OBSERVING GALAXIES IN DIFFERENT WAVELENGTHS

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OBSERVING GALAXIES IN DIFFERENT WAVELENGTHS

• Direct imaging surveys of external galaxies
  provides information on the distribution and
  relative numerical abundances of all types of
  stars, over the entire galaxy, which can be
  compared with similar information for our own
  galaxy.
• Ground-based telescopes have been limited to
  wavelength ranges which can penetrate Earths
  atmosphere, hence excluding most of the UV and a
  significant part of the IR spectral ranges.
• Since the stars in a distant galaxy are (nearly)
  all at the same distance, the relative
  brightnesses of stars at each wavelength are
  directly proportional to their absolute
  luminosities (but can be affected by local
  extinction by dust and gas within the observed
  galaxy).
• Likewise, if stars in an external galaxy can be
  determined to be of the same type and luminosity
  as local stars in our own galaxy, this can be
  used to determine the distance to the external
  galaxy.
• Observations in the UV and IR ranges, of external
  galaxies as well as of stars in our own galaxy,
  provide a broader spectral range for
  determination of stellar temperatures and total
  luminosities, and of interstellar dust
  distributions and compositions (from dust
  extinction of starlight).

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Ground-based visible-light image of the Large
Magellanic Cloud (left), and Apollo-16 far-UV
image (right). Note that the central bar,
consisting mostly of relatively cool stars and
which produces most of the visible light in the
ground-based image, is not prominent in the
far-UV image, but the opposite is true of the
more sparsely distributed but very hot stars in
the outer regions of the galaxy.
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Ground-based visible-light image of the spiral
galaxy M101 (left) and far-UV image, taken with
the Astro-2 Ultraviolet Imaging Telescope (right).
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Comparison of far-UV images of galaxies, taken
with the NASA Goddard Space Flight Centers
Ultraviolet Imaging Telescope during the Astro-2
space shuttle mission (top row), with
ground-based visible light images (bottom row).
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Andromeda Galaxy (M31) Observed in Visible Light
(Left) and in Far Ultraviolet Light by Galex
Explorer Satellite (Right)
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Infrared Images of the Andromeda Galaxy by Spitzer
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M81- Ground Based Visible Light Image
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TYPES OF GALAXIES

• Many galaxies, including our own, appear to have
  more mass than can be accounted for by visible
  stars and interstellar material. This missing
  mass is relatively more prominent in the outer
  regions of galaxies.
• This mass is evident by the fact that the
  rotation velocities at different radii from the
  center of a galaxy cannot be accounted for (under
  the previously discussed laws of gravity and
  orbital mechanics) by the observable mass (stars
  and interstellar material).
• Candidates for this missing mass include very low
  luminosity red dwarf stars, brown dwarfs (stars
  born with masses too low to support thermonuclear
  fusion in their cores), and cooled-off white
  dwarf stars
• Other, more non-conventional candidates proposed
  include dark matter of unknown properties, and
  exotic particle radiation (such as massive
  neutrinos).

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INTERACTING GALAXIES

• The distances between galaxies in our Universe
  are typically very large (millions of light years
  on the average).
• However, unlike stars in our Galaxy (and most
  others), the sizes of galaxies, on the average,
  are not negligibly small compared to the
  distances between them.
• Therefore, galaxies interact with each other to a
  much greater extent than do stars in our Galaxy
  (other than those created in binary or multiple
  systems).
• As a result, not only do large galaxies (such as
  our own and the Andromeda galaxy) have satellite
  galaxies, but originally isolated galaxies can
  gravitationally disrupt or even collide with each
  other.

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Tadpole Galaxies Observed with HST
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The Mice Galaxies (NGC 4676) Observed with HST
Advanced Camera for Surveys (ACS)
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HST Advanced Camera for Surveys Image of
Tadpole Galaxy UGC10214
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The Black Eye Galaxy, M64 (HST Image)
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ACTIVE GALAXIES AND QUASARS

• Some galaxies, the active galaxies, show highly
  energetic phenomena and light emission in their
  central regions.
• These include the Seyfert galaxies and the
  quasi-stellar objects (quasars).
• Quasars are among the most luminous objects in
  the Universe - much brighter than the total
  brightnesses of most galaxies, but their light
  sources are only comparable to our solar system
  in size!
• Because of their brightness and compactness,
  quasars can be seen to greater distances than can
  the brightest normal galaxies.
• The sources of this energy are believed to be
  supermassive black holes in the centers of
  galaxies, having as much as 108 times the Suns
  mass.
• The radiated energy is produced by matter being
  sucked in to the black hole by way of an
  intermediate accretion disk.

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HST Image of Jet from Core of Active Galaxy M87
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GALAXIES AND THE UNIVERSE

• The most distant galaxies observed to date, with
  the Hubble Space Telescope and large ground-based
  telescopes, are more than 5 billion light years
  away.
• The entire Universe, as we know it, contains
  about as many galaxies as our Galaxy contains
  stars - about 100 billion!
• Galaxies are observed to be moving away from our
  Galaxy, and from each other, at velocities
  proportional to distance (Hubbles Law), which
  demonstrates that the Universe is expanding.
• The observed rate of expansion infers an age of
  the Universe (originating in a Big Bang), of at
  least 10 billion years (the most current age
  determination is 13.7 billion years).
• The subject of cosmology deals with the origin,
  evolution, and large-scale structure of the
  Universe.
• Until the recent advent of the Hubble Space
  Telescope, other space flight investigations, and
  of larger and more powerful ground-based
  telescopes, cosmology was mostly a theoretical
  field of study, based largely on Einsteins
  theory of general relativity.

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This distant cluster of galaxies in the
constellation Hercules includes a wide variety of
sizes and shapes among its members. (Palomar
Observatory Image)
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A Day in the Lives of Galaxies (from HST Advanced
Camera for Surveys)