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A Diversity of Galaxies
Courtesy of NASA, ESA, S. Beckwith (STScI) and the HUDF Team
Chapter 17
17-1 The Hubble Classification
1. In 1924, Hubble found Cepheid variables in three
spiral nebulae, showing they were actually spiral
galaxies.
– The evidence that galaxies existed outside the Milky Way expanded
our appreciation of the size of the universe.
2. Hubble divided galaxies into three basic types:
spiral, elliptical, and irregular.
– Each major classification contains subdivisions.
3. An elliptical galaxy is one of a class of galaxies that
have smooth spheroidal shapes.
An irregular galaxy is a galaxy of irregular shape that
cannot be classified as spiral or elliptical.
Courtesy of NOAO/AURA/NSF
Spiral Galaxies
1. Hubble divided spiral galaxies
into two groups: ordinary spirals
and barred spirals.
2. Ordinary spirals are designated
with an S; barred spirals are
designated with an SB.
Fig. 17.01b: M77 is a type Sb spiral galaxy
Courtesy of NOAO/AURA/NSF
3. A barred spiral galaxy is a spiral
galaxy in which the spiral arms
come from the ends of a bar
through the nucleus rather than
from the nucleus itself.
Figure 17.02c: NGC1365 is a SBc barred spiral galaxy
4. Each type of spiral galaxy is then further
subdivided into categories a, b, c, depending on
how tightly the spiral arms are wound around
the nucleus.
Galaxies with the most tightly wound arms are
type a; they also have the most prominent
nuclear bulges.
5. Up to 2/3 of all spirals contain bars. The bar
system provides an efficient mechanism for
fueling star birth at the center of an SB galaxy.
Courtesy of NASA, ESA, and The Hubble Heritage Team (STScI/AURA)
6. Galaxies that seem to
have the nuclear
bulge and disk of a
spiral, but no arms,
are called are called
lenticular (or S0)
galaxies.
Figure 17.03: The S0 galaxy
NGC5866
7. Type c spirals contain more gas and dust than
type a, resulting in a larger proportion of their
mass being involved in star formation.
Courtesy of Todd Boroson/NOAO/AURA/NSF
Figure 17.01c: M51, the Whirlpool "nebula," comprises a type Sc galaxy and its
smaller, barred and more amorphous companion.
8. Most spiral galaxies are from 50,000 to
2,000,000 million light-years across and
contain from 109 to 1012 stars.
Courtesy of ESO
Figure 17.01a: The Sombrero galaxy (M104, in Virgo) is a type Sa galaxy
Courtesy of NOAO/AURA/NSF
Elliptical Galaxies
1. Elliptical galaxies are
ellipsoids; they are
classified from round (E0)
to very elongated (E7).
2. Most of the galaxies in
existence are ellipticals,
but most of these are
smaller than spiral
galaxies.
3. A few giant elliptical
galaxies have 21013 stars
and are thus larger than
any spiral galaxy.
Figure 17.04a: NGC5128 (or
Centaurus A) is a type E0 peculiar
elliptical galaxy.
Irregular Galaxies
1. Fewer than 20% of all galaxies fall in the category
of irregulars, and they are all small, normally
having fewer than 25% of the number of stars in
the Milky Way.
Courtesy of AURA/NOAO/NSE
Figure 17.5a: The Large Magellanic Cloud, the nearest galaxy to the Milky Way, is an irregular galaxy.
– On the other hand,
stars in a galaxy
rarely collide since
they are separated
by distances that
are millions of time
their diameter.
Courtesy of Brad Whitmore (STScI) and NASA
2. Collisions between
galaxies are not
unusual because
on average
galaxies are
separated by
distances only
about 20 times
their diameter.
Figure 17.06a: The Antennae, colliding galaxies
3. Because of their great distances, galaxies exhibit
no proper motion.
Evidence of past collisions has to come from
present appearance.
4. Computer simulations show that colliding galaxies
actually pass through one another with few
collisions between individual stars.
