Chapter 25: Galaxies

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Chapter 25 Galaxies

• What are galaxies?
• How are they distributed in space?

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Discovery of other Galaxies

• Using large telescopes one can see clouds of dust
  and gas inside the Galaxy.
• One can also see other peculiar milky nebulae
  scattered among the stars.
• Some of these milky nebulae have spiral shapes
• Others look like squashed spheres or tortured
  messes of material.
• Three of the milky nebulae are visible as fuzzy
  patches to the naked eye
• one is in the constellation Andromeda
• two others (called the Large and Small Magellanic
  Clouds after the first European explorer to see
  them, Ferdinand Magellan) are in the southern sky
  in the constellations Mensa and Hydrus.

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Galaxies Discovery Pioneers

• Work by
• Edwin Hubble (lived 1889--1953)
• Milton Humason (lived 1891--1972)
• in the 1930's established that each of the spiral
  nebulae was another huge star system, called a
  galaxy
• Galaxy is from the Greek galactos'', meaning
  milk.
• Hubble and Humason used large high resolution
  telescopes to measure the distances to the
  galaxies.

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Other Galaxies

• Galaxies are
• organized systems
• thousands to hundreds of thousands of light years
  across
• made of tens of millions to trillions of stars
• sometimes mixed with gas and dust all held
  together by their mutual gravity.

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Star Count in a Galaxy

• One gets an estimate of the number of stars in a
  galaxy by dividing the total luminosity of the
  galaxy by a typical star's luminosity.
• A more accurate value would result if you use the
  galaxy's luminosity function (a table of the
  proportion of stars of a given luminosity).
• Or you could divide the total mass of the galaxy
  by a typical star's mass (or use the mass
  function to get the proportions right).

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Distances to other Galaxies

• Distances between galaxies are large and are
  often measured in megaparsecs.
• A megaparsec is one million parsecs
• or about 3.3 million light years.
• Example
• distance between the Milky Way and the closest
  large galaxy, the Andromeda Galaxy, is about
  0.899 megaparsecs.

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Shapley-Curtis debate

• Big controversy in the 1910's and early 1920's
  over whether the nebulae called galaxies were
  outside the Milky Way or were part of it.
• National Academy of Sciences held a debate
  between the opposing sides in 1920.
• Those favoring a large Milky Way with the spiral
  nebulae inside it were represented by Harlow
  Shapley.
• Those favoring the spiral nebulae as separate
  groups of stars outside the Milky Way were
  represented by Heber Curtis.
• The Shapley-Curtis debate did not decide much
  beyond the fact that both sides had powerful
  evidence for their views.

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Resolution

• Edwin Hubble and Milton Humanson set out to
  resolve the debate
• by using the largest telescope at the time,
• the 100-inch telescope on Mount Wilson,
• to study the large spiral nebula in the Andromeda
  constellation.

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Resolution (2)

• Because of its large mirror, the telescope had
  sufficient resolving power and light-gathering
  power to spot individual stars in the Andromeda
  Galaxy.
• In the mid-1920's they discovered Cepheid
  variables in the galaxy and used the
  period-luminosity relation to find that the
  distance to the galaxy was very much greater than
  even the largest estimates for the size of the
  Milky Way.
• Galaxies are definitely outside the Milky Way and
  our galaxy is just one of billions of galaxies in
  the universe.
• Their discovery continued the process started by
  Copernicus long ago of moving us from the center
  of the universe.

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Types of Galaxies

• Edwin Hubble divided the galaxies into three
  basic groups
• ellipticals,
• spirals,
• irregulars.

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Types of Galaxies (2)

• Ellipticals are smooth and round or elliptical.
• Spirals are flat with a spiral pattern in their
  disk.
• Irregulars have stars and gas in random patches.
• Most galaxies are small and faint so only the
  luminous galaxies are seen at great distances.
• Very luminous galaxies tend to be either the
  elliptical or spiral type, so they are the ones
  often displayed in astronomy textbooks.

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Sequence of Galaxy Classification

• Hubble (1936) put these groups onto a two-pronged
  sequence that looks like a tuning fork.
• He thought (incorrectly) that galaxies evolved
  from left to right in diagram.

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Not All Ideas are Correct

• Astronomers now know that it is NOT an
  evolutionary sequence because each type of galaxy
  has very old stars.
• The oldest stars in any galaxy all have about the
  same age of around 15 billion years.
• This means that spirals form as spirals,
  ellipticals form as ellipticals, and irregulars
  form as irregulars.
• However, the tuning fork diagram remains
  convenient for classifying galaxies.

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Ellipticals

• Smooth and elliptical in appearance.
• Have four distinguishing characteristics
• much more random star motion than orderly
  rotational motion
• star orbits are aligned in a wide range of angles
  and have a wide range of eccentricities.
• Little dust and gas left between the stars
• No new star formation occurring now and no hot,
  bright, massive stars in them.
• No spiral structure.

