PHYSICS 105
The Formation of Galaxies
Lucas Talavan-Becker
4/27/2011
Image 1. Bright knots of glowing gas light
up the arms of spiral galaxy Messier 74,
indicating a rich environment of star
formation.
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Image 2. Located in Hawaii near the
summit of Mauna Kea, these twin
telescopes which are part of the Keck
observatory are providing optical
spectroscopy of the faint Coma cluster
galaxies.
Graph 1. Due to dark matter’s strong presence in the
universe relative to stars and intergalactic gas, it plays an
integral part in the formation and existence of all galaxies.
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Introduction
Astronomers are still unsure as to how the galaxies
form because evidence is scarce and hard to obtain due
to the fact that early galaxies are billions of light years
away from us. However, the advent of stronger
telescopes is allowing us to observe galaxies formed
during the early years of the universe. For example, in
2007, the Keck Telescope, made by a team from the
California Institute of Technology found six stars that
dated back 13.2 billion years ago and therefore created
when the universe was only 500 million years old. Despite the universe’s enigmatic properties,
astronomers have agreed on a general idea of how galaxies formed and clustered into their present
states. But before we address how galaxies formed, we need to define what they are.
A galaxy is a massive
gravitationally bound system
composed of stars, star remnants,
gas dust, and an important but
poorly understood component
called dark matter (poorly
understood due to the fact that we
cannot observe anything beyond the
Schwarzschild radius and therefore the
collapse of a massive star which in
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Image 3. This is a picture of Zwicky
18 (lower left portion of the
image) a dwarf irregular galaxy
located 59 million light years away
in the constellation Ursa Major.
turn creates dark matter). The word galaxy comes from the Greek word galaxias, literally meaning
“milky”, which is why our galaxy is called the Milky Way. Galaxies range from dwarfs with as little as ten
million stars to giants with as many as one hundred trillion stars. Everything in a galaxy orbits about the
galaxies center of mass which is why dark matter is so important to the structure of a galaxy; dark
matter is extremely massive and the accretion of dark matter alters the location of the center of mass.
Galaxies may contain star systems, star clusters, and various interstellar clouds. The Sun is an example of
a star in the Milky Way galaxy and the Solar System, which includes the Earth and everything else that
orbits the Sun, is an example of a star system.
Historically, galaxies have been categorized by their shape.
A common form is an elliptical galaxy which has an ellipse-
shaped light profile. Another common galaxy is a spiral
galaxy. Galaxies with incoherent shapes are called
irregular galaxies. Usually irregular galaxies form by the
gravitational interactions of two or more galaxies. Such
interactions between galaxies may ultimately result in the
merging of galaxies which induces episodes of significant
star formation. Consequently, often times merging galaxies
are called starburst galaxies. Small, newly formed galaxies
that have not yet assumed either the spiral or elliptical
formation are called irregular galaxies as well.
There are probably more than one hundred and seventy billion galaxies in the observable universe.
Most galaxies range from one thousand to one hundred thousand parsecs in diameter. The distance
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Image 4. The prominent concentration
of galaxies running diagonally across
the northern (that is, upper) portion of
the image above has been termed the
Great Wall.
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between galaxies is on the order of millions of parsecs. Intergalactic space (the space between galaxies)
is made of extremely thin gas of an average density of one atom per cubic meter.
Galaxies usually form into clusters which make up super
clusters which are generally arranged into sheets or
filaments and surround immense voids in the universe.
The Great Wall is one of the largest known
superstructures in the universe. It is a massive cluster of
galaxies approximately two hundred million light years
away and its observable dimensions are five hundred
million light years long, three hundred million light years
wide, and fifteen million light years thick. It is not known
how much further the Great Wall extends because
intergalactic dust in the Milky Way obscures the view and
makes it impossible to see beyond what we know. Such structures like the Great Wall form along and
follow web-like strings of dark matter. It is hypothesized that dark matter dictates the structure of the
universe on the grandest scale.
Dark matter can account for ninety percent of all galaxies and usually, if not always, exists at the center
of galaxies. For example, the Milky Way is hypothesized to harbor a super massive black hole at its
center.
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Image 5. The image above is a newly formed
proto-galaxy.
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The evolution of galaxies
Edwin Hubble's observations and subsequent Hubble Law led to the idea that the universe is expanding.