However, large dust and gas clouds in the
galaxies make them more likely targets, resulting
in increased star formation rates.
Courtesy of NASA/ESA/ and the Hubble Heritage Team (STScI)
Figure 17.06c: Tidal distortion as a result of a grazing encounter.
5. Bursts of star formation may also occur as a
result of tidal interactions among neighboring
galaxies.
6. Galactic cannibalism often occurs as a result of
collisions.
Hubble’s Tuning Fork Diagram
1. Hubble’s tuning fork diagram relates the
various types of galaxies.
In his plan, S0 galaxies form the connecting
link, because they have characteristics of both
elliptical and spiral galaxies.
2. Astronomers once also thought the diagram
represented an evolutionary sequence, but this
interpretation has been discarded as old stars
have been found in all three types.
Hubble’s Tuning Fork Diagram
Fig. 17-7
17-2 Measuring Galaxies
1. The most important properties of a galaxy that we
can measure are its
– distance,
– mass,
– and motion.
Distances Measured by Various Indicators
1. Cepheid variables are excellent distance
indicators but can be seen in only relatively
nearby galaxies, out to perhaps 200 million lightyears.
2. Bright stars (giants, supergiants, novae) can also
be used as distance indicators.
3. Large globular clusters and supernovae are of
consistent brightness so they, too, can be used
to determine distances to more distant galaxies.
4. These objects allow astronomers to determine
distances out to about 1000 million light-years.
5. Starting with the period-luminosity relationship of
Cepheids, astronomers are able to follow a chain
of reasoning and observation that allows them to
determine the distances to galaxies too far away
for their Cepheids to be visible.
Fig. 17-9
6. As one distance measurement builds on another in
a series of steps, constant checks are always being
made as new data arrive.
– Otherwise, an error in the first step will propagate up
through the chain of steps and lead to wrong conclusions.
7. In this analysis we are assuming that galaxies in our
neighborhood are basically the same as those
farther away.
– This may seem reasonable but keep in mind that we are
seeing distant galaxies as they were in the past, not as
they are today.
The Hubble Law
1. In 1912, Slipher found
that spiral nebulae had
redshifted spectra
indicating that they were
moving away from us at
tremendous velocity.
Figure 17.09
2. In 1920s, Hubble and
Humason showed that
there is a relationship
between the recessional
velocities of galaxies
and their distances.
Figure 17.11: The data on Type Ia supernovae provide a dramatic
confirmation of the Hubble and Humason.relationship.
Figure 17.10: Redshifts of galaxies
Courtesy of California Institute of Technology
3. Hubble showed that the universe is expanding,
and his work is the foundation for today’s
theories of cosmology—the study of the nature
and evolution of the universe as a whole.
4. The redshift that Hubble observed is not due to
the Doppler effect.
5. The Hubble law states that a galaxy’s recessional
speed () is directly proportional to its distance (d):
 = H0d,
where H0 is the Hubble constant (the proportionality
constant in the Hubble law; the ratio of recessional
velocities of galaxies to their distances).
6. Modern day measurements of the Hubble constant
place it between 15 and 25 km/s per million lightyears (MLY) or between 50 and 80 km/s per
megaparsec (Mpc).
7. The latest observations of the radiation left over
from the hot big bang indicate a value of
H0 = 73  3 (km/s)/Mpc
or about 22.5 (km/s)/Mly.
8. Determining a precise value for the Hubble
constant is difficult because accurate
measurements of distances to galaxies far away
are hard to obtain.
9. The value of H0 changes with time.
It is simply the slope of the line in the graph of
recessional velocity of galaxies versus their
distance as measured during this period of time in
the universe’s life.
The Hubble Law Used to Measure Distance
1. For the most distant galaxies, most of our distance
indicators can be seen.
Therefore, the Hubble law can be used to determine
their distances.
The Tully-Fisher Relation
1. The Tully-Fisher relation holds that the wider the 21centimeter spectral line, the greater the absolute
luminosity of a spiral galaxy.
