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Chapter 18
Our Expanding Universe
Chapter 18 makes the “grand leap” to the universe as a
whole, and introduces ideas about other galaxies that
are less than a century old. The existence of other
galaxies of stars was debated without resolution in 1920,
but is now common fact.
1. The nature of the universe is revealed by galaxy
redshifts and the 3K microwave background.
2. At present the term “redshift” is synonymous with
“distance,” although there is still some debate in lesser
quarters.
Until after the Great Debate of 1920 between Harlow
Shapley and Heber Curtis into the nature of the spiral
nebulae, it was not obvious that there were other galaxies
beyond the Milky Way. Shapley, the “winner” of the
debate, actually argued that spiral nebulae were part of
our Galaxy. Curtis, the loser ultimately proven correct,
argued that the spiral nebulae were other galaxies lying
beyond the borders of the Milky Way.
In 1920 the best views of “spiral nebulae” were ambiguous
about whether or not they contained stars.
This is one of the best “pictures” of our Galaxy from the
Sun’s location as sketched by Sergei Gaposhkin from
Australia (1957),. The lower view is Sergei’s attempt to
step outwards by 1000 parsecs from the Sun.
Bulge
Disk
Since the Sun is located just above the central line
of the main disk of the Galaxy, only lines of sight
upwards from the main disk give the best views of
the surrounding universe of galaxies.
The rectangular area below lies upwards from the
Galactic plane, thereby sampling the universe.
A close-up of the region of sky where the Hubble
Space Telescope Ultra Deep Field image lies.
Every “fuzzy”
object in this
image is a
distant galaxy.
Foreground
stars in our
own Galaxy
have
associated
diffraction
spikes because
they are pointlike images.
Star
Schematics of the meanings for “homogeneous”
and “isotropic.”
Bubbles, Voids, and Strands.
Surveys of the spatial distribution of galaxies indicate
that they are not as “homogeneous” as sometimes
thought. The location and brightness of galaxies in this
view indicate the patchy nature of their distribution.
Distance surveys in
selected bands of
sky further
accentuate the
patchy nature of
the distribution of
galaxies, which is
marked by bubbles
containing very few
galaxies
surrounded by
denser strands rich
in clusters and
superclusters of
galaxies.
The CfA survey (with “stickman”) of Huchra and Geller.
The CfA survey of Huchra and Geller.
The 2dF Galaxy Redshift Survey. The largest voids
measure 100 Mpc across.
How are distances to galaxies determined? Distances to
nearby solar system objects are measured by radar,
nearby stars by parallax, then cluster main sequence
fitting, the Cepheid PL relation, and finally Type Ia
supernovae.
The standard candles used to measure distances
to galaxies are:
For “nearby” galaxies, Cepheid variables, since
their periods of pulsation correlate with their
intrinsic luminosities (period-luminosity relation).
(similar to using the “watts” label on a light bulb
to determine how bright it is)
For distant galaxies, Type Ia supernovae, since
their peak brightness is always the same. They
are also the brightest class of supernovae.
(similar to, say, all supernovae appearing as
bright as a “150 watt” light bulb)
Determining distances
to galaxies using
Cepheid variables.
Step 1. Identify a
Cepheid from its
changes in brightness
with time.
Step 2. Measure its
mean brightness and
period of variability.
Step 3. Use the Cepheid
period-luminosity
relation to establish its
intrinsic brightness and
calculate its distance.
Lastly, there is a relationship between a galaxy’s redshift
and its distance. Cluster elliptical galaxies of different
redshift, and how they correlate with distance for an
outdated Hubble constant of 50 km/s/Mpc.
Note how size and redward shift of the spectral
lines correlate exactly for bright ellipticals.
Redshift-distance relation.
Conclusion:
We live in an
expanding
universe.
A baking raisin bread analogy is often used to picture an
expanding universe. It does not matter which raisin
represents the Sun. From every raisin the nearest raisins
all appear to increase their distances with time, at a rate
proportional to the distance from the reference raisin.