We can estimate the age of the universe based on the rate of expansion. Because some galaxies are
billions of light years away from us, we can discern that they formed fairly soon after the Big Bang.
Right after the Big Bang, the universe was fairly
homogenous. That is, when the universe was
young, before the formation of stars and planets,
the cosmic microwave background radiation filled
the universe with a uniform glow from its white-
hot fog of hydrogen plasma. So how did the
universe change from its homogenous origins to
its clumpy heterogeneous form that we know of it
today? As the universe cooled, clumps of dark
matter began to condense and within that matter, gas condensed as well. The formation of galaxies can
largely be explained due to primordial fluctuations which are variations in density of the universe. The
higher density regions gravitationally attracted dark matter and gas, and therefore created the first
proto-galaxies. The helium and hydrogen in these clusters began to condense and form stars; thus the
first galaxies were formed. This explains how galaxies formed, but how do we explain the distribution of
galaxies about the universe?
When the universe was young, galaxies formed quickly, evolving and expanding by the accretion of
smaller galaxies. Galaxies aren’t uniformly distributed about the universe but rather distributed in a
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Figure 1. Here is a summary of star
formation.
great cosmic web of filaments, and where these filaments meet are dense clusters of galaxies that
began as the small density fluctuations to the universe.
Star Formation
Stars make up an integral part of all galaxies, so it’s only appropriate that we address how stars form. A
star is formed out of a cloud of cool, dense molecular gas. In order for it to become a potential star, the
cloud needs to collapse and increase in density. There are two common ways this can happen: it can
either collide with another dense molecular cloud or it can be near enough to encounter the pressure
caused by a giant supernova. Several stars can be born at once with the collision of two galaxies. In both
cases, heat is needed to fuel this reaction, which comes from the mutual gravity pulling all the material
inward.
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Image 6. This Hubble image of
the Antennae galaxies is the
sharpest yet of this merging pair
of galaxies. As the two galaxies
smash together, billions of stars
are born, mostly in groups and
clusters of stars. The brightest
and most compact of these are
called super star clusters.
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What happens next is dependent upon the size of the newborn star; called a protostar. Very small
protostars will usually not have high enough temperatures to perpetuate hydrogen burning necessary to
maintain hydrostatic equilibrium in a star. The small protostar will cool slowly over billions of years to
become the background temperature of the universe.
Medium to large protostars can take one of two paths depending upon their size: if they are smaller
than the sun, they undergo a proton-proton chain reaction to convert hydrogen to helium. If they are
larger than the sun, they undergo a carbon-nitrogen-oxygen cycle to convert hydrogen to helium. The
difference is the amount of heat involved. The CNO cycle happens at a much, much higher temperature
than the PP chain cycle. Whatever the route, a new star has formed.
How galaxies interact and
what forms from these
interactions
Galaxies do not act alone. The
distances between galaxies do
seem large, but the diameters
of galaxies are also large.
Compared to stars, galaxies are relatively close to one another. They can interact and, more importantly,
collide. When galaxies collide, they actually pass through one another, however, the stars inside don't
run into one another because of the enormous interstellar distances. But collisions do tend to distort a
galaxy's shape. Gravitational interactions between colliding galaxies could cause new waves of star
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Figure 2. Summary of spiral galaxy formation.
formation, supernovae, and or stellar collapses that form the black holes or supermassive black holes in
active galaxies.
Three main types of galaxies
Spiral Galaxies
There are three main types of
galaxies: disk galaxies, which are
also commonly called spiral
galaxies, elliptical galaxies and
lenticular galaxies. Disk galaxies
have a thin, rapidly rotating spiral
structure. The formation of disk
galaxies is still unclear but early
scientists hypothesized that the
collapse of a monolithic gas cloud
triggers the formation of disk
galaxies. As the gas cloud
collapses, the gas settles into a
rapidly rotating disk. However,
some astronomers claim that all processes in the universe occur bottom-up which is defined as smaller
parts grouping to bigger parts, rather than top to bottom.
