large amounts.
This method measures the total mass (visible and
dark) in the cluster, and gives results consistent
with those obtained from other techniques.
Again, we conclude that dark matter dominates
most clusters of galaxies.
16.5 Gravitational Lensing
16.5 Gravitational Lensing


Projects in which the apparent brightness of millions of stars in the Large
and Small Magellanic Clouds are systematically monitored have revealed
that occasionally, a star brightens and fades over the course of a few
weeks. (These galaxies provide a nice background field of stars that are
out of the Galactic plane.)
The light curve has exactly the shape expected if a compact lens were to
pass between the star and us, temporarily focusing the star’s light toward
us (see figure).

Moreover, the shape and height (amplitude) of the light curve is independent
of the filter through which it was obtained, as predicted for gravitational
lensing and unlike the case for intrinsically variable stars.
16.5 Gravitational Lensing



Some of these searches for lensed stars in the
Magellanic Clouds were motivated by the
opportunity of finding so-called “massive compact
halo objects,” or MACHOs.
In principle, such objects would be too massive to
be brown dwarfs, yet could not be normal mainsequence stars because we would see them.
White dwarfs are a possible candidate, but in this
case the population of stars that produced them
must have been devoid of very low-mass stars,
since we do not see enough stars in the halo,
where astronomers expect that MACHOs would be
found.
16.5 Gravitational Lensing

The searches have already detected hundreds of lensed
stars in the Magellanic Clouds.


In a few especially favorable cases, the distance of the
lens has been determined.


There is considerable controversy about the interpretation of
these detections.
In two cases, the lens turns out to be in the Large
Magellanic Cloud itself, rather than in the halo of the Milky
Way Galaxy.
Another Hubble discovery of a lens in the direction of the
Large Magellanic Cloud turns out to be an ordinary star in
the disk of the Milky Way Galaxy.

So far, the searches have not revealed any definitive lenses
in the halo of our Galaxy.
16.5 Gravitational Lensing

If most of the lenses are not in our Galaxy’s halo, then the
evidence for dark, compact objects greatly decreases.


Thus, it appears that much of the dark matter in the halo
may consist of subatomic particles like the WIMPs
mentioned above.


Indeed, the most recent estimates (late 2005) suggest that
no more than about 10 or 20 per cent of the halo’s mass
consists of MACHOs.
It is exciting to think that astronomical observations may
end up providing the crucial evidence for the existence of
tiny, otherwise undetectable particles.
Gravitational lensing also helps us find out about dark
matter by revealing how mass is distributed in galaxies.

Just how centralized mass is in galaxy cores can, in
principle, be revealed by the study of gravitational lenses
with multiple images.
16.6 The Birth and Life of Galaxies


It is difficult or impossible to determine what any given
nearby galaxy (or our own Milky Way Galaxy) used to look
like, since we can’t view it as it was long ago.
However, as we discussed in Chapter 1, the finite speed of
light effectively allows us to view the past history of the
Universe: We see different objects at different times in the
past, depending on how long the light has been travelling
toward us.

At least in a statistical manner we can explore galactic
evolution by examining galaxies at progressively larger
distances or lookback times, and hence progressively
farther back in the past.
16.6 The Birth and Life of Galaxies

An important but likely valid assumption is that we live in a
typical part of the Universe, so that nearby galaxies are
representative of galaxies everywhere.


The refurbished Hubble Space Telescope has led to the most
progress in this field, since it provides detailed images of faint,
distant galaxies.


Hence, galaxies several billion light-years away, viewed as they
were billions of years ago, probably resemble what today’s nearby
galaxies used to look like.
Also, the Chandra X-ray Observatory has revealed objects that
might be very primitive, distant galaxies; the seemingly uniform xray glow that previous x-ray telescopes had detected is actually
produced by many individual discrete sources.
It is crucial to know the distances of the very distant galaxies,
but they are so far away that no Cepheid variables or other
normal stars can be seen and compared with nearby examples.

So, an indirect technique is used: Hubble’s law, as described below.
16.7 The Expanding Universe


Early in the 20th century, Vesto Slipher of the Lowell Observatory in
Arizona noticed that the optical spectra of “spiral nebulae” (later
recognized by Edwin Hubble to be separate galaxies) almost always show
a redshift (recall our discussion of redshifts and blueshifts in Chapter
11).
The absorption or emission lines seen in the spectra have the same
patterns as in the spectra of normal stars or emission nebulae, but these
patterns are displaced (that is, shifted) to longer (redder) wavelengths
(see figure).

