ch24

							Roger A. Freedman • William J. Kaufmann III




   Universe
           Eighth Edition
           CHAPTER 24
              Galaxies
The two galaxies NGC 1531 and NGC 1532 are so close
together that they exert strong gravitational forces on each
other. Both galaxies are about 17 million pc (55 million ly)
from us in the constellation Eridanus.
A Modern View of the Spiral Galaxy M51 : This galaxy, also called NGC
5194, has spiral arms that are outlined by glowing H II regions. These
reveal the sites of star formation. One spiral arm extends toward the
companion galaxy NGC 5195.
The Andromeda “Nebula” : The Great Nebula in Andromeda, also known as M31,
can be seen with even a small telescope. Edwin Hubble was the first to
demonstrate that M31 is actually a galaxy that lies far beyond the Milky Way. M32
and M110 are two small satellite galaxies that orbit M31.
   The Hubble Classification: Galaxies can be grouped
    into four major categories: spirals, barred spirals,
    ellipticals, and irregulars.
   The disks of spiral and barred spiral galaxies are sites of
    active star formation.
   Elliptical galaxies are nearly devoid of interstellar gas
    and dust, and so star formation is severely inhibited.
   Lenticular galaxies are intermediate between spiral and
    elliptical galaxies.
   Irregular galaxies have ill-defined, asymmetrical shapes.
    They are often found associated with other galaxies.
Spiral Galaxies : Edwin Hubble classified spiral galaxies according to the
texture of their spiral arms and the relative size of their central bulges. Sa
galaxies have smooth, broad spiral arms and the largest central bulges,
while Sc galaxies have narrow, well-defined arms and the smallest central
bulges.
Barred Spiral Galaxies: As with spiral galaxies, Hubble classified barred
spirals according to the texture of their spiral arms (which correlates to
the sizes of their central bulges). SBa galaxies have the smoothest spiral
arms and the largest central bulges, while SBc galaxies have narrow, well-
defined arms and the smallest central bulges
Elliptical Galaxies : Hubble classified elliptical galaxies according to how
round or flattened they look. A galaxy that appears round is labeled E0,
and the flattest-appearing elliptical galaxies are designated E7.
Giant Elliptical Galaxies : The Virgo cluster is a rich, sprawling collection
of more than 2000 galaxies about 17 Mpc (56 million ly) from Earth. Only
the center of this huge cluster appears in this photograph. The two
largest members of this cluster are
the giant elliptical galaxies M84 and M86. These galaxies have angular
sizes of 5 to 7 arcmin.
A Dwarf Elliptical Galaxy : This diffuse cloud of stars is a nearby E4
dwarf elliptical called Leo I. It actually orbits the Milky Way at a distance
of about 180 kpc (600,000 ly). Leo I is about 1 kpc (3000 ly) in diameter
but contains so few stars that you can see through the galaxy’s center.
A Lenticular Galaxy : NGC 2787 is classified as a lenticular galaxy
because it has a disk but no discernible spiral arms. Its nucleus displays
a faint bar (not apparent in this image), so NGC 2787 is denoted as an SB0
galaxy. It lies about 7.4 Mpc (24 million ly) from Earth in the constellation
Ursa Major.
Hubble’s Tuning Fork Diagram : Edwin Hubble’s classification of regular
galaxies is shown in his tuning fork diagram. An elliptical galaxy is
classified by how flattened it appears. A spiral or barred spiral galaxy is
classified by the texture of its spiral arms and the size of its central bulge.
A lenticular galaxy, is intermediate between ellipticals and spirals. Irregular
galaxies do not fit into this simple classification scheme.
The Large Magellanic Cloud (LMC) At a distance of only 55 kpc (179,000 ly), this Irr
I galaxy is the third closest known companion of our Milky Way Galaxy. One sign
that star formation is ongoing in the LMC is the Tarantula Nebula, whose diameter
of 250 pc (800 ly) and mass of 5  106 M make it the largest known H II region.