One bottom-up galaxy formation hypothesis involves the interactions of matter that composed the early
universe, dark matter, and gas. Dark matter halos and gas formed the early galaxies, and as smaller
galaxies accreted with larger galaxies, the dark matter stayed mostly on the outer parts of the galaxy
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Image 7. The above image of Elliptical
Galaxy M87 was taken recently by the
Canada-France-Hawaii Telescope on top of
the dormant volcano Mauna Kea in
Hawaii, USA.
because dark matter only reacts gravitationally and cannot dissipate. The gas, however, contracts, and
as it does so it rotates faster until it forms a spirally disk.
Astronomers still do not know what stops the contraction. Some hypothesize that radiation from newly
formed stars or active galactic nuclei halts the contraction. Others believe that the dark matter can pull
the galaxy and thus stop disk contraction. Regardless, believers of the disk contraction process still
cannot correctly predict the speed at which the disk rotates or the size of the galaxy.
Elliptical galaxies
An elliptical galaxy is a galaxy that has an ellipsoid
shape and a smooth, nearly featureless brightness
profile. Elliptical galaxies range in shape from nearly
spherical to almost flat and can have as few as a
hundred million to as many as a trillion stars. An
elliptical galaxy is often the result of two galaxies colliding
and merging together. Most elliptical galaxies are
composed of older, low-mass stars with sparse
interstellar medium (the space between star systems in a
galaxy) and minimal star formation activity. They are surrounded by large numbers of globular clusters,
which are tightly bound groups of stars that orbit a galactic core as satellites. Elliptical galaxies make up
approximately ten to fifteen percent of the local universe but are by no means the dominant type of
galaxy in the overall universe. They are typically found near the centers of galaxy clusters and are less
common in the early universe.
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Elliptical galaxies have several properties that make them distinct from other classes of galaxies. Stars in
elliptical galaxies orbit about a central point in the galaxy whereas the motion of stars in spiral galaxies is
characterized by revolutions. Because star formation is minimal due to the lack of gas, dust, and space,
elliptical galaxies get their glow and color from aging stars. Therefore, they tend to be yellow-red, which
contrasts with the white, blue color of spiral galaxies which are much more conducive to star formation
activity and have hotter, youngers stars that radiate brighter colors.
There is a wide range in size and mass for elliptical galaxies: some are as small in diameter as a tenth of a
kilo parsec while others as big in diameter as one thousand kilo parsecs. Some small elliptical galaxies
are as big as globular clusters but contain a considerable amount of dark matter at their centers
differentiating them from globular clusters.
There are two main types of elliptical galaxies: the boxy giant elliptical galaxies which get their shape
from the random motion of stars and the “disc-like” low luminosity galaxies that are also characterized
by the random motion of stars but are flattened due to rotation.
Dwarf elliptical galaxies have properties that are intermediate between globular clusters and normal
elliptical galaxies. Dwarf spheroid galaxies are similar in shape and composition to dwarf elliptical
galaxies but generally have lower luminosity and are recognized only as satellite galaxies.
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Image 8. This is Hubble image of an actual
sideswipe of galaxies called The Mice.
The formation of an elliptical galaxy is thought to occur
due to the collision and merging of two galaxies. These
major galactic mergers were thought to occur more
frequently in the early universe. Nonetheless, minor
galactic mergers continue to occur often. In fact, our very
own Milky Galaxy is ingesting smaller galaxies right now.
Furthermore, our Milky Way galaxy is on collision course
with the Andromeda galaxy. This collision is expected to take
place in three to four billion years and the result of the
collision of the two spiral galaxies will most likely be an elliptical galaxy.
Every bright elliptical galaxy is believed to contain a super massive black hole at its center which limits
star formation and in turn the growth of elliptical galaxies.
Lenticular galaxies
Lenticular galaxies are an intermediate between elliptical galaxies and spiral galaxies. Lenticular galaxies
have a disk-like shape similar to spiral galaxies but have
lost or consumed most of their interstellar matter
essential to star formation. Therefore, they are similar to
elliptical galaxies in the sense that they’re mostly
comprised of aging stars and because of their ill-defined
spiral arms, when inclined face-on, it is often difficult to
distinguish between them and elliptical galaxies.
Image 9. NGC 5866 is a lenticular
galaxy discovered by Charles Messier
in 1781.