2. Using the Tully-Fisher relation, astronomers can
determine the absolute magnitude of a galaxy and
use it as a distance indicator.
Figure 17.13: A more massive
galaxy would be expected to be
bright and to have a wide 21centimeter spectral line.
Figure 17.14: A review of the various methods of measuring distance in
astronomy.
17-3 The Masses of Galaxies
1. A galaxy’s mass can be determined by observing
the rotation periods of some parts of it (using
Doppler shift data) and then applying Kepler’s third
law.
2. Another method is to use a pair of galaxies
revolving around each other.
– The problem with this method is that it is difficult to
determine the angle of the plane of revolution to our line of
sight.
Clusters of Galaxies; Missing Mass
1. Most galaxies are part of clusters.
A cluster of galaxies is a gravitationally linked
assemblage.
2. The local group is a cluster of 20 or so galaxies that
includes the Milky Way Galaxy, the Andromeda
galaxy, and the two Magellanic Clouds.
3. A third method of measuring the masses of
galaxies takes advantage of their clustering.
It uses the Doppler effect to find the speed (and
thus period) of a galaxy at the outskirts of a cluster.
4. The cluster method gives mass values for clusters
that are much greater than is accounted for by the
visible stars within the galaxies in the cluster.
5. For the Milky Way we can account for as little as
1/10 of the total mass of the Galaxy.
6. Missing mass is the difference between the mass
of clusters of galaxies as calculated from
Keplerian motions and the amount of visible
mass.
7. Several possibilities have been proposed for the
nonluminous matter.
– (i) Ordinary “nonluminous” matter; composed of ordinary
matter but not easily observed (e.g., planets, brown dwarfs,
very old white dwarfs, etc.)
– (ii) Hot dark matter; neutrinos and other exotic particles
(introduced by theories but not observed yet) moving at
very high speeds
– (iii) Black holes
– (iv) Cold dark matter; an exotic form of matter, moving at
relatively slow speed, which can be detected only by its
gravitational interactions; it appears to be quite abundant
throughout the universe.
8. It seems that the universe is
about 4% normal matter
23% dark matter
73% dark energy.
9. Dark matter is distributed in galaxies and clusters
of galaxies in a way similar to visible matter, as
shown by the rotation curves of galaxies.
10. Galactic halos may contain much of the missing
matter.
11. A supercluster is a group of clusters of galaxies.
Our local supercluster contains the local group
and the Virgo cluster. Between superclusters are
great voids with no galaxies.
12. It seems that matter in the universe forms a
cosmic web in which galaxies are formed along
filaments of normal and dark matter, and clusters
are formed at the intersections of these filaments.
Figure 17.17: The cosmic web revealed by the Two Degree Field (2dF)
Galaxy Redshift Survey.
Colless et al. (2001), MNRAS, 328, 1039. Image courtesy of the 2dF Galaxy Redshift Survey team and Professor Matthew Colless, Director of Anglo-Australian Observatory
17-4 The Origin of Galactic Types
1. Two modern theories
– the cloud density theory
– the merger theory
purport to explain why galaxies exist in various
types
The Cloud Density Theory
1. Elliptical galaxies formed from the densest
gas/dust clouds.
Rapid star formation then used up the gas/dust
before a disk had a chance to form.
2. Clouds with lower density would have formed
stars less frequently, and the dust and gas would
have collapsed into a disk before star formation
used it all up.
The Merger Theory
1. According to this theory, spiral galaxies formed
before elliptical galaxies, and ellipticals are the
result of mergers of spirals.
2. In clusters where galaxies are packed close
together, ellipticals dominate, supporting the
notion of frequent mergers.
– In loosely packed clusters of galaxies, ellipticals are fairly
rare.
3. At this point neither theory explains irregular
galaxies well.
Some irregulars are seen to be pairs of galaxies in
collision.
Look-Back Time
1. We have observed objects that may be as far away
as 13 billion light-years.
– This means that the light we see left these objects 13
billion light-years ago.
2. Look-back time is the time light from a distant
object has traveled to reach us.
