Download DTU Chap 16 Galaxies v2

Neil F. Comins • William J. Kaufmann III
Discovering the Universe
Eighth Edition
CHAPTER 16
Galaxies
Hubble Deep Field Image
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http://hubblesite.org/newscenter/archive/releases/2012/37/
Hubble’s Tuning Fork Diagram
Hubble summarized his classification scheme for galaxies with this tuning fork
diagram. Elliptical galaxies are classified by how oval they appear, whereas spirals
and barred spirals are classified by the sizes of their central bulges and the
correlated winding of their spiral arms. An S0 or SB0 galaxy, also called lenticular
galaxy, is an intermediate type between ellipticals and spirals. It has a disk but no
spiral arms.
Spiral Galaxies (Nearly Face-on Views)
Edwin Hubble classified spiral galaxies according to the tightness of their spiral
arms and the sizes of their nuclear bulges. Sa galaxies have the largest nuclear
bulges and the most tightly wound spiral arms, whereas Sc galaxies have the
smallest nuclear bulges and the least tightly wound arms. The images are different
colors because they were taken through filters that pass different colors.
Andromeda (M31)
Andromeda is a beautiful spiral galaxy and the only galaxy visible to the naked
eye from Earth’s northern hemisphereLocated only 2.5 Mly (0.77 Mpc) from us,
Andromeda is gravitationally bound to the Milky Way, and it covers an area in
the sky roughly 5 times as large as the full Moon. Two other galaxies, M32 and
M110, are also labeled on this photograph.
Spiral Galaxies Seen Nearly Edge-on from the Milky Way
a) Because of its large nuclear bulge, this galaxy (called the Sombrero Galaxy)
is classified as an Sa. If we could see it face-on, the spiral arms would be
tightly wound around a voluminous bulge. (b) Note the smaller nuclear bulge in
this Sb galaxy. (c) At visible wavelengths, interstellar dust obscures the
relatively insignificant nuclear bulge of this Sc galaxy.
Ripples in Water
The usual circular ripples
expanding from the place where a
rock was thrown into the water.
Ripples in rotating water creating
spiral arms, as do ripples in the gas
and dust of a disk galaxy.
Dynamics of a Grand-Design Spiral Galaxy
Barred Spiral Galaxies
As with spiral galaxies, Edwin Hubble classified barred spirals according to
the tightness of their spiral arms (which correlates with the sizes of their
nuclear bulges). SBa galaxies have the most tightly wound spirals and
largest nuclear bulges, SBb have moderately tight spirals and mediumsized nuclear bulges, and SBc galaxies have the least tightly wound spirals
and the smallest nuclear bulges.
Giant Elliptical Galaxies
The Virgo cluster is a rich, sprawling collection of more than 2000
galaxies about 50 million light-years from Earth. Only the center of this
huge cluster appears in this photograph. The two largest galaxies in the
cluster are the giant elliptical galaxies M84 and M86.
Elliptical Galaxies
Hubble classified elliptical galaxies according to how round or elongated
they appear. An E0 galaxy is round; a very elongated elliptical galaxy is
an E7. Three examples are shown.
Irregular Galaxies
(a) At a distance of only 179,000 light-years, the Large Magellanic Cloud
(LMC), an Irr I irregular galaxy, is the third closest known companion of our
Milky Way Galaxy. About 62,000 light-years across, the LMC spans 22°
across the sky, about 44 times the angular size of the full Moon.
Galaxy Distribution
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Galaxies group into clusters rather than being randomly
scattered through the universe.
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A rich cluster contains at least a thousand galaxies
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A poor cluster may contain only a few dozen up to a
thousand galaxies.
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A regular cluster has a nearly spherical shape with a
central concentration of galaxies; in an irregular cluster,
the distribution of galaxies is asymmetrical.
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Our Galaxy is a member of a poor, irregular
cluster, called the Local Group.
Superclusters in Our Neighborhood
This diagram shows the distances and relative positions
of superclusters within 950 million light-years of Earth.
Note also the labeling of some of the voids, which are
large, relatively empty regions between superclusters.
Structure in the Universe
This map shows the distribution of 62,559 galaxies in two wedges
extending in opposite directions from Earth out to distances of 2 billion
light-years. Note the prominent voids surrounded by thin areas full of
galaxies.
Foamy Structure of the Universe
A sponge that recreates the distribution of bright clusters of galaxies
throughout the universe. The empty spaces in the foam are analogous to the
voids found throughout the universe. The spongy regions are analogous to
the locations of most of the galaxies.