The meaning of “redshift.” Spectral features are shifted
to longer wavelengths according to a simple formula:
(λobserved – λrest)/λrest = velocity/c.
The spectrum is “stretched” to the red, not simply
shifted. There is no shift for λ = 0.
The Hubble Law is not a peculiarity of our local
galactic neighbourhood, but reflects an actual
expansion of space, much like the expanding
raisin bread, referred to as the “Hubble flow.”
Velocities and distances are often found with the
standard Doppler equation:

obs  rest 
v
cz
rest
and
cz
d
H0
For large velocities (approaching c), the
relativistic velocity and distance equations
become:
2
 1  z 2  1 

c 1  z   1 
and d 
v
c


2
2
H 0  1  z   1
 1  z   1
For reference purposes only.
The simple formula z = v/c breaks down as speeds
approach the velocity of light, and the relativistic
version must be used.
How observed redshift z correlates with recession velocity v
when the relativistic formula is used. Note that z = 5 does
not mean a galaxy is moving at 5 times the speed of light. It
is actually moving at 95% of the speed of light.
Redshift-distance relation (again).
Slope =
Rise/Run
= H0
The inverse of the Hubble constant (which is the
slope of the Hubble relation) has units of time and
is called the Hubble Time. It is an estimate of the
age of the universe (backwards extrapolation),
provided that the expansion began at some point
in the past and has been continuing at the same
rate ever since.
1
978 billion seconds

 13.8 billion years
H0
H 0 ( km/s/Mpc )
For H0 = 71 km/s/Mpc.
The textbook uses H0 = 22 km/s/Mly (the same).
The Milky Way’s globular clusters are all less than
14 billion years old.
Penzias and Wilson with the radio horn used to
discover the 3K microwave background radiation.
The 3K background revealed by the COBE satellite,
displays a “Doppler shift” attributed to mass asymmetry
in the early universe when matter separated from
radiation.
The “Doppler effect” seen in COBE
measurements of the 3K background.
The 3K microwave background with the Doppler
shift removed, as recorded by WMAP.
The 3K microwave background matches the radiation
from a black body with T = 2.728 K.
The 2.728 K background is the constant faint glow from
the universe when T = 3000 K, now redshifted by z ≈ 1000.
Conclusion:
We live in an
expanding universe
that has a
background glow.
Next Step?
Big Bang
cosmology?
Predicted versus
Observed
element
abundances for a
Big Bang Model
Origin of
the 3K
microwave
background
How the “scale factor”
changes with time
The temporal development of the universe according to the
Big Bang model (logarithmic units).
Astronomical Terminology
Shapley-Curtis Debate. A 1920 debate in Washington
about the nature of the spiral nebulae.
Hubble Law. The relationship correlating increasing
distance with larger recession velocity for galaxies.
Hubble Constant. The slope of the Hubble relation: H0 =
71 km/s/Mpc = 22 km/s/Mly.
Hubble Time. The inverse slope of the Hubble relation,
yielding an estimate of 13.8 billion years for the
age of the universe.
Relativistic Doppler Shift. How redshift z = Δλ/λ is
converted to recession speed v for velocities near c.
3K Microwave Background. The faint glow from the
entire sky (black body T = 2.728 K) attributed to
the recombination era following the Big Bang.
Big Bang. Fred Hoyle’s derogative term for the supposed
origin of the universe in an explosive fireball 14
billion years ago.
Astronomical Terminology (continued)
Homogeneity. A term describing the similar appearance
of the universe in all directions for all observers.
Isotropy. A term describing how the universe appears to
be roughly the same no matter in which direction
one looks.
Distance Ladder. A term describing the various methods
by which astronomers determine distances to
objects in space, beginning with the closest and
ending with the most distant.
Scale Factor. The constant of proportionality describing
the expansion of the universe, namely the
separation of galaxies in the past in terms of their
present separations.
Light Element Production. A model of the early universe
describing the production of various isotopes of
hydrogen (1H and 2H, deuterium), helium (3He and
4He), and lithium (7Li) in the Big Bang.