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Elliptical Sub-classification

• Most elliptical galaxies are small and faint.
• The dwarf ellipticals may be the most common type
  of galaxy in the universe
• (or maybe the dwarf irregulars are).
• Examples of elliptical galaxies are M32 (an E2
  dwarf elliptical next to the Andromeda Galaxy)
  and M87 (a huge elliptical in the center of the
  Virgo cluster).

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Elliptical Galaxies Example 1

• Messier 32 a dwarf elliptical (E2) satellite
  galaxy of the Andromeda Galaxy.

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Elliptical Galaxies Example 2

• Messier 87
• giant elliptical (E1)
• at the core of the Virgo Cluster
• Grown very large by eating'' other galaxies.

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Elliptical Galaxies Example 3

• Leo I
• dwarf elliptical
• E3
• Local Group.

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Elliptical Galaxies Example 4

• Messier 110
• dwarf elliptical
• E6
• satellite of Andromeda Galaxy.

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Spiral Galaxies

• Flattened disks with a spiral pattern in the
  disk.
• Spiral arms can go all of the way into the bulge
  or be attached to the ends of a long bar of gas
  and dust that bisects the bulge.

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Spiral Galaxies Characteristics

• Four distinguishing characteristics of the
  spirals are
• More orderly, rotational motion than random
  motion
• the rotation refers to the disk as a whole and
  means that the star orbits are closely confined
  to a narrow range of angles and are fairly
  circular.
• Lot of gas and dust between the stars.
• New star formation occurring in the disk,
  particularly in the spiral arms.
• A spiral structure.

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Spiral Galaxies Example 1

• Andromeda Galaxy
• M31
• large spiral galaxy (Sb)
• near the Milky Way.
• Note
• M32 just above it
• M110 below it.

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Spiral Galaxies Example 2

• Triangulum Galaxy
• M33
• Small spiral galaxy (Scd)
• in the Local Group.

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Spiral Galaxies Example 3

• Messier 81
• Large spiral galaxy (Sb).

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Spiral Galaxies Example 4

• NGC 2997
• Large face-on spiral galaxy (Sc).

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Spiral Galaxies Example 5

• NGC 1365
• barred spiral galaxy (SBbc).

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Spiral Galaxies Example 6

• NGC 3351
• (M95)
• Barred spiral galaxy (SBb).

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Irregular Galaxies

• Irregular galaxies have no definite structure.
• Stars bunched up but the patches are randomly
  distributed throughout the galaxy.
• Some irregulars have a lot of dust and gas so
  star formation is possible.
• Some are undergoing a burst of star formation
  now, many H II regions are seen in them.
• Others have very little star formation going on
  in them (even some of those with a lot of gas and
  dust still in them).

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Irregular Galaxies (2)

• Most irregulars are small and faint.
• The dwarf irregulars may be the most common type
  of galaxy in the universe (or maybe the dwarf
  ellipticals are).
• Dwarf galaxies far away are faint and hard to
  see.
• Perhaps if the dwarf galaxies were brighter, E.
  Hubble would have arranged the galaxies in a
  different sequence instead of the two-pronged
  sequence.
• Examples of irregular galaxies are the Large and
  Small Magellanic Clouds (two small irregulars
  that orbit the Milky Way).

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Irregular Galaxies Example 1

• Large Magellanic Cloud
• Dwarf irregular satellite galaxy of the Milky
  Way.

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Irregular Galaxies Example 2

• Small Magellanic Cloud
• Dwarf irregular satellite galaxy of the Milky
  Way.

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Irregular Galaxies Example 3

• NGC 6822
• Dwarf irregular galaxy in the Local Group.

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Irregular Galaxies Example 4

• IC 5152
• Dwarf irregular galaxy in the Local Group.

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Irregular Galaxies Example 5

• NGC 1313
• starburst galaxy
• also called a barred spiral galaxy (SBc).

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Irregular Galaxies Example 6

• Messier 82
• starburst galaxy.

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Distribution in the Sky

• Galaxies are distributed fairly uniformly across
  the sky.
• Approximately the same number of galaxies are
  seen in every direction

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More about Galaxy Distributions

• The distribution of galaxies is not perfectly
  smooth.
• They clump together into loose groups.
• Three-dimensional maps of the universe have
  revealed surprisingly large structures in the
  universe.
• Galaxies like to group together and those groups,
  in turn, congregate together.

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Distances To Galaxies

• As for the determination of stellar properties,
  finding the distance to galaxies is essential for
  comparing the galaxies against each other.
• In order to determine the luminosities and masses
  of the galaxies and the distribution of the mass
  inside the galaxies, one must know their distance
  from our own Galaxy.

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Galaxy Distance Determination

• Use the period-luminosity relation of Cepheid
  variable stars to derive the distance from the
  apparent brightness of the Cepheids.
• Works only for the nearby galaxies.
• For galaxies farther away, other standard candle
  techniques involving objects more luminous than
  Cepheids like supernova explosions or supergiant
  stars are used.
• Luminosities are not as well known or uniform,
• Greater uncertainty in the derived distances to
  the very distant galaxies.