Under the assumption that the redshift
results from the Doppler effect, we can
conclude that most galaxies are
moving away from us, regardless of
their direction in the sky. (In Chapter
18, we will see that the redshift is
actually caused by the stretching of
space, but the equation is the same as
that for the Doppler effect, at least at
low redshifts.)
16.7 The Expanding Universe


In 1929, using newly derived distances to some of these galaxies (from
Cepheid variable stars), Hubble discovered that the displacement of a
given line (that is, the redshift) is proportional to the galaxy’s distance.
(In other words, when the redshift we observe is greater by a certain
factor, the distance is greater by the same factor.)
Thus, under the Doppler-shift interpretation, the recession speed, v, of a
given galaxy must be proportional to its current distance, d (see figure).

This relation is known as Hubble’s
law, v=H0d, where H0 (pronounced
“H naught”) is the present-day value
of the constant of proportionality, H
(the factor by which you multiply d
to get v). H0 is known as Hubble’s
constant.
16.7 The Expanding Universe

For various reasons, Edwin
Hubble’s original data were
suggestive but not conclusive.

Subsequently, Hubble’s
assistant and disciple Milton
Humason joined Hubble in
very convincingly showing the
relationship (see figure).
(Interestingly, Humason had
first come to Mt. Wilson as a
mule-team driver, helping to
bring telescope parts up the
mountain. He worked his way
up within the organization.)
16.7 The Expanding Universe


This behavior is similar to that produced by an explosion:
Bits of shrapnel are given a wide range of speeds, and
those that are moving fastest travel the largest distance in
a given amount of time.
Although Edwin Hubble himself initially resisted this idea,
the implication of Hubble’s law is that the Universe is
expanding!


As we shall see in Chapter 18, however, there is no unique
center to the expansion, so in this way it is not like an
explosion.
Moreover, the expansion of the Universe marks the
creation of space itself, unlike the explosion of a bomb in
a preexisting space.
16.7 The Expanding Universe

One of the greatest debates in 20th century astronomy
has been over the value of Hubble’s constant.


In Chapter 18 we will explain in detail how it is determined.
Measurements of the recession speeds of galaxies at
known distances showed that H0=50 to 80 km /sec/Mpc;
as of late 2005, the value is known to be 71 km /sec/Mpc
to within 10 per cent. (We will discuss these definitive
measurements, from NASA’s Wilkinson Microwave
Anisotropy Probe spacecraft, in Chapter 19.)

For example, a galaxy 10 Mpc (32.6 million light-years) away
recedes from us with a speed of about 710 km/sec, and a
galaxy 20 Mpc away recedes with a speed of about 1420 km
/sec.
16.7 The Expanding Universe

Hubble’s constant is always quoted in the strange units of
km /sec/Mpc.
 The value H0=71 km/sec/Mpc simply means that for each
megaparsec (3.26 million lightyears) of distance, galaxies
are receding 71 km /sec faster.


The expansion of the Universe will be the central theme in
Chapters 18 and 19; we will discuss its implications and
associated phenomena.
For now, however, let us simply use Hubble’s law to
determine the distances of very distant galaxies and other
objects.
 Knowing the value of H0, a measurement of a galaxy’s
recession speed v then gives the distance d, since d=v/H0.
16.7 The Expanding Universe

Note that Hubble’s law cannot be used to find the
distances of stars in our own Galaxy, or of galaxies in our
Local Group; these objects are gravitationally bound to us,
and hence the expansion of the intervening space is
overcome.
 Moreover, Hubble’s law does not imply that objects in the
Solar System or in our Galaxy are themselves expanding;
they are bound together by forces strong enough to
overcome the tendency for empty space to expand.

Hubble’s law applies to distant galaxies and clusters of
galaxies; the space between us and them is expanding.
16.8 The Search for the
Most Distant Galaxies

With the Hubble Space Telescope, we obtained relatively clear
images of faint galaxies that are suspected to be very distant.

The main imaging camera of the time (the Wide Field/Planetary
Camera 2) exposed on a small area of the northern sky for 10 days
in December 1995.

Though it covers only about one 30millionth of the area of the sky (roughly
the apparent size of a grain of sand held
at arm’s length), this Hubble Deep
Field contains several thousand
extremely faint galaxies (see figure).

If we could photograph the entire sky
with such depth and clarity, we would
see about 50 to 100 billion galaxies, each
of which contains billions of stars!
16.8 The Search for the
Most Distant Galaxies

Three years later, the Hubble Space
Telescope got very deep images of another
region, this time in the southern celestial
hemisphere: the Hubble Deep Field—South.


It looks similar to the northern field, even
though it is nearly in the opposite direction in
the sky, providing some justification for our
assumption that the Universe is reasonably
uniform over large scales.
Later, after the Advanced Camera for
Surveys was installed on Hubble, it was used
to make a Hubble Ultra Deep Field (see
figure).