Sample Questions
13. What are peculiar velocities, and how
do they affect our ability to measure H0?
Answer. Peculiar velocities are speeds that
galaxies exhibit in addition to those
expected from the general expansion of the
universe. They result from the gravitational
attraction of galaxies to each other over
large scales, and need to be taken into
account when determining how a galaxy is
moving relative to us in the absence of local
gravitational effects.
21. What is the origin of the cosmic
background radiation (CBR)?
Answer. It is the leftover energy from the
early universe. At early times the universe
is assumed to have been very hot, but a few
hundred thousand years after the Big Bang
the fireball had cooled considerably,
although still at high temperatures. When
the fireball had cooled to T ≈ 3000 K it
became transparent to energy. The thermal
energy of the hot universe then escaped into
space as black body radiation and is seen
now as the highly-redshifted light from that
early epoch.
Sun: March 9, 2011 from Atrium
Chapter 19
Galaxies
Now that the grand nature of the universe has been
described, Chapter 19 steps backwards to look into the
nature of individual galaxies and their environs.
1. The main galaxy types are described, pointing out
how they differ in terms of the relative frequency of old
and young stars, and interstellar gas, within them.
2. Imaging of galaxies and galaxy clusters reveals that it
is a “violent” universe. Evidence for collisions between
galaxies is everywhere, although keep in mind that by
“collisions” we are talking about events that take place
over billions of years, not mere seconds or minutes.
The Hubble Sequence:
The familiar “tuning fork” diagram developed by Edwin
Hubble is an attempt to link the main galaxy types of
elliptical (E), spiral (S), barred spiral (SB), and irregular
(Ir) classes. The lenticular galaxies (S0, SB0), added later,
were a supposed link between spheroidal E galaxies and
flattened S and SB galaxies, but unfortunately the
diagram was also pictured as an evolutionary sequence.
Thus, elliptical galaxies are often referred to as “earlytype” galaxies, much like “early-type” OB stars.
Elliptical galaxies contain old low-mass stars/no gas or
dust and are denoted E0, = circular to E6 = elongated.
Sample: IC 4296, E0, NGC 777, E1, NGC 1549, E2,
NGC 4365, E3, NGC 4564, E6, NGC 4623, E7.
Some are actually lenticulars, S0.
Ellipticals are not all the same shape galaxy seen at
various angles of projection. Many seem to be oblate (top)
or prolate (bottom) spheroids viewed at various angles.
Early examples of lenticular = lens-like galaxies.
List: NGC 3245, S0, NGC 4251, S0, NGC 4179, S0:,
NGC 5422, S0:, NGC 3203, S0:, NGC 4429, S0/Sa.
Lenticular galaxies get their name because they are “lenslike,” but are more like spiral and barred spiral galaxies
in possessing a flattened disk, rather than like elliptical
galaxies which mostly possess an ellipsoidal symmetry.
The distinguishing feature of lenticulars relative to spiral
galaxies is that they do not have spiral arms or dust. For
example, lenticular galaxies seen edge-on do not have
dust lanes.
van den Bergh suggested a possible origin for lenticular
galaxies (S0s) via collisions between spirals. That results
in the dust and gas being swept out of the galaxies by ram
pressure. Because they have been swept clean of material
for making new stars, lenticular galaxies contain only old
stars, like elliptical galaxies, and lack the distinguishing
features of spirals. The fact that most lenticulars are
found in rich clusters of galaxies supports the idea.
Hubble Atlas examples of “early-type” spiral galaxies.
NGC 3269, Sa, NGC 7096, Sa, NGC 1350, Sa,
NGC 1371, Sa, NGC 488, Sab, NGC 2460, Sab.
Hubble Atlas examples of “late-type” spiral galaxies.
NGC 1566, Sbc, NGC 5247, Sc, NGC 2997, Sc,
NGC 3184, Sc, NGC 3938, Sc, NGC 5055, Sbc.
The effect of inclination to the line of sight on the
appearance of spiral galaxies. Note how dust lanes show
up for edge-on spirals.