3. The look-back time complicates our interpretation
of galaxies because the farther out we look, the
earlier in time we are seeing them.
– Our assumption that distant clusters are similar to nearby
clusters may not be valid, since we have observed galactic
cannibalism in large clusters of galaxies.
17-5 Active Galaxies
1. All galaxies emit radio waves; for a normal galaxy,
radio waves constitute only about 1% of the
galaxy’s total luminosity.
2. A radio galaxy is a galaxy having greatest
luminosity at radio wavelengths.
– A typical radio galaxy emits millions of times more energy
in radio waves than does a normal galaxy.
3. Cygnus A, the first radio galaxy, was discovered in
1951 and has a double-lobed radio source
associated with the visible light image.
Courtesy of NRAO/AUI
Figure 17.19a: A false-color
radio image of Cygnus A
shows a jet extending from
the source (an AGN) toward
one of the radio lobes.
4. Most of the galaxies associated with double-lobed
radio sources are either giant ellipticals or spirals.
5. The radio lobes are enormous and mark the
positions where the outflows (jets) start interacting
with the intergalactic medium.
Figure 17.20a: Centaurus A with hot gas arcs
X-ray (NASA/CXC/M. Karovska et al.); Radio 21-cm image (NRAO/VLA/J. Condon et al.); Optical (Digitized Sky Survey U.K. Schmidt Image/STScI)
6. Radio galaxies often appear unusual when viewed
in visible light.
7. Radio galaxies are one type of a group of highenergy galaxies called active galaxies.
– An active galaxy is a galaxy with an unusually luminous
nucleus.
– Because the energy of an active galaxy comes from its
nucleus, astronomers often refer to active galactic
nuclei (AGNs) rather than active galaxies.
Figure 17.21a: Virgo A (M87), a giant E0 elliptical radio galaxy
Courtesy of NOAO/AURA/NSF
Figure 17.21c: A jet is formed within a few tenths of a light-year from the core of the galaxy
Courtesy of NASA, NRAO, and J. Biretta (STScI)
8. Jets seem to be a universal phenomenon.
– They are the natural byproducts of accretion onto a
compact objects, emanating at right angles to the disk that
surrounds the object.
– They are mostly well-collimated and transfer energy,
matter, momentum, and magnetic fields from the central
region to the surrounding environment.
Quasars
1. In 1960 an unusual star-like object—3C 273—was
discovered that emitted intense radio waves.
The object appeared to be very small, it had a small jet
protruding from it, and the radio waves were
emanating from the jet and the main body of the
object.
Fig. 17-22
WFPC2 image: NASA and J. Bahcall (IAS); ACS image: NASA, A. Martel (JHU), H. Ford
(JHU), M. Clampin (STScI), G. Hartig (STScI), G. Illingworth (UCO/Lick Observatory), the
ACS Science Team and ESA
2. The spectra of 3C 273 and 3C 48 (the second
unusual object discovered in 1960) showed
emission lines, which could not be identified.
Because of their star-like appearance and strong
radio emission, the objects were named quasars.
3. A quasar (quasi-stellar radio source) is a small,
intense celestial source of radiation with a very
large redshift.
Figure 17.22b: The jet emanating from the core of quasar 3C 273, as seen
at different wavelengths.
Optical: NASA/STScI; X-ray: NASA/CXC; Radio: MERLIN
4. In 1963, the unusual spectral lines found in 3C
273 and 3C 48 were shown to be highly
redshifted hydrogen lines.
If the redshifts are caused by the Doppler effect,
the quasars are moving at 15% and 30% of the
speed of light, respectively.
5. Since the early 1960s, more than 23,000 quasars
have been discovered.
– Unlike 3C 273 and 3C 48, most quasars are not sources
of radio waves.
– Most are blue-white objects and X-ray emitters.
6. Many quasars vary in intensity in an irregular
way, changing intensity in weeks or months.
– This observation confirms their small size.
7. Redshifts for quasars range from 0.06 up to 6.41
for the farthest known quasar.
