A Universe of Galaxies

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Chapter 15

• A Universe of Galaxies

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The Hubble Deep Field10 day exposure field
located in the Big Dipper
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Edwin Hubble
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Edwin Hubble
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Hubbles Galaxy Classification
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Spiral Galaxies

• galaxies like the Milky Way with arcing
  structures lying in a plane and emanating from
  the nuclear bulge

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Lenticular Galaxies

• Galaxies that have disks but no spiral arms.

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Barred Spiral Galaxies

• galaxies with a bar of stars running through the
  nuclear bulge

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Elliptical Galaxies

• galaxies with an elliptical shape, no spiral
  arms, and little interstellar matter

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Irregular Galaxies

• galaxies that are asymmetrical and are sometimes
  just two or more galaxies colliding

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Large Magellanic Cloud a small irregular galaxy
that orbits the MMilky Way
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Measuring Cosmic Distances

• Distances to other galaxies are measured using
  Main Sequence Fitting.
• This entails the use of a light source of known,
  standard luminosity called a standard candle.
• The distance can be found using the
  luminosity-distance formula
• Apparent brightness luminosity/4?d2

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Comparison of the apparent brightness of stars in
the Hyades Cluster with those of the Pleiades
Cluster
The Pleiades Cluster is 2.75 times farther away
because (2.75)2 7.5 times dimmer The same
luminosities are assumed for all main sequence
stars of the same color.
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RR Lyrae and Cepheid Variable Stars

• These are both pulsating variable stars.
• Their pulsation periods are on the order of a few
  days.
• Using the period-luminosity relationship,
  distances to other galaxies can be estimated

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Cepheid Period-luminosity Relation. Cepheids of
a particular period have very nearly the same
luminosity.
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Edwin Hubble
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Edwin Hubble
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http//www.seds.org
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Andromeda Nebula M31, is actually another spiral
galaxy
Andromeda Nebula M3, is actually another spiral
galaxy
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Tully-Fisher Relation using galaxies as
standard candles

• Astronomers discovered that the faster a spiral
  galaxy rotates, the more luminous it is.
• This relationship is called the Tully-Fisher
  relation, after its discoverers

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The Tully-Fisher relation.
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Galaxy Observations

• During the 1920's Edwin Hubble and Milton Humason
  photographed the spectra of many galaxies with
  the 100 inch telescope at Mount Wilson.
• They found that most of the spectra contained
  absorption lines with a large redshift.

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Red Shift and Distance
24 Mpc
1200 km/s
300 Mpc
15,000 km/s
780 Mpc
39,000 km/s
1220 Mpc
61,000 km/s
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Galaxy Observations

• Using the Doppler effect, Hubble calculated the
  velocity at which each galaxy is receding from
  us.
• Using the period and brightness of Cepheid
  variables in distant galaxies, Hubble estimated
  the distances to each of the galaxies.

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The Tully-Fisher Relation
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Hubbles Law

• Hubble noticed that there was a linear
  relationship between the recessional velocity and
  the distance to the galaxies.
• This relationship is know as Hubbles Law
• V H D
• recessional velocity Hubbles Constant ?
  Distance

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Hubbles Law

• H is known as the Hubble constant.
• Its true value appears to be somewhere between
  55 to 75 km/s/Mpc.
• This means that a galaxy that is 1 megaparsec
  from Earth will be moving away from us at a speed
  somewhere between 55 to 75 km/s.

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The Distance Chain or Ladder
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Measuring Cosmic Age
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Measuring Cosmic Age
Raisin Cake Model
Like raisins in rising raisin cake, galaxies move
away from each other in our expanding universe.
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Typical Cube of Galaxies
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Homogeneous, isotropic universe?
NO!
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The Birth of The Universe-The Big Bang
A very rough estimate for the age of the universe
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All of space and time were created in the Big
Bang, which then expands. Analogous to the
surface of a balloon.
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Cosmological Red Shift
As the universe expands, photons of radiation are
stretched in wavelength, giving rise to the
cosmological redshift.
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Elliptical, Spiral and irregular galaxies at
different ages.
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Modeling Galaxy Birth

• The most successful models are based on the
  following assumptions
• Hydrogen and helium gas filled all of space
  fairly uniformly early in the universe.
• The near uniformity had small perturbations which
  allowed for dense regions to exist.