These regions of the deep fields and ultra
deep field have since been observed by many
other telescopes on the ground and in space,
notably the Chandra X-ray Observatory.
16.8 The Search for the
Most Distant Galaxies


Other deep surveys further strengthen our conclusion that we live in a
rather typical place in the Universe.
Spectra obtained with large
telescopes, especially the two
Keck telescopes in Hawaii,
confirm that many galaxies in
the Hubble Deep/Ultra Deep
Fields and other deep surveys
have large redshifts and hence
are very far away.

Though a few of the galaxies
have relatively low redshifts,
typical redshifts of the faintest
objects are between 1 and 4
(see figure).
16.8 The Search for the
Most Distant Galaxies


Light that we observe at visible wavelengths
actually corresponds to ultraviolet radiation
emitted by the galaxy, but shifted redward by 100
per cent to 400 per cent!
If we convert these redshifts to “distances” (or,
more precisely, lookback times—see Table 16 –1),
we find that the galaxies are billions of light-years
away.


We see them as they were billions of years in the
past, when the Universe was much younger than it
is now.
Note that when astronomers say that light from a
high-redshift galaxy comes from “the distant
universe,” what they really mean is “distant parts
of our Universe.”

We do not receive light from other universes, even if
they exist!
16.8 The Search for the
Most Distant Galaxies

In the past few years, many galaxies with redshifts
exceeding 5 (that is, all the spectral lines are shifted by over
500 per cent, putting them at more than 600 per cent of
their original values) were discovered (see figure), and
there are quite a few with redshifts over 6.



As of late 2005, at least one galaxy has been reported with a
redshift of about 7.
Several objects are suspected of having even higher redshifts,
though the spectra are not yet good enough to be certain.
The lookback time
corresponding to
redshift 6 is about 12.7
billion years (Table 16 –
1); we are seeing
denizens of an era
shortly (a billion years)
after the birth of the
Universe.

They are probably
newly formed galaxies.
16.8 The Search for the
Most Distant Galaxies



Out of the thousands of galaxies found in images such as
the Hubble Deep and Ultra Deep Fields, how do we go
about choosing those that are likely to be at high redshift?
(After all, with limited time for spectroscopy using large
telescopes such as Keck, it is important to improve the odds
if the goal is to find the highest redshifts.)
One very effective technique is to first measure the “color”
of each galaxy (see figure).
For two reasons, those that are likely to be very far away
look very red, and have little if any of the blue or ultraviolet
light normally emitted by stars.


First, their redshift moves light to redder (longer)
wavelengths.
Second, there is often another galaxy or large cloud of gas
along the way, and the hydrogen gas within it completely
absorbs the ultraviolet light.
16.9 The Evolution of Galaxies

Comparisons of the appearance of distant
galaxies and nearby galaxies provide clues to the
way in which galaxies evolve.


These comparisons must be done carefully to avoid
wrong conclusions.
For example, a visible-light image of a highredshift galaxy actually corresponds to ultraviolet
radiation emitted by the galaxy and shifted into
the visible band, so it would not be fair to
compare the image with the visible-light
appearance of a nearby (and hence essentially
unshifted) galaxy.


One way around this problem is to compare the
visible-light images of high-redshift galaxies with
ultraviolet images of nearby galaxies, as obtained
with the Hubble Space Telescope.
These images emphasize regions containing hot,
massive, young stars that glow brightly in the
ultraviolet (see figure).
16.9 The Evolution of Galaxies


Another technique is to obtain infrared images of the
high-redshift galaxies, and compare them with the visiblelight images of nearby galaxies.
Such images tend to be dominated by light from older,
less massive stars that more accurately reflect the overall
shape of the galaxy rather than pockets of recent, intense
star formation (see figure).

So far, the clearest infrared images have been
made with the Hubble Space Telescope. (Its
infrared camera ran out of solid nitrogen coolant
in 1998, sooner than anticipated. A new method
of cooling the camera was used in equipment
installed during the servicing mission in 2002,
and it is now working even better than before.)
16.9 The Evolution of Galaxies



These data, together with various types of analysis such
as computer simulations (for example, of what happens
when two galaxies collide and merge), provide many
interesting results.
One spectacular conclusion is that most spiral galaxies
used to look quite peculiar; there were essentially no large
galaxies with distinct, well-formed spiral arms beyond
redshift 2.
By redshift 1 there were quite a few of them, but many
took on their current, mature shapes more recently, in the
past 5 billion years (that is, at redshifts below about 0.5).
16.9 The Evolution of Galaxies


Another conclusion is that
there used to be a large
number of small, blue,
irregular galaxies that
formed stars at an
unusually high rate (see
figure).
Their strange shape might be partly caused by an irregular distribution of
young star clusters.


However, they appear peculiar even at infrared wavelengths, which are more
sensitive to older stars, so they must be structurally disturbed.
Some of them probably later merged together to form larger galaxies,
including disturbed spirals.

Perhaps others faded and are now difficult to find because they are so dim.
16.9 The Evolution of Galaxies

It appears that most elliptical galaxies formed early in the
Universe, beyond redshift 2 (lookback time 10 billion years);
there are many old-looking, well-formed ellipticals between
redshifts of 1 and 2.