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Rotational Velocities and Luminosity

• In 1977 Brent Tully and Richard Fisher discovered
  a relation between the rotational velocity of the
  disk and the luminosity of a spiral galaxy.
• Rotational velocity is found from the 21-cm
  emission of the neutral atomic hydrogen gas in
  the outer parts of the disk.
• Rotation curve is flat in the outer parts of most
  galactic disks (dark matter!).

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Rotation
42

• Parts of a spiral galaxy's disk rotating toward
  us have lines blue-shifted.
• Part of the disk rotating away from us have lines
  red-shifted.
• The 21-cm emission from a galaxy of small angular
  size is the blended result of the emission from
  all parts of the disk.
• The faster the disk rotates, the broader the
  21-cm emission line will be.

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Tully-Fisher relation

• The Tully-Fisher relation for the infrared
  luminosity is
• circular velocity 220 x (L/L)0.22.
• Infrared is used to lessen the effect of the dust
  in our galaxy and in the other spiral galaxy.
• The luminosity of the galaxy is found from the
  width of the 21-cm emission line and the distance
  is then derived using the apparent brightness and
  the inverse square law.

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Distance and Redshift

• In 1914, Vesto Slipher (1870--1963) announced
  results from spectra of over 40 spiral galaxies.
• He found that over 90 of the spectra showed
  redshifts which meant that they were moving away
  from us.
• Edwin Hubble and Milton Humason found distances
  to the spiral nebulae.
• When Hubble plotted the redshift vs. the distance
  of the galaxies, he found a surprising relation
• more distant galaxies are moving faster away from
  us.

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Hubble Law

• Hubble and Humason announced their result in
  1931
• the Galactic recession speed H ? distance,
• where H is a number now called the Hubble
  constant.
• This relation is called the Hubble Law and the
  Hubble constant is the slope of the line.

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Determination of the Hubble Constant

• With distances measured in megaparsecs (Mpc) and
  the recession speed in kilometers/second
  (km/sec), the Hubble constant is between 60 and
  70 km/sec/Mpc.
• Value found by using the galaxies that have
  accurate distances measured (Cepheids, etc.) and
  dividing their recession speeds by their
  distances.

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Benefits of the Hubble Law

• It is easy to find the recession speeds of
  galaxies from their redshifts.
• The Hubble law provides an easy way to measure
  the distances to even the farthest galaxies from
  the (recession speed/H).
• For example, if a galaxy has a redshift of 20,000
  km/sec and H is set to 70 km/sec/Mpc, then the
  galaxy's distance (20,000 km/sec)/(70
  km/sec/Mpc) 20,000/70 (km/sec)/(km/sec) Mpc
  286 megaparsecs.

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The Center of the Universe ?

• At first glance,
• it looks like the Milky Way is at the center of
  the universe
• it committed some galactic social blunder because
  all of the other galaxies are rushing away from
  it (there are a few true galactic friends like
  the Andromeda galaxy that are approaching it).

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Copernican principle

• Hubble law shows that there is actually not a
  violation of the Copernican principle.
• More distant galaxies move faster.
• Galaxies (or galaxy clusters) are all moving away
  from each other
• The universe is expanding uniformly.
• Every other galaxy or galaxy cluster is moving
  away from everyone else.
• Every galaxy would see the same Hubble law.

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Expansion
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Masses of Galaxies

• Masses of galaxies are found from the orbital
  motion of their stars.
• Stars in a more massive galaxy orbit faster than
  those in a lower mass galaxy because the greater
  gravity force of the massive galaxy causes larger
  accelerations of its stars.
• By measuring the star speeds, one finds out how
  much gravity there is in the galaxy.
• Since gravity depends on mass and distance,
  knowing the size of the star orbits enables you
  to derive the galaxy's mass.

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Masses from Rotation Curvefor Spiral Galaxies

• The rotation curve shows how orbital speeds in a
  galaxy depend on their distance from the galaxy's
  center.
• The mass inside a given distance from the center
  (orbital speed)2 (distance from the
  center)/G.
• Obital speed is found from the doppler shifts of
  the 21-cm line radiation from the atomic hydrogen
  gas.

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A Mass Problem

• The stars and gas in most galaxies move much
  quicker than expected from the luminosity of the
  galaxies.
• In spiral galaxies, the rotation curve remains at
  about the same value at great distances from the
  center (it is said to be flat).
• This means that the enclosed mass continues to
  increase even though the amount of visible,
  luminous matter falls off at large distances from
  the center.
• In elliptical galaxies, the gravity of the
  visible matter is not strong enough to accelerate
  the stars as much as they are.
• Something else must be adding to the gravity of
  the galaxies without shining.

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Dark Matter Problem

• That something else is called dark matter.
• It is material that does not produce detectable
  amounts of light but it does have a noticeable
  gravitational effect.
• Astronomers are not sure what the dark matter is
  made of.
• Possibilities range from large things like
  planets, brown dwarfs, white dwarfs, black holes
  to huge numbers of small things like neutrinos or
  other particles that have not been seen in our
  laboratories yet.
• The nature of dark matter is one of the central
  problems in astronomy today.