External Galaxies

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External Galaxies
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The Great Debate
In the early 1920s, two great astronomers,
Harlow Shapley and Heber Curtis had a great
debate about the nature of the spiral nebulae
Shapley
Curtis
Shapley believed that the spiral nebulae were
objects in our own Milky Way Galaxy, and thus
were small and nearby. Curtis believed that they
were island universes, each as large as our own
Milky Way, and thus large and far away.
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The Great Debate
This debate was resolved by Edwin Hubble who, in
1923, discovered Cepheid Variable stars in the
Andromeda
Nebula. He used the Period-Luminosity
relationship to determine the distance to
Andromeda, and thus showed that it was a true
external galaxy, of similar size to the Milky
Way. He was able to show that it was composed
of stars.
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The Classification of Galaxies
Hubble went on to devise a classification system
for external galaxies. There is a great variety
of galaxies, but most fall into three classes
elliptical galaxies, spiral galaxies and
irregular galaxies.
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Elliptical Galaxies
Elliptical galaxies range from Dwarf
Ellipticals 1/100 the size of the
Milky Way
To Giant Ellipticals, 5 10X the size of
the Milky Way
All elliptical galaxies are characterized by very
little gas and dust, and no new star formation.
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Spiral Galaxies

• Come in two types regular and barred. Both
  types
• are classified according to
• Size of central region
• Tightness of spiral arms

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• Spiral galaxies are characterized by
• Lots of Gas and Dust
• Current Star formation
• A Disk and Nuclear Bulge

The amount of gas and dust and the rate of star
formation increase in the sequence
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Star Formation in Spiral Galaxies
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The Hubble Galaxy Classification Scheme
The Tuning Fork Diagram
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S0 Galaxies
S0 galaxies show a disk nuclear bulge
structure, but very little gas and dust.
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Determining the Masses of Galaxies
The Rotation Curve Method
Using Newtons form of Keplers 3rd law, this
method will give the mass of the luminous part of
the galaxy. Typically, this method gives Mgalaxy
? few X 1011M?
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Determination of Galaxy Masses (continued)
The Double Galaxy Method
The Cluster Method
This method uses the Virial Theorem, which asks
Given
the velocities of the galaxies in the cluster,
what must the total mass of the cluster be in
order for it to be gravitationally bound?
Both methods measure the total mass of the
galaxy, and both give total masses of a few X
1012 M?. ? Missing Mass/Dark Matter Problem!
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Characterizing the Missing/Dark Matter
Astronomers speculate that at least some of the
Dark Matter is in the form of brown dwarfs, cool
white dwarfs and black holes floating freely in
our galaxy. If one of These objects passes
directly between us and a distant star, the
distant star will appear to momentarily brighten
because of the gravitational lensing effect.
Such events are called micro-lensing events.
The MACHO project (MAssive Compact Halo
Objects) was set up to try to detect these
micro-lensing events. Detection of just a few of
these events will allow us to determine the
proportion of the Dark Matter in the form of
brown dwarfs, white dwarfs and black holes.
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A Micro-lensing event in the direction of
the Bulge of our Galaxy.
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The preliminary conclusion from the MACHO
project (and other similar projects) is that
sub-luminous stars, like cool M-dwarfs, brown
dwarfs, freely-floating planets and cool white
dwarfs probably cannot account for more than 5
10 of the dark matter.
The rest of the dark matter must be in a more
bizarre form, perhaps in the form of sub-atomic
particles, such as neutrinos, or more exotic
particles.
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Determining Distances to Galaxies
The distances to the nearest galaxies can be
determined using the Cepheid Period-Luminosity
relationship. This method can be used out to a
distance of about 20Mpc.
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Example finding the distance to the LMC
m(av) 15.56 P 4.7 days
MV -2.81log(P) 1.43 gt MV -3.32
Mv mv 5log(d/10) log(d/10) (mv Mv)/5
18.88/5 3.78 d/10 5970pc gt d 59700pc
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Distances to Galaxies by the Bootstrap Method.
Beyond 20Mpc, we must use other means of
determining distances. We proceed in the
following way Using the 100 or so galaxies
within 20 Mpc, we can find and calibrate other
Standard Candles which can be used to even
greater distances.
Some of these standard candles include 1)
the most luminous stars in a galaxy, 2) the
brightest globular clusters in a galaxy 3)
Type Ia Supernovae. The sizes of the largest H
II regions (emission nebulae) in a galaxy can
also be used. For the most distant
galaxies, the brightest
galaxy in a cluster can be used.
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Distance Indicators
Globular Clusters
Largest H II Regions
Supernovae
Brightest Galaxy in cluster
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The Determination of Galaxy Distances