How to get trailing
spiral arms from any
elongated structure in
a disk-dominated
galaxy: differential
rotation in the disk
naturally stretches
such features into
spiral arms.
Note that 2-armed
spirals appear to be
most common.
The rotation of stars in the disks of spiral galaxies is
expected to follow standard orbital speeds for
gravitational influences, namely declining speeds with
distance from the galactic centre, but all spirals display
“flat” rotation curves instead.
Spiral galaxies are sometimes classified on the basis of the
length of the spiral arms and their degree of “tightness.”
However, a more universal criterion is the relative size of
the central bulge, which is always largest in the Sa
galaxies and smallest at Sc. van den Bergh suggested
using disk to bulge ratio, D/B, as a good indicator, with Sa
galaxies characterized by D/B ≤ 3, Sb galaxies by 3 ≤ D/B
≤ 10, and Sc galaxies by D/B ≥ 10.
Certainly, the dimensions of the bulge are the important
characteristic of spiral galaxies, and bulges containing old
low-mass stars account for much of their mass. The
length of the spiral arms, where young stars and gas/dust
are found, is another feature that appears to depend upon
the dimensions of the disk. Again, van den Bergh suggests
that this characteristic can be used as a luminosity
indicator for spirals. See van den Bergh 1960, ApJ, 131,
215 & 558.
Early examples of barred lenticular galaxies. Here a
central bar of old stars replaces the central bulge.
NGC 3384, SB0:, NGC 5473, SB0:, NGC 1574, SB0,
NGC 4340, RSB0, NGC 2859, RSB0, NGC 3945, RSB0.
van den Bergh (1960) developed a luminosity
classification scheme for galaxies using members
of clusters of galaxies, a scheme that distinguishes
galaxies in the same fashion as stars: i.e., I, II, III,
IV, V, corresponding to supergiant galaxies, bright
giant galaxies, giant galaxies, subgiant galaxies,
and dwarf galaxies.
van den Bergh applied the scheme to late-type
spirals and barred spirals, as well as irregular
galaxies, but could not apply it to elliptical
galaxies. The types applying to well-known
objects: M31 (Sb I-II), LMC (SBm III), SMC (Im
IV-V), M33 (Sc II-III), are listed in the Observer’s
Handbook. The Milky Way is a supergiant galaxy
by this criterion.
Some of van den Bergh’s classifications for Sc and SBb
galaxies.
The Large Magellanic Cloud (SBm III = giant barred
spiral) from UK Schmidt plates. An irregular galaxy that
is an incipient barred spiral in the making?
The Small Magellanic Cloud (Im IV-V = irregular dwarf)
from UK Schmidt plates. An “inverse C” shape.
The Andromeda Galaxy NGC 224 (M31, Sb I-II =
supergiant spiral with large bulge). The companions are
NGC 205 (M110, S0/E5pec) and NGC 221 (M32, E2).
M32
M110
The Triangulum Galaxy NGC 598 (M33, Sc II-III = giant
spiral with small bulge). Note the small bulge and
restricted length of the spiral arms.
The Sculptor dwarf spheroidal galaxy (dE) contains only
old low-mass stars and no gas or dust.
The Leo II dwarf spheroidal galaxy (dE or dSph).
van den Bergh’s classification scheme for galaxies (1976).
An alternate schematic for van den Bergh’s scheme
linking the a, b, c types to Disk/Bulge ratio.
NGC 4921 in the Coma cluster is an example of what van
den Bergh refers to as an anemic spiral galaxy, since it is
depleted in young high-mass stars, i.e. “anemic” looking.
Luminosity Differences Among E Galaxies.
The luminosity differences between elliptical galaxies is
tied to overall dimensions:
cD galaxies. Huge elliptical galaxies dominating some
clusters of galaxies. “cD” stands for “cluster dominating”
galaxy. Some are incredibly large, massive, and luminous.
Normal ellipticals. Standard E galaxies comparable in
luminosity to supergiant spiral galaxies.