A Supernova in a Spiral Galaxy: These images from the Very Large Telescope
show the spiral galaxy M100 (a) before and (b) after a Type Ia supernova exploded
within the galaxy in 2006. Such luminous supernovae, which can be seen at
extreme distances, are important standard candles used to determine the
distances to faraway galaxies. The distance to M100 is also known from
observations of Cepheid variables (see Figure 24-4), so this particular supernova
can help calibrate Type Ia supernovae as distance indicators.
The Distance Ladder : Astronomers employ a variety of techniques for
determining the distances to objects beyond the solar system. Because
their ranges of applicability overlap, one technique can be used to
calibrate another. The arrows indicate distances to several important
objects. Note that each division on the scale indicates a tenfold increase
in distance, such as from 1 to 10 Mpc.
Measuring the Distance to a Galaxy Using Masers : This drawing shows
interstellar clouds called masers (the colored dots) moving from position
1 to 2 to 3 as they orbit the center of a galaxy. The redshift and blueshift of
microwaves from the masers shown in red and blue tell us their orbital
speed. By relating this to the angle through which the masers shown in
green appear to move in a certain amount of time, we can calculate the
distance to the galaxy.
The above figure: Relating the Distances and Redshifts of Galaxies
These five galaxies are arranged, from top to bottom, in order of increasing
distance from us. All are shown at the same magnification. Each galaxy’s
spectrum is a bright band with dark absorption lines; the bright lines above
and below it are a comparison spectrum of a light source at the observatory
on Earth. The horizontal red arrows show how much the H and K lines of
singly ionized calcium are redshifted in each galaxy’s spectrum. Below
each spectrum is the recessional velocity calculated from the redshift.
The more distant a galaxy is, the greater its redshift.
The Hubble Law and the
Relativistic Redshift :
The accompanying graph
displays this relationship
between redshift z and
speed v. Note that z
approaches infinity as v
approaches the speed of
light.
   The Hubble Law: There is a simple linear relationship
    between the distance from the Earth to a remote galaxy
    and the redshift of that galaxy (which is a measure of the
    speed with which it is receding from us). This
    relationship is the Hubble law, v = H0d.
   The value of the Hubble constant, H0, is not known with
    certainty but is close to 73 km/s/Mpc.
   Clusters and Superclusters: Galaxies are grouped into
    clusters rather than being scattered randomly throughout
    the universe.
   A rich cluster contains hundreds or even thousands of
    galaxies; a poor cluster, often called a group, may
    contain only a few dozen.
   A regular cluster has a nearly spherical shape with a
    central concentration of galaxies; in an irregular cluster,
    galaxies are distributed asymmetrically.
The Hercules Cluster : This irregular cluster of galaxies is
about 200 Mpc (650 million ly) from Earth. The Hercules
cluster contains many spiral galaxies, often associated in
pairs and small groups.
   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 spiral,
    barred spiral, and irregular galaxies along with ellipticals.
   Giant elliptical galaxies are often found near the centers
    of rich clusters.
The Local Group : This illustration shows the relative positions of the galaxies
that comprise the Local Group, a poor, irregular cluster of which our Galaxy is
part. (The blue rings represent the plane of the Milky Way’s disk; 0° is the
direction from Earth toward the Milky Way’s center. Solid and dashed lines point
to galaxies above and below the plane, respectively.) The largest and most
massive galaxy in the Local Group is M31, the Andromeda Galaxy; in second
place is the Milky Way, followed by the spiral galaxy M33. Both the Milky Way and
M31 are surrounded by a number of small satellite galaxies.
Nearby Clusters of Galaxies
This illustration shows a sphere of space: 800 million ly (250 Mpc) in diameter
centered on the Earth in the Local Group. Each spherical dot represents a cluster
of galaxies. To better see the three-dimensionality of this figure, colored arcs are
drawn from each cluster to the green plane, which is an extension of the plane of
the Milky Way outward into the universe. Note that clusters of galaxies are
unevenly distributed here, as they are elsewhere in the universe.
