8/17/2015
1
UniverseTenth Edition
Chapter 23
Galaxies
Roger Freedman • Robert Geller • William Kaufmann III
23‐1 How astronomers first observed other galaxies
23‐2 How astronomers determined the distances to other galaxies
23‐3 The basic types of galaxies
23‐9 How galaxies formed and evolved
23‐7 What happens when galaxies collide
23‐4 What techniques astronomers use to determine distances to remote
galaxies
23‐5 How the spectra of remote galaxies tell us that the universe is expanding
23‐6 How galaxies are grouped into clusters and larger structures
23‐8 What observations indicate the presence of dark matter in other galaxies
and clusters
By reading this chapter, you will learn
N.B. Order of sections has been altered
A Pioneering View of Another Galaxy
23‐1: When galaxies were first discovered, it was not clear that they lie far beyond the Milky Way
8/17/2015
2
Harlow Shapley (left) Mt. Wilson Observatory/ Heber D. Curtis Lick Observatory
Positions• The galaxy is approximately
300,000 light years in diameter and the sun is located far from the center.
• The spiral nebulae are associated with the galaxy, although outside the main body. The nature of the spirals is not known, but is probably some combination of gas and faint stars.
• The Milky Way and its “halo” of globular clusters and spiral nebulae is all there is to the universe.
• The galaxy is approximately 30,000 light years in diameter and the sun is located near the center.
• The spiral nebulae are “island universes”, i.e., other galaxies comparable in size to the Milky Way.
• The universe contains a large, indeterminate, number of galaxies spread out over a large, indeterminate volume of space.
HS HC
Shapley’s Picture of the Universe
8/17/2015
3
Curtis’ Island Universe
The Andromeda Galaxy
23‐2: Hubble proved that the spiral nebulae are far beyond the Milky Way
The tie‐breaker
• In 1923 Edwin Hubble (1899‐1953) used the new 100‐inch telescope at the Mt. Wilson observatory to photograph M31.
• For the first time ever, individual stars could be distinguished on the photographic plate
8/17/2015
4
Var!
• Hubble scratched “N” on the plate, first believing the Cepheid was a nova
• His discovery of a Cepheid in M31 made it possible to apply Henrietta Leavitt's period‐luminosity method and determine that M31 is located at the remarkable distance of 1 million light‐years (300,000 parsecs)
• For most Astronomers this settled the issue of the spiral nebulae and the island universe theory, and also gave a clue as to how really big the cosmos is
Measuring Galaxy Distances with Cepheid Variables
8/17/2015
5
Who was right about what• The galaxy is approximately
300,000 light years in diameter and the sun is located far from the center.
• The spiral nebulae are associated with the galaxy, although outside the main body. The nature of the spirals is not known, but is probably some combination of gas and faint stars.
• The Milky Way and its “halo” of globular clusters and spiral nebulae is all there is to the universe.
• The galaxy is approximately 30,000 light years in diameter and the sun is located near the center.
• The spiral nebulae are “island universes”, i.e., other galaxies comparable in size to the Milky Way.
• The universe contains a large, indeterminate, number of galaxies spread out over a large, indeterminate volume of space.
HS HC
Spiral Galaxies
23‐3: Galaxies are classified according to their appearance
The Hubble Tuning Fork
8/17/2015
6
Spiral Galaxies
Variety in Spiral Arms
8/17/2015
7
Variety in Spiral Arms *fluffy like dust bunnies
Barred Spiral Galaxies
Elliptical Galaxies
8/17/2015
8
Giant Elliptical Galaxies
A Lenticular Galaxy
NGC 5128 in Visible and Infrared Wavelengths
8/17/2015
9
A Dwarf Elliptical Galaxy
The Large MagellanicCloud (LMC)
8/17/2015
10
The Small MagellanicCloud (SMC)
Hubble’s Tuning Fork Diagram (again)
The Building Blocks of Galaxies
23‐9: Galaxies formed from the merger of smaller objects
8/17/2015
11
8/17/2015
12
A Starburst Galaxy
23‐7: Colliding galaxies produce starbursts, spiral arms, and other spectacular phenomena
The M81 Group
8/17/2015
13
“Some 4¾ billion years from now, the Andromeda Galaxy (M31) will loom large in the night sky as its fateful collision with our Milky Way nears.”
NASA / ESA / Z. Levay / R. van der Marel / T. Hallas / A. Mellinger
• M33 will be involved also
• The Solar System will remain intact but hurled to another location
• 2 Gyr after impact the three galaxies will merge into one big elliptical
Kelly Beatty, Sky and Telescope
Collision Between Andromeda and the Milky Way
Collision Between Andromeda and the Milky Way
8/17/2015
14
Antenna Galaxy showing intense star forming regions
8/17/2015
15
Cartwheel Galaxy Formation
And the galaxy itself
Stephan’s Quintet
8/17/2015
16
Simulation of Future Interactionfor the Quintet
The Distance Ladder
23‐4: Astronomers use various techniques to determine the distances to remote galaxies
A Supernova in a Spiral Galaxy
8/17/2015
17
Measuring the Distance to a Galaxy Using Masers
23‐5: The Hubble law relates the redshifts of remote galaxies to their distances from Earth
The Hubble Law
Relating the Distances and Redshifts of Galaxies
8/17/2015
18
The Hercules Cluster
23‐6: Galaxies are grouped into groups, clusters and superclusters
Groups• Usually 2 or so large galaxies with about 2X1010 Solar
Masses
• Several dwarf companion galaxies
• More likely to have spirals than ellipticals
• Hot gas (T ~ 107K), dark matter
• Relatively slow moving components 300‐400 kps
• Core radius about 20‐100 kpc
The Local Group
8/17/2015
19
Local Group
Clusters
• More than 50 large galaxies
• Many more ellipticals than spirals
• Often have a H‐U‐G‐E cD type galaxy in the center of the cluster
• Core radius ~ 1Mpc
• Poor clusters have galaxy speeds ~500‐800 kps, rich clusters as
much as 1500kps
• Less hot gas within the ellipticals
• Always dark matter within and among
The Coma Cluster
8/17/2015
20
Why Spirals are in Groups, but Ellipticals are in Clusters
• Spirals are delicate, easily disrupted
• Ellipticals are less organized, entropy
Nearby Clusters of Galaxies
X‐ray Emission from a Cluster of Galaxies
8/17/2015
21
Clusters to Superclusters
Structure in the Nearby Universe
The Large‐Scale Distribution of Galaxies
8/17/2015
22
*6dFGS: 100,000 galaxies out to 1E9 light years; 80% of the Southern sky
*Six Degree Field Galaxy Survey
Fly Through
8/17/2015
23
The Rotation Curves of Four Spiral Galaxies
23‐8: Most of the matter in the universe is mysterious dark matter
Abell 2218 (note gravitational lensing)
Einstein Cross
8/17/2015
24
Gravitational Lensing by a Cluster of Galaxies
8/17/2015
25
Hubble 3D Map of Largest single object: dark matter structure 270 million LY across
Seen from ‘Outside’
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.
8/17/2015
26
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.
8/17/2015
27
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. Particles called WIMPs are
the favored hypothesis for dark matter.
• Gravitational lensing of remote galaxies by a foreground cluster enables
astronomers to glean information about the distribution of dark matter in
the foreground cluster.
8/17/2015
28
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.