Dwarf ellipticals, dE. Lower luminosity elliptical galaxies
comparable in absolute magnitude to giant and subgiant
spiral galaxies.
Dwarf spheroidal galaxies, dSph. Very low luminosity
elliptical galaxies, like rich globular clusters, found in
nearby regions of the Local Group.
Blue compact dwarf galaxies, BCD. Compact elliptical
galaxies that contain lots of gas and massive blue stars.
Gas and Dust Properties of Galaxies.
Gas and dust content increases towards “later-type”
galaxies.
Likewise, the content of young stars also increases
monotonically towards “later” types of galaxies. The
exceptions are interesting objects in their own right.
Hickson Compact Group 87. How would you classify the
galaxies?
Examples of interactions between galaxies.
Evidence for Interactions of Galaxies:
The centre of the Coma cluster of galaxies, a rich cluster.
The distribution of elliptical galaxies (filled circles) in the
Coma cluster relative to spirals (open circles) peaks
towards the centre of the cluster, i.e. galaxy collisions
produce elliptical, lenticular, or anemic spiral galaxies.
The Coma cluster of galaxies as viewed in X-rays, i.e. gas
has been swept out of the galaxies through collisions.
The lower density Hercules cluster, on the other hand,
appears to be more heavily populated by spiral galaxies
in its central regions, i.e. few collisions.
The Hercules cluster of galaxies, in X-rays, has less gas.
The Whirlpool Galaxy (M51) and its companion (M52).
The Whirlpool Galaxy and its companion modeled by
Toomre & Toomre (1972) as a collision between galaxies.
The Antennae (NGC 4038/39), optical image.
The Antennae as modeled by Toomre & Toomre (1972) as
a collision between two galaxies.
The ring galaxy II Hz 4, optical images.
The ring galaxy II Hz 4 modeled by Lynds & Toomre
(1976) as a head-on collision between galaxies.
The polar ring galaxy NGC 4650A. Is it a collision in
progress between two galaxies?
Centaurus A. Dust lanes do not belong in ellipticals.
Starburst Galaxies.
These are a group of strongly interacting galaxies that are
bluer in colour than isolated galaxies, presumably
because of the presence of recently-created hot young
stars. It is believed that a tidal interaction with another
galaxy has induced star formation, although the resulting
excess luminosity is hidden behind obscuring clouds of
gas and dust. Such galaxies are bright at infrared
wavelengths, however.
Although starburst activity was initially discovered in
galaxy nuclei, some spiral galaxies also exhibit disk-wide
starburst activity. The star formation is assumed to have
been induced by shock waves generated by a
gravitational interaction.
Starburst galaxies often exhibit strong X-ray emission.
Does it originate from gas falling into a deep potential
well at the centre of the galaxy, perhaps a “black hole”?
The starburst galaxy M82, which interacts with M81.
Star formation appears to have been induced in M82 as a
result of its near encounter with M81. The gas and dust
clouds in the galaxy must have been induced to contract
and form stars by the gravitational interaction with M81.
The rich galaxy cluster Abell 2199 and the multiple
nucleus (cannibalistic?) cD galaxy near its centre.
The curious elliptical galaxy NGC 3923 and the multiple
concentric rings (left, gas?) that surround it.
The butterfly galaxy NGC 6240 as viewed by the Hubble
Space Telescope (left) and by the Chandra Orbiting XRay Observatory (right). What are the multiple strong
sources of X-rays near the galaxy’s centre? How were the
surrounding streams of gas produced?
There is an entire class of galaxies that appears to have
something peculiar about their nuclei. Some are called
Seyfert galaxies, after their discoverer, others are referred
to as AGN = Active Galactic Nuclei, others still display
oppositely-directed radio jets.
Quasars are considered to be extreme examples of the
phenomenon: star-like objects with very large redshifts
representing distant galaxies with their jets directed
towards us, so that the light from the object is dominated
by material in the jet.
