Chapter 15, Galaxies

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Chapter 15, Galaxies

• Galaxies come in different size and shape. In
  the previous chapter, we talked about how
  galaxies provide an environment for the stars to
  be born and die, and enrich the heavy element
  content of the galaxy. In this chapter, we will
  talk about
• Galaxy Classification
• Location of Galaxies in the Universe
• Galaxy Evolution
• Quasars and AGN (Active Galactic Nuclei)

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Spiral Galaxies

• NGC (New General Catalog) 4594

• NGC 1300 barred spiral

• NGC 4594 Sombrero Galaxy. Large bulge, small
  disk

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Elliptical and Irregular

• M87 Elliptical Galaxy in Virgo

• Large Magellanic Cloud, irregular galaxy

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Galaxy Classification

• Galaxies come in different size and shape
• Spiral Galaxies
• Barred Spiral Spiral galaxy with a bar.
• Lenticular galaxy Spiral galaxy without spiral
  arms.
• Formed by gas clouds with large initial angular
  momentum.
• Elliptical Galaxies
• Similar to the bulge of the spiral galaxies.
• Formed by gas clouds with small initial angular
  momentum.
• Formed by high density cloudsmore efficient
  cooling, faster star formation, exhausting the
  gas supply before the galaxy has time to collapse
  into the disk.
• Formed by collision and merging of galaxies.
• Irregular Galaxies
• Usually more distant galaxies.
• Starburst galaxies
• Galaxies with high star forming rate. Merging
  galaxies?
• Quasars, Active Galactic Nuclei
• Distant objects. Extremely luminouscould be
  1,000 times the luminosity of the Milky Way.
• Supermassive Black holes?

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Galaxy Facts

• Grouping
• Spiral galaxies are usually found in loosely
  associated small group (tens) of galaxies
• Ellipticals are commonly found in large cluster
  of galaxies containing hundreds or thousands of
  galaxies extending over tens of millions of
  light-years.
• Size
• Most of the large galaxies are spiral galaxies
• While some of the largest galaxies are giant
  elliptical galaxies, the most common type of
  galaxy in the universe is small elliptical
  galaxy.
• Very small ellipticals (or dwarf spheroidals,
  less than a billion stars) are often found near
  large spirals our Milky Way galaxy has 10 or
  more nearby

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Where are the Galaxies?

• Where are the galaxies located? Are they located
  within the Milky Way, or are they much further
  away from us than the stars?
• Before the 1920s, there were no reliable methods
  of measuring the distance to the galaxies. Many
  people believed that the galaxies were located
  within the Milky Way
• How do we measure the distance of objects far
  away in the universe, much farther than the
  distance that can be measured by stellar
  parallax?
• Measurement of distance farther than the reach of
  stellar parallax rely on our ability to find
  objects with known luminosity
• In 1924, Edwin Hubble determined the distance to
  the Andromeda galaxy using Cepheid variables,
  thus proving that the galaxies are located far
  beyond the stars in the Milky Way galaxy.

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Measuring Cosmological Distance

• The principle method of measuring astronomical
  distance is the
• distance-luminosity relation
• However, the apparent brightness B is the only
  thing we can measure accurately in most of the
  cases.
• If we know the distance D, we can determine the
  luminosity L.
• If we know the luminosity L, then we can
  determine the distance D.
• Need to find Standard Candles astronomical
  objects with known luminosity.
• Main sequence stars.
• Cepheid variables.
• White dwarf supernovae.
• Galaxies (using Tully-Fisher Law).

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The Cosmological Distance Ladder

• Methods of Measuring Distance and their useful
  range
• Radar ranging D lt 10-4 light-years
• Parallax D lt 103 light-years
• Standard Candles
• Main sequence stars D lt 105 light-years
• Cepheid variables D lt 107 light-years
• White dwarf supernovae D lt 1010 light-years
• Hubbles Law 1010 ly and beyond

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Calibrating the Cosmic Tape Measure

• We rely heavily on the standard candles for
  the measurement of the cosmological distance. How
  do we make sure that these standard candles are
  truly standard?
• Use independent measurements to check the
  luminosity of the standard candle.
• For example, we can use parallax measurements of
  the distance to main sequence stars to check
  measurements of distance using main-sequence
  fitting. If we do this for a few of them, then we
  can verify the assumption that the main sequence
  stars are good standard candle. However, this
  method works only for stars that are relatively
  close by.
• The standard candles we had verified in close
  range can now be used (by extrapolation) to
  measure the distance to more remote objects. This
  new distance measure then allows us to calibrate
  the next standard candle.
• For example we used the distance measured by
  observation of Cepheid variables to check the
  assumption of the constancy of the white dwarf
  supernovae.
• Keep going