Structure in the Nearby Universe : This composite infrared image from
the 2MASS (Two-Micron All-Sky Survey) project shows the light from 1.6
million galaxies. In this image, the entire sky is projected onto an oval;
the blue band running vertically across the center of the image is light
from the plane of the Milky Way.
The Large-Scale Distribution of Galaxies: (a) This map shows the
distribution of 62,559 galaxies in two wedges extending out to redshift z =
0.25. Note the prominent voids surrounded by thin regions full of
galaxies. (b) The two wedges shown in (a) lie roughly perpendicular to the
plane of the Milky Way. These were chosen to avoid the obscuring dust
that lies in our Galaxy’s plane.
a) An X-ray image of this cluster of galaxies shows emission from hot gas
between the galaxies. The gas was heated by collisions between galaxies
within the cluster. (b) The galaxies themselves are too dim at X-ray
wavelengths to be seen in (a), but are apparent at visible wavelengths.
   Galactic Collisions and Mergers: When two galaxies
    collide, their stars pass each other, but their interstellar
    media collide violently, either stripping the gas and dust
    from the galaxies or triggering prolific star formation.
   The gravitational effects during a galactic collision can
    throw stars out of their galaxies into intergalactic space.
   Galactic mergers may occur; a large galaxy in a rich
    cluster may tend to grow steadily through galactic
    cannibalism, perhaps producing in the process a giant
    elliptical galaxy.
A Simulated Collision Between Two Galaxies These frames from a
supercomputer simulation show the collision and merger of two galaxies
accompanied by an ejection of stars into intergalactic space. Stars in the
disk of each galaxy are colored blue, while stars in their central bulges
are yellow-white. Red indicates dark matter that surrounds each galaxy.
The frames progress at 125-million-year intervals.
Although galaxies can collide at very high speeds by Earth standards,
they are so vast that a collision can last hundreds of millions of years.
Gravitational Lensing : (a) A massive object such as a galaxy
can deflect light rays like a lens so that an observer sees
more than one image of a more distant galaxy.
Gravitational Lensing : (b), (c), (d) Three examples of
gravitational lensing. In each case a single, distant blue
galaxy is “lensed” by a closer red galaxy or galaxies.
Isolated Dark Matter in a Cluster of Galaxies (a) This visible-light image of
the galaxy cluster 1E0657-56 shows more than a thousand galaxies. The
superimposed image in red shows the distribution of the cluster’s hot, X-
ray emitting gas, and the blue image shows the distribution of dark matter
as determined by gravitational lensing
The Building Blocks of Galaxies
The Building Blocks of Galaxies (b) If these objects were to
coalesce, the result would be a full-sized galaxy such as we
see in the nearby universe today.
   Formation and Evolution of Galaxies: Observations
    indicate that galaxies arose from mergers of several
    smaller gas clouds.
   Whether a protogalaxy evolves into a spiral galaxy or an
    elliptical galaxy depends on its initial rate of star
    formation.
The Formation of Spiral and Elliptical Galaxies (a) If the
initial star formation rate in a protogalaxy is low, it can
evolve into a spiral galaxy with a disk.
The Formation of Spiral and Elliptical Galaxies (b) If the
initial star formation rate is rapid, no gas is left to form a disk.
The result is an elliptical galaxy.
The Formation of Spiral and Elliptical Galaxies (c) This graph
shows how the rate of star birth (in solar masses per year)
varies with age in spiral and elliptical galaxies.
The image shows the Small Magellanic Cloud (SMC), an
irregular galaxy that orbits the Milky Way.
   The Dark-Matter Problem: The luminous mass of a
    cluster of galaxies is not large enough to account for the
    observed motions of the galaxies; a large amount of
    unobserved mass must also be present. This situation is
    called the dark-matter problem.