On the other hand, we also think that an elliptical galaxy can be
produced by the collision and merging of two spiral galaxies.
Many new stars are created from the interstellar gas in the
spirals.
Computer models of the merging process also show that long
“tails” of material are sometimes temporarily formed (see figure
on next slide), just as we see in nearby examples of interacting
galaxies (see figures, left).


Later these tails disappear, leaving a more normal looking elliptical
galaxy, but with a population of stars younger than in the really
ancient ellipticals.
In fact, the Milky Way Galaxy and the Andromeda Galaxy are
approaching each other and may collide (or barely miss each other)
in about 5 or 6 billion years. Subsequently, they are likely to merge
and become an elliptical galaxy over the course of billions of years.
16.9 The Evolution of Galaxies
16.9 The Evolution of Galaxies


The total rate at which stars currently form in the
Universe is rather small compared with what it was billions
of years ago.
We see that the star formation rate has decreased since a
redshift of 1 (8 billion years ago) to the present time.



The rate may have been constant at still larger redshifts, up
to about 4 or 5 (12 billion years ago), though we are unsure
because many high-redshift galaxies are cloaked with dust.
The dust seems to have been produced by the first few
generations of stars, making it difficult to detect highredshift galaxies at visible wavelengths.
But infrared and submillimeter telescopes are finding them
in progressively larger numbers, so we can expect a more
accurate census in the near future.
16.9 The Evolution of Galaxies

As galaxies age, they evolve chemically, primarily because
of supernovae that create many of the heavy elements
through nuclear reactions and disperse them into the
cosmos.


Large, massive galaxies, whose gravitational fields don’t
allow much of the gas to escape, tend to become more
chemically enriched than small galaxies that are not able to
retain the hot gas.
So, we don’t expect to find many rocky, Earth-like planets
in small (dwarf ) galaxies like the Magellanic Clouds.

The formation of massive galaxies like the Milky Way seems
to be a critical step for the existence of humans.
16.10 Evolution of Large-Scale
Structure


We have also studied the evolution of large-scale
structure in the Universe.
By getting the redshifts of hundreds of thousands
of galaxies over large regions of the sky (see figure,
right), the growth of clusters, superclusters, and
voids can be traced (see figure, below).

As mentioned earlier, it appears
that superclusters and giant walls
of galaxies are the largest
structures in the Universe; we
have no clear evidence for supersuperclusters of galaxies.
16.10 Evolution of Large-Scale
Structure


Galaxies generally preceded clusters, and then
gravitationally assembled themselves into clusters.
Many clusters formed relatively recently, within the past 5
billion years (that is, at redshifts below 0.5), and in fact
are still growing now.


However, cluster formation did begin earlier.
Some very large, well-formed clusters have been found at
redshift 1 (8 billion light-years away), and evidence exists
for substantial concentrations of matter (which later
formed clusters) at a redshift of 4, corresponding to a
lookback time of about 12 billion years.
16.10 Evolution of Large-Scale
Structure



The observed distribution of
superclusters and voids (as the 2dF
mapping project showed in the
figure at top) can be compared with
the predictions of various theoretical
models using computer simulations
(see figures).
One important conclusion is that
dark matter pervades the Universe;
otherwise, it is difficult to produce
very large structures.
Galaxies and clusters seem to form
at unusually dense regions (“peaks”)
in the dark matter distribution, like
snow on the peaks of mountains.
16.10 Evolution of Large-Scale
Structure



Agreement with observations seems best when most of
the dark matter used in the simulations is “cold”—that is,
moving relatively slowly compared with the speed of light.
Work is in progress to determine what specific type of
cold dark matter is likely to account for most of the
material.
Simulations that use primarily hot dark matter (such as
neutrinos, with speeds close to that of light) do not
produce galaxy distributions that resemble those
observed.

Specifically, hot dark matter has a hard time clustering on
small scales, like those of individual galaxies and small
clusters.
16.10 Evolution of Large-Scale
Structure

Results announced in 2003 from NASA’s Wilkinson
Microwave Anisotropy Probe (WMAP; see Chapter 19)
show that about 15 per cent of the matter in the Universe
consists of protons, neutrons, and electrons—although
only about one-quarter of this normal matter resides in
stars and other visible objects, the rest being in the form
of relatively hot gas, MACHOs, and other constituents.
 The remaining 85 per cent is matter that does not consist of


normal particles; moreover, it is dark.
This is the cold dark matter discussed above.
It may be mostly WIMPs (see Section 16.4c), but we don’t
yet know this for sure, and no actual WIMPs have ever
been directly detected in a laboratory.

Again, we emphasize that the nature of dark matter is one
of the outstanding mysteries of modern astrophysics!