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Galactic Collisions
NGC 4038/4039 are a pair of colliding spiral
galaxies
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Hubble Space Telescope Photos Of Distorted Young
Galaxies.
The larger number of distorted galaxies in the
past suggests that collisions between galaxies
were common during the first few billion years.
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Star- Burst galaxies
While the Milky Way forms a new star about once
per year, starburst galaxies can form over 100
new stars per year
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Quasars and Active Galactic Nuclei
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• Active galaxies are galaxies which are much more
  luminous than normal galaxies and have spectra
  that are nonstellar in nature.

• This indicates that the energy they emit is not
  simply the accumulated light of many stars.

• Most of the energy from active galaxies is in the
  radio and infrared portions of the spectrum.

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Planck curves for Active and Normal Galaxies
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Seyfert Galaxies

• Look like normal spiral galaxies except with
  extremely bright central galactic nucleus.

• The luminosity of the nucleus can exceed that of
  the rest of the galaxy.

• Spectral lines are very broad, indicating rapid
  rotation.

• Luminosities can vary by large amounts in
  fractions of a year.

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Active galactic nucleus in the elliptical galaxy
M 87.
Jet of particles shooting outward from the
nucleus at nearly the speed of light
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Radio Galaxies

• Active galaxies that emit most of their energy in
  the radio part of the spectrum.

• Comparable to Seyferts in total energy output.

• Usually associated with elliptical galaxies.

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Two Types Of Radio Galaxy

• Core- Halo Radio Galaxy Energy is emitted from
  a small central nucleus, as with a Sayfert Galaxy.

• Lobe Radio Galaxy Energy is emitted from
  enormous radio lobes. These lobes usually lie far
  beyond the galactic nucleus and are usually much
  larger than the visible part of the galaxy.

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Radio image of the radio galaxy Cygnus A taken
with the VLA.
400,000 light-years
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Active Galaxies show some or all of the following
properties.

• High Luminosities.
• Energy emission is nonstellar.
• Energy output can be highly variable.
• Often exhibit jets and other signs of explosive
  activity.
• Spectra show broad emission lines - indicate
  rapid internal motions.

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Central Engine of Active Galaxy
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NGC 1461 in Virgo Cluster
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Energy Emission

• Although the rotating supermassive black hole
  model is now widely accepted, the actual
  mechanism for the energy production is uncertain.
• One popular model which explains some features is
  the synchrotron radiation model.

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Synchrotron Radiation

• A type of nonthermal radiation produced by
  high-speed charged particles, such as electrons,
  as they are accelerated in a strong magnetic
  field.

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Synchrotron Radiation
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Quasi-stellar Objects (QUASARS)

• Circa 1960, astronomers observe what appear to be
  faint blue stars identified with radio sources.
• These objects had odd spectral lines which
  appeared broadened and extremely redshifted.

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• Quasars are
• believed to be some of the oldest objects in the
  universe.
• some of the most distant objects from us.
• the most luminous objects known.

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Radio Jet in the Quasar 3C 345, shows a blob of
plasma moving away from the core at nearly the
speed of light
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Active Galaxy Formation

• Possible evolutionary track for galaxies may be
  as follows
• Quasars ---gt
• Radio/Sayfert Galaxies ---gt
• Normal spiral and elliptical galaxies.
• Black holes are always present, but reduce over
  time as they run out of fuel.

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Doppler Shift of the emission lines in the
nucleus of the elliptical galaxy M 87 indicates a
2-3 billion solar mass black hole
Artists conception of an accretion disk
surrounding a super-massive black hole.
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End of Section