• Out to 20 Mpc Cepheid Period-Luminosity
  Relationship
• Out to 100 Mpc The Brightest Stars and Globular
  Clusters
• Out to 300 Mpc The sizes of the largest H II
  regions
• Out to 2500 Mpc Type Ia Supernovae
• To the Edge of the Observable Universe The
  Brightest
• galaxy in a cluster of galaxies.

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Velocities of Galaxies
Vesto Slipher, and later Edwin Hubble determined
that all galaxies, except for a few nearby
galaxies show redshifts in their spectra. This
implies that all these galaxies are moving away
from us. Hubble found that the velocities of
recession were proportional to the distances of
the galaxies. This is the famous Hubbles Law.
Where vr is the radial velocity of the galaxy, D
is the Distance in Megaparsecs, and H is Hubbles
Constant. The current value of Hubbles constant
is about 70 km/s/Mpc
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Hubbles Law implies that the entire Universe
is expanding in a uniform expansion. We will
examine this profound result in more detail when
we discuss cosmology.
Please note that this does not imply that the sun
is at the center of the Universe!
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Hubbles Law
The current accepted value of Hubbles constant,
H, is 70 km/s/Mpc. This number tells us the rate
of the expansion of the Universe.
As a consequence, the quantity 1/H gives us
information on the age of the Universe. The
value of 70 km/s/Mpc gives an age of about 14
billion years.
The value of Hubbles constant has been uncertain
for many years. For some decades, astronomers
were divided into two warring camps over the
value of H. Some supported 50 km/s/Mpc, others
100 km/s/Mpc.
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Clusters of Galaxies

• Most Galaxies are found in clusters. Clusters
  are
• classified as
• Poor Clusters Generally contain 100 or fewer
  galaxies
• The Local Group contains about 2 dozen
  galaxies,
• including the Milky Way, Andromeda,
  SMC, LMC,
• M33, a number of dwarf ellipticals,
  etc.
• Poor clusters generally contain a high
  proportion of
• spiral galaxies.

• Rich Clusters Can contain more than 1000
  galaxies.
• Generally contain mostly elliptical and S0
  galaxies, with
• few spiral galaxies. Most contain one or
  more Giant
• Elliptical Galaxies near their centers

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The Local Group
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Members of the Local Group
Milky Way
Andromeda, M32 NGC 205
M33
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More members of the Local Group
LMC
NGC 6822
NGC 185
SMC
Leo I
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Some other Poor Clusters
Fornax Cluster
Hickson Compact Group
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Some Rich Clusters
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Why are there different kinds of galaxies in Rich
and Poor clusters?
Elliptical galaxies are formed in rich clusters
where galaxy collisions are frequent. Galaxy
collisions lead to star formation, implying that
ellipticals formed their stars in one initial
burst.
Collapsing galaxies collide
Violent initial burst of star formation
Elliptical galaxies as we now see them
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• Spiral Galaxies, on the other hand, are found
  mostly in
• poor clusters where collisions were much less
  frequent.
• Star formation in spiral galaxies has been much
  more
• gradual, and still continues until today.

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Star Formation in Colliding Galaxies
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What about those mysterious S0 galaxies?
S0 galaxies are almost always found in rich
clusters.

• Rich clusters have a hot intergalactic medium
  (gas).
• The velocities of galaxies in rich clusters is
  higher than
• in poor clusters.

X-ray View of same cluster
Optical View of Hydra A Galaxy cluster
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Ram Pressure due to movement of the galaxy
through the intergalactic gas strips gas from a
spiral galaxy, converting it to an S0 galaxy.
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This image shows a galaxy in a rich cluster
being stripped of gas by ram pressure
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Superclusters
Even galaxy clusters are clustered together into
bigger Clusters called Superclusters.

• Superclusters take the form of sheets or bubbles
  with
• enormous voids in between.
• The entire Universe is like an enormous
  bubble-bath

Galaxies reside on the surface of the
bubbles with nothing inside (Voids).
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