The Local Group
Our Galaxy belongs to a poor, irregular cluster that consists of about 40 galaxies,
called the Local Group. This map shows the distribution of about three-quarters of
the galaxies. The Andromeda Galaxy (M31) is the largest and most massive
galaxy in the Local Group. The second largest is the Milky Way itself. M31 and
the Milky Way are each surrounded by a dozen satellite galaxies. The recently
discovered Canis Major Dwarf Galaxy is the Milky Way’s nearest known neighbor.
Interacting and Colliding Galaxies
Pairs of colliding galaxies often exhibit long “antennae” of stars ejected by the
collision. This particular system is known as NGC 4676 or “the Mice” (because of
its tails of stars and gas). It is 300 million light-years from Earth in the
constellation Coma Berenices. The collision has stimulated a firestorm of new
star formation, as can be seen in the bright blue regions. Mass can also be seen
flowing between the two galaxies, which will eventually merge.
Interacting and Colliding Galaxies
These two galaxies, NGC 2207 (right) and IC 2163, are orbiting and tidally
distorting each other. Their most recent close encounter occurred 40 million
years ago when the two were perpendicular to each other and about 1
galactic diameter apart. Computer simulations indicate that they should
eventually coalesce.
Simulated Galactic Cannibalism
This computer simulation shows a small galaxy (yellow stars)
being devoured by a larger, disk-shaped galaxy (blue stars, white
gas). Note how spiral arms are generated in the disk galaxy by
its interaction with the satellite galaxy.
The Rotation Curves of Four Spiral Galaxies
This graph shows how the orbital speed of material in the disks
of four spiral galaxies varies with the distance from the center
of each galaxy. If most of each galaxy’s mass were concentrated
near the center of the galaxy, these curves would fall off at
large distances. But these and many other galaxies have flat
rotation curves that do not fall off. This indicates the presence
of extended halos of dark matter.
Gravitational Lensing of Extremely Distant Galaxies
Schematic of how a gravitational lens works. Light from the
distant object changes direction due to the gravitational
attraction of the intervening galaxy and underlying dark
matter. The more distant galaxy appears in two places to the
observer on the right.
Gravitational Lensing of Extremely Distant Galaxies
Three examples of gravitational lensing: (1) The bluer arc is a
galaxy that has been lensed by the redder elliptical galaxy. (2) A
pair of bluish images of the same object lensed symmetrically by
the brighter, redder galaxy between them. (3) The lensed object
appears as a blue arc under the gravitational influence of the group
of four galaxies.
Gravitational Lensing of Extremely Distant Galaxies
Superimposed in blue on this image of the galaxy cluster
CL 002417 is the location of dark matter that is
gravitationally lensing the galaxies behind it.
Five Galaxies and Their Spectra
The photographs of these five
elliptical galaxies were all taken at the
same magnification. They are labeled
according to the constellation in which
each galaxy is located. The spectrum
of each galaxy is the hazy band
between the comparison spectra at
the top and bottom of each plate. In
all five cases, the so-called H and K
lines of calcium are seen. The
recessional velocity (calculated from
the Doppler shifts of the H and K
lines) appears below each spectrum.
Note that the fainter— and thus more
distant—a galaxy is, the greater is its
redshift.
The Hubble Law
The distances and recessional velocities of distant galaxies are
plotted on this graph. The straight line is the “best fit” for the data. This
linear relationship between distance and speed is called the Hubble
law. For historical reasons, distances between galaxies, clusters of
galaxies, and superclusters of galaxies are usually given in
megaparsecs, Mpc, rather than millions of light-years.
Two Supernovae in NGC 664
In 1997 the rare occurrence of two supernovae in the same galaxy at
the same time was observed in the spiral galaxy NGC 664, located
about 300 Mly (90 Mpc) from Earth. Supernovae observed in remote
galaxies are important standard candles used by astronomers to
determine the distances to these faraway objects.
Techniques for Measuring Cosmological Distances
Astronomers use different methods to determine different distances in
the universe. All of the methods shown here are discussed in the text.
Distant Galaxies
(a) The young cluster of galaxies MS1054-03, shown on the left, contains many
orbiting pairs of galaxies, as well as remnants of recent galaxy collisions.