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Main Sequence Fitting

• Main sequence stars with the same color
  should have the same luminosity. So, if we
  compare the (pseudo) H-R diagram of a star
  cluster with unknown distance (using their
  apparent brightness instead of luminosity) to
  that of a group of main sequence stars with known
  distance, then we can determine the distance to
  this new cluster.
• For example
• Hyades (in constellation Taurus) is a open
  cluster about 150 light years away. Its distance
  is close enough to be measured by stellar
  parallax.
• Comparing the H-R Diagrams of Hyades with that of
  Pleiades, we can determine that the distance of
  Pleiades should be 2.75 times farther than
  Hyadesor, 1502.75 410 light years.
• New parallax measurement of Hyades by Hipparcos
  (ESO Space Interferometry mission) yielded a
  distance of 438 light years

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Cepheid Variables

• Cepheid variable stars are population I
  (metal-rich) yellow giant stars with periodic
  luminosity variation.
• Their periods range from a few days to over 100
  days,
• Their luminosities range from 1000 to 30,000 L?,
• The high luminosity makes it possible to identify
  them from a large distance
• Their luminosity and period are strongly
  correlated. Therefore, we can determine their
  luminosity by simply measuring their periods!

The luminosity of Cepheid variabls are strongly
correlated to their periodicity
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Cepheid Variables in M100

• The period-luminosity relation of Cepheid
  variables were discovered Henrrietta Leavitt in
  1912. Edwin Hubble identified Cepheid variables
  in Andromeda galaxy (about 2.5 million
  light-years away) in 1924, and used the
  luminosity-distance relation to demonstrate that
  galaxies are much farther than the stars.
• There are well over 1,000 Cepheid variables known
  todayfor example, the Polaries!
• In 1994, Hubble Space Telescope observed a
  Cepheid variable in the face-on spiral galaxy
  M100 in Virgo Cluster located at a distance of 56
  million light-yearsthis is the most distant
  distant Cepheid observed so far.
• http//hubblesite.org/newscenter/archive/releases/
  1994/49/

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Tully-Fisher Relation

• Although this is not discussed in our text
  book, the luminosity of the spiral galaxies are
  related to their rotational speed,. This was
  discovered by B. Tully (of UH/IfA) and J.R.
  Fisher in 1977. Therefore, the luminosity of the
  spiral galaxies can be determined simply by
  measuring their rotational speed
• Spiral galaxies are good standard candles also!
• The slope of the luminosity-rotation rate curve
  is different for different type of spiral
  galaxies

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White Dwarf Supernova

• Every time the hydrogen shell is ignited, the
  mass of the white dwarf may increase (or
  decrease, we dont know for sure yet).
• The mass of the white dwarf may gradually
  increase,
• At about 1 M?, the gravitation force overcomes
  the electron degenerate pressure, and the white
  dwarf collapses, increasing temperature and
  density until it reaches carbon fusion
  temperature.
• The carbon inside the white dwarfs are
  simultaneously ignited. It explodes to form a
• White dwarf supernova. (Type I).
• Nothing is left behind from a white dwarf
  supernova explosion (In contrast to a
  massive-star supernova, which would leave a
  neutron star or black hole behind). All the
  materials are dispersed into space.

White Dwarf Supernova is a very important
standard candle for measuring cosmological
distance
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White Dwarf and Massive Star Supernovae
Because the mass of white dwarfs when they
explode as supernovae is always around 1.0 M?,
its luminosity is very consistent, and can be
used as a standard candle for the measurement of
distance to distant galaxies (Chapter 15). The
amount of energy produced by white dwarf
supernovae and massive star supernovae are about
the same. But the properties of the light emitted
from these two types of supernovae are
intrinsically different, allowing us to
distinguish them from a distance.

• Massive star supernovae spectrum is rich with
  hydrogen lines (because they have a large outer
  layer of hydrogen).
• White dwarf supernovae spectra do not contain
  hydrogen line (because white dwarfs are mostly
  carbon, with only a thin shell of hydrogen).
• The light curve is different.

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Supernovae from Distant Galaxies

• These snapshots, taken by NASA's Hubble Space
  Telescope, reveal five supernovae, or exploding
  stars, and their host galaxies.
• The arrows in the top row of images point to the
  supernovae. The bottom row shows the host
  galaxies before or after the stars exploded. The
  supernovae exploded between 3.5 and 10 billion
  years ago.

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Distance and Redshift

• In addition to distance, Hubble also measured
  the redshift of the galaxiesand when combined
  with distances derived from observation of
  Cepheid variables and the brightest stars in
  galaxies, Hubble found that, the more distant a
  galaxy, the greater its redshift is, and hence
  the faster it is moving away from us
• the universe is expanding!

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Hubbles Law

• From the redshift and distance measurements,
  we can express the recession speed V of a galaxy
  located at a distant d away from us by
• V d ? H0
• The value of the Hubbles Constant is
• H0 2024 km/sec / million light-year
• Once the value of H0 is determined, we can use
  measured recession speed to infer the distance of
  galaxies using the formula
• d V / H0