   Hot intergalactic gases in rich clusters account for a
    small part of the unobserved mass. These gases are
    detected by their X-ray emission. The remaining
    unobserved mass is probably in the form of dark-matter
    halos that surround the galaxies in these clusters.
   Gravitational lensing of remote galaxies by a foreground
    cluster enables astronomers to glean information about
    the distribution of dark matter in the foreground cluster.
                      Key Ideas
   The Hubble Classification: Galaxies can be grouped
    into four major categories: spirals, barred spirals,
    ellipticals, and irregulars.
   The disks of spiral and barred spiral galaxies are sites of
    active star formation.
   Elliptical galaxies are nearly devoid of interstellar gas
    and dust, and so star formation is severely inhibited.
   Lenticular galaxies are intermediate between spiral and
    elliptical galaxies.
   Irregular galaxies have ill-defined, asymmetrical shapes.
    They are often found associated with other galaxies.
                     Key Ideas
   Distance to Galaxies: Standard candles, such as
    Cepheid variables and the most luminous supergiants,
    globular clusters, H II regions, and supernovae in a
    galaxy, are used in estimating intergalactic distances.
   The Tully-Fisher relation, which correlates the width of
    the 21-cm line of hydrogen in a spiral galaxy with its
    luminosity, can also be used for determining distance. A
    method that can be used for elliptical galaxies is the
    fundamental plane, which relates the galaxy’s size to its
    surface brightness distribution and to the motions of its
    stars.
                      Key Ideas
   The Hubble Law: There is a simple linear relationship
    between the distance from the Earth to a remote galaxy
    and the redshift of that galaxy (which is a measure of the
    speed with which it is receding from us). This
    relationship is the Hubble law, v = H0d.
   The value of the Hubble constant, H0, is not known with
    certainty but is close to 73 km/s/Mpc.
                      Key Ideas
   Clusters and Superclusters: Galaxies are grouped into
    clusters rather than being scattered randomly throughout
    the universe.
   A rich cluster contains hundreds or even thousands of
    galaxies; a poor cluster, often called a group, may
    contain only a few dozen.
   A regular cluster has a nearly spherical shape with a
    central concentration of galaxies; in an irregular cluster,
    galaxies are distributed asymmetrically.
                      Key Ideas
   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 spiral,
    barred spiral, and irregular galaxies along with ellipticals.
   Giant elliptical galaxies are often found near the centers
    of rich clusters.
                       Key Ideas
   Galactic Collisions and Mergers: When two galaxies
    collide, their stars pass each other, but their interstellar
    media collide violently, either stripping the gas and dust
    from the galaxies or triggering prolific star formation.
   The gravitational effects during a galactic collision can
    throw stars out of their galaxies into intergalactic space.
   Galactic mergers may occur; a large galaxy in a rich
    cluster may tend to grow steadily through galactic
    cannibalism, perhaps producing in the process a giant
    elliptical galaxy.
                      Key Ideas
   The Dark-Matter Problem: The luminous mass of a
    cluster of galaxies is not large enough to account for the
    observed motions of the galaxies; a large amount of
    unobserved mass must also be present. This situation is
    called the dark-matter problem.
   Hot intergalactic gases in rich clusters account for a
    small part of the unobserved mass. These gases are
    detected by their X-ray emission. The remaining
    unobserved mass is probably in the form of dark-matter
    halos that surround the galaxies in these clusters.
   Gravitational lensing of remote galaxies by a foreground
    cluster enables astronomers to glean information about
    the distribution of dark matter in the foreground cluster.
                     Key Ideas
   Formation and Evolution of Galaxies: Observations
    indicate that galaxies arose from mergers of several
    smaller gas clouds.
   Whether a protogalaxy evolves into a spiral galaxy or an
    elliptical galaxy depends on its initial rate of star
    formation.