Yet even nearby galaxies, including our own Milky Way,
display peculiarities. Most massive galaxies appear to
contain a supermassive “black hole” at their centre, a
place where hundreds of thousands to millions of solar
masses (M) of matter is squeezed into a volume of space
only a few parsecs or light years across.
Examples of (left) quasars = quasi-stellar radio sources
(i.e., lots of radio noise) and (right) QSOs = quasi-stellar
objects (very little radio noise, if any).
Double QSO
0957+561
Many radio
galaxies have jets of
high-speed gas
ejected
symmetrically
about the centre of
the galaxy, well
away from the
optical galaxy.
Many such galaxies
are referred to as
having active
galactic nuclei, and
are termed AGN
galaxies.
Apparent motion of
some galactic jets at
speeds exceeding the
speed of light are
projection effects
only. The jets also
have motion in the
line of sight that
produces such an
anomaly.
Can you now see the evidence for galaxy collisions?
Astronomical Terminology
Elliptical galaxy (E). A spheroidal galaxy containing
millions to billions of old low-mass stars and no
gas or dust.
Spiral Galaxy (S). A galaxy with a spheroidal bulge of
several million old low-mass stars and a flattened
pancake-like disk of billions of old low-mass and
young high-mass stars, along with gas or dust,
dominated by two (or more) spiral arms.
Irregular galaxy (Ir). A galaxy containing millions to
billions of young and old high-mass and low-mass
stars and lots of gas and dust.
Lenticular galaxy (S0). A disk galaxy of billions of old
low-mass stars and no gas or dust with the
appearance of a lens when viewed edge-on.
Barred spiral galaxy (SB). A spiral galaxy where the
central bulge is an elongated bar structure, from
the ends of which the spiral arms originate.
Astronomical Terminology (continued)
Anemic Spiral galaxy (A). A spiral galaxy with anemiclooking spiral arms, as if much of the gas and dust
in the disk used to create new stars has been swept
out of the galaxy.
Ring galaxy. An unusual galaxy with a ring-like
appearance that appears to be the dynamical
consequence of a head-on collision between two
normal galaxies.
Tidal tails. Elongated streamers of stars and gas from
normal galaxies that appear to have been
generated through gravitational encounters
between galaxies.
Starburst galaxy. A galaxy that is dominated by lots of
recently-formed young blue stars.
Rich galaxy cluster. A cluster of thousands of galaxies
dominated by ellipticals (E) and lenticular (S0)
galaxies, i.e. galaxies devoid of interstellar matter.
Astronomical Terminology (continued)
Quasar. A star-like object superposed on a distant
galaxy, discovered by its strong radio emission,
blue colour, and high redshift emission lines.
QSO = quasi-stellar object. A quasar, i.e. high redshift
star-like galaxy of blue colour, lacking detectable
radio emission.
Galaxy jets. Streams of oppositely directed high-speed
gas from active galaxies ejected from the accretion
disks surrounding supermassive black holes at
their centres.
Supermassive black hole. The term used to describe the
central regions of most large galaxies, where
millions of solar masses of matter occupy a very
small volume only a few parsecs across.
Superluminal motion. A term used to describe the
apparent outwards motion of matter in galaxy jets
in excess of the speed of light.
Sample Questions
8. Some galaxies have regions that are
relatively blue in colour, while other
regions appear redder. Aside from colour,
what can you say about the differences
between these regions?
Answer. The luminosity of the blue regions
is dominated by young blue stars, which
implies that they are regions of active star
formation that must contain significant
amounts of gas and dust. The luminosity of
the red regions, on the other hand, is
dominated by cool red giant stars, which
implies that they are regions of old stars
lacking active star formation, although gas
and dust may or may not be present.
20. The nearest quasar is almost a billion
light years distant according to its redshift.
Why do we not see any closer quasars?
Answer. If their redshifts are representative
of Hubble flow in the universe, then
quasars must be associated with the early
stages of galaxy formation. Since far-away
galaxies represent an earlier epoch in the
universe than nearby galaxies, we need to
look at very distant galaxies to find objects
representing earlier stages of galaxy
evolution.