– The latter quasar is receding at about 96.4% the speed
of light.
8. At such great distances, a bright quasar must be
1000 times more luminous than a galaxy like
ours, while being much smaller than a typical
galaxy.
Competing Theories for the Quasar Redshift
1. The local hypothesis proposal states that
quasars are much nearer than a cosmological
interpretation of their redshifts would indicate.
2. If quasars were local, then we would see at least
some highly blueshifted ones, but we don’t.
3. Most astronomers now agree that quasars’
redshifts do fit the Hubble law.
Seyfert Galaxies
1. Observations of quasars suggest that there are
many similarities between quasars and active
galaxies.
2. A Seyfert galaxy is one of a class of spiral
galaxies having active nuclei and spectra
containing emission lines.
3. It now appears that quasars may be at the nuclei
of some type(s) of galaxies.
Quasars and Gravitational Lenses
1. Twin quasars were discovered in 1979, having the
same luminosity, the same redshift, and identical
spectra.
2. A gravitational lens is the phenomenon in which the
gravity of a massive body between a distant object
and the viewer bends light from the distant object
and causes it to be seen as two or more objects
(according to the general theory of relativity)
Figure 17.28d: Five arc-shaped images of one galaxy.
Courtesy of W.N. Colley (Princeton University), E. Turner (Princeton University), J.A. Tyson (AT&T Bell Labs),
and NASA
3. Since 1979 many examples of gravitational lensing
have been found.
4. When the alignment between the viewer, distant
object and massive body is perfect, we observe a
ring (called an Einstein ring).
5. Gravitational lenses are important not only
because they provide another confirmation of the
general theory of relativity but also because they
indicate that quasars are indeed very distant.
6. A graph of the density of quasars as a function of
distance shows that most quasars appear at a fairly
specific distance from us.
Since distance is proportional to time, this indicates
that quasars existed during a relatively short period
of time in the distant past.
Fig. 17-29b
Quasars, Blazars, and Superluminal Motion
1. Blazars (BL Lac objects) are especially luminous
AGNs that vary in luminosity by a factor of up to 100
in just a few months.
2. Radio observations of blazers indicated they were
double radio sources oriented so that one jet is
coming straight (or nearly so) at us.
– This was supported by observations of superluminal
motion, motion that appears to occur faster than the speed
of light.
3. Superluminal motion is also observed in some
quasars.
– It is a projection effect, but does indicate very high speeds
for the material in the jets.
17-6 The Nature of Active Galactic Nuclei
1. According to present theory, the tremendous
energy that comes from an AGN is caused by an
immense black hole at the nucleus of the galaxy.
The black hole is surrounded by an accretion
disk heated by infalling material.
2. The leading theory on the nature of AGNs holds
that the different observed types of AGNs are
basically the same, and that they appear
different depending upon their orientation with
respect to us.
Fig. 17-34b
3. According to this unification theory,
– when an AGN is viewed edge-on we see it as a radio
galaxy (radio lobes and jets)
– when an AGN is viewed at a small angle, we see it as a
quasar.
– when the jet is aimed directly toward us, we see the AGN
as a blazar.
4. To test this theory we must detect radiation from
AGNs that isn’t blocked and isn’t affected by the
orientation of the dust torus surrounding the
accretion disk around the central black hole.
Far-infrared radiation fulfills this requirement.
5. In 2001, ESA’s Infrared Space Observatory showed
that very hot and luminous quasar cores are found
even in weak radio galaxies at large distances.
6. AGNs are not found in our neighborhood because
previous AGNs are the ancestors of today’s galaxies.
7. A census of many nearby galaxies suggests that
nearly all of them harbor supermassive black holes
that once powered quasars.
– The mass of the black hole is proportional to the mass of the
host galaxy, and the number and masses of the black holes
are consistent with what would have been required to power
the quasars.
8. It seems that galaxies have progressed from having
quasars or blazars at their centers, to Seyferts or
radio galaxies, to normal spiral or elliptical galaxies.