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There are two hypotheses that describe how lenticular galaxies form. One is that lenticular galaxies are
the remnants of faded spiral galaxies whose spiral arms disappeared. Another hypothesis is that
lenticular galaxies are the result of galaxies merging. The faded spiral galaxy hypothesis is supported by
the fact that lenticular galaxies are characterized by the following properties: the absence of gas, the
presence of dust, the lack of star formation rotational support, which are all attributes one might expect
for a spiral galaxy that has consumed all of its interstellar matter. This hypothesis is also enhanced by
the existence of gas poor or “anemic” spiral galaxies. If the spiral pattern disappeared, the resulting
galaxy would be very similar to many lenticular galaxies.
The merging hypothesis is supported by the fact that lenticular galaxies are more luminous than spiral
galaxies and have higher bulge-to-disk ratios than spiral galaxies. Merging galaxies would form a galaxy
with increased stellar matter and therefore more stars to increase luminosity. Furthermore, a merger
would explain the spiral, arm-less structure of lenticular galaxies.
Galaxy Morphological Classification
Galaxy morphological classification is a system used by astronomers to divide galaxies into groups based
on their visual appearance. There are several schemes in use by which galaxies can be classified
according to their morphologies.
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Figure 3. Summary of Hubble’s classification scheme.
The Hubble sequence is a
morphological classification scheme
for galaxies invented by Edwin Hubble
in 1936. Hubble’s scheme divides
galaxies into 3 broad classes based on
their visual appearance. These broad
classes can be extended to enable
finer distinctions of appearance and to
encompass other types of galaxy, such as
irregular galaxies, which have no obvious regular structure either disk-like or ellipsoidal.
The Hubble sequence is often represented in the form of a two-pronged fork as shown above, with the
ellipticals on the left with the degree of ellipticity increasing from left to right and the barred and
unbarred spirals forming the two parallel prongs of the fork. Lenticular galaxies are placed between the
ellipticals and the spirals, at the point where the two prongs meet the handle.
To this day, the Hubble sequence is the most commonly used system for classifying galaxies.
The de Vaucouleurs system for classifying galaxies is a widely used extension to the Hubble sequence.
De Vaucouleurs argued that Hubble's two-dimensional classification of spiral galaxies based on the
tightness of the spiral arms and the presence or absence of a bar did not adequately describe the full
range of observed galaxy morphologies. In particular, he argued that rings and lenses were important
structural components of spiral galaxies.
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Which came first: the Black Hole or the Galaxy?
Do galaxies form first and then a black hole springs up in the center, or possibly, do galaxies form around
an already existing black hole?
Previous studies of galaxies and their central black holes in the nearby Universe revealed an intriguing
connection between the masses of the black holes and of the central “bulges” of stars and gas in the
galaxies. For central black holes from a few million to many billions of times the mass of our Sun, the
black hole’s mass is about one one-thousandth of the mass of the surrounding galactic bulge. This
constant ratio indicates that the black hole and the bulge affect each other’s growth in some sort of
interactive relationship. The big question has been whether one grows before the other or if they grow
together, maintaining their mass ratio throughout the entire process.
We finally have been able to measure black-hole and bulge masses in several galaxies seen as they were
in the first billion years after the Big Bang, and the evidence suggests that the constant ratio seen nearby
may not hold in the early Universe. The black holes in these young galaxies are much more massive
compared to the bulges than those seen in the nearby Universe. The implication is that the black holes
started growing first.
Conclusion
Less than a century ago astronomers knew only about our own galaxy, the Milky Way, which they
believed held about 100 million stars. Then observers discovered that some of the fuzzy blobs in the sky
weren't in our own galaxy, but were galaxies in their own right—collections of stars, gas, and dust bound
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together by gravity. Today we know that the Milky Way contains more than 100 billion stars and that
there are some 100 billion galaxies in the universe, each harboring an enormous number of stars. Our
view of the universe is changing completely with the introduction of stronger telescopes, new
technology, and a deeper understanding of the fundamental laws of the universe.
Hopefully future discoveries can clarify or unravel some of the enigmatic properties of the universe.
Dark matter, for example, fundamentally determines the structure of the universe but is poorly
understood due to the tentative fact that we cannot observe it. Getting a better understanding of dark
matter may lead to an understanding of how galaxies are distributed about the universe. The apparent
incomprehensibility of the universe may at first appear negative, but the beauty is that there’s always
something new to discover and therefore an infinite amount of knowledge to gain.
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