Several of these systems are shown at the right .This cluster is located 8 billion
light-years away from Earth. (b) This image of more than 300 spiral, elliptical,
and irregular galaxies contains several that are an estimated 12 billion lightyears from Earth. Two of the most distant galaxies are shown in the images on
the right, in red,at the centers of the pictures.
The Expanding Chocolate Chip Cake Analogy
The expanding universe can be compared to a chocolate chip cake baking
and expanding in the International Space Station. Just as all of the
chocolate chips move apart as the cake rises, all of the superclusters of
galaxies recede from each other as the universe expands.
Types of Galaxies
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The Hubble classification system groups galaxies into
four major types: spiral, barred spiral, elliptical, and
irregular.
The arms of spiral and barred spiral galaxies are sites of
active star formation.
According to the theory of self-propagating star
formation, spiral arms of flocculent galaxies are caused
by the births and deaths of stars over extended regions
of a galaxy. Differential rotation of a galaxy stretches the
star-forming regions into elongated arches of stars and
nebulae that we see as spiral arms.
Types of Galaxies
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According to the spiral density wave theory, spiral arms
of grand-design galaxies are caused by density waves.
The gravitational field of a spiral density wave
compresses the interstellar clouds that pass through it,
thereby triggering the formation of stars, including OB
associations, which highlight the arms.
Elliptical galaxies contain much less interstellar gas and
dust than do spiral galaxies; little star formation occurs in
elliptical galaxies.
Clusters and Superclusters
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Galaxies group into clusters rather than being randomly
scattered through the universe.
A rich cluster contains at least a thousand galaxies; a
poor cluster may contain only a few dozen up to a
thousand galaxies. A regular cluster has a nearly
spherical shape with a central concentration of galaxies;
in an irregular cluster, the distribution of galaxies is
asymmetrical.
Our Galaxy is a member of a poor, irregular cluster,
called the Local Group.
Rich, regular clusters contain mostly elliptical and
lenticular galaxies; irregular clusters contain more spiral
and irregular galaxies. Giant elliptical galaxies are often
found near the centers of rich clusters.
Clusters and Superclusters
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No cluster of galaxies has an observable mass large
enough to account for the observed motions of its
galaxies; a large amount of unobserved mass must be
present between the galaxies.
Hot intergalactic gases emit X rays in rich clusters.
When two galaxies collide, their stars initially pass each
other, but their interstellar gas and dust collide violently,
either stripping the gas and dust from the galaxies or
triggering prolific star formation. The gravitational effects
of a galactic collision can cast stars out of their galaxies
into intergalactic space.
Galactic mergers occur; a large galaxy in a rich cluster
may grow steadily through galactic cannibalism,
sometimes producing a giant elliptical galaxy.
Superclusters in Motion
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A simple linear relationship exists between the distance
from Earth to galaxies in other superclusters and the
redshifts of those galaxies (a measure of the speed at
which they are receding from us). This relationship is
the Hubble law: recessional velocity = Ho x distance,
where Ho is the Hubble constant.
Astronomers use standard candles—Cepheid variables,
the brightest supergiants, globular clusters, H II regions,
supernovae in a galaxy, and the Tully-Fisher relation—
to calculate intergalactic distances. Because of
difficulties in measuring the distances to remote
galaxies, the value of the Hubble constant, Ho, is not
known with complete certainty.
Key Terms
barred spiral galaxy
cluster (of galaxies)
elliptical galaxy
galactic merger
gravitational lensing
Hubble classification
Hubble constant
Hubble flow
Hubble law
intergalactic gas
irregular cluster (of galaxies)
irregular galaxy
lenticular galaxy
Local Group
poor cluster (of galaxies)
regular cluster (of galaxies)
rich cluster (of galaxies)
spiral density wave
spiral galaxy
standard candle
starburst galaxy
supercluster (of galaxies)
trailing-arm spiral galaxy
Tully-Fisher relation
WHAT DID YOU THINK?
Are most of the stars in spiral galaxies
located in their spiral arms?
 No. The spiral arms contain only 5% more
stars than the regions between the arms.
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WHAT DID YOU THINK?
Do all galaxies have spiral arms?
 No. Galaxies may be either spiral, barred
spiral, elliptical, or irregular. Only spirals
and barred spirals have arms.
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WHAT DID YOU THINK?
Are galaxies isolated objects?
 No. Galaxies are grouped in clusters, and
clusters are grouped in superclusters.
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WHAT DID YOU THINK?
Is the universe contracting, unchanging in
size, or expanding?
 The universe is expanding.
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