Galaxy SurveyThis largescale galaxy survey, carried out at the Las Campanas Observatory in Chile, co

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Galaxy Survey This large-scale galaxy survey,
carried out at the Las Campanas Observatory in
Chile, consists of 23,697 galaxies within about
1000 Mpc, in two 80º   4.5º wedges of the sky.
Many voids and walls on scales of up to 100200
Mpc can be seen, but no larger structures are
evident, suggesting that the universe is roughly
homogeneous on sufficiently large scales.
Preliminary results from even more extensive
surveys, now underway are in general agreement
with he findings reported here.
Coma
Great Wall
Zone of Avoidance
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• The results of these large-scale studies strongly
  suggest that the universe is homogeneous (the
  same everywhere) on scales greater than a few
  hundred megaparsecs. In other words, if we took a
  huge cube300 Mpc on a side, sayand placed it
  anywhere in the universe, its overall contents
  would look much the same no matter where it was
  centered. Some of the galaxies it contained would
  be clustered and clumped into fairly large
  structures and some would not, and we would see
  numerous walls and voids, but the total numbers
  of these objects would not vary much as the cube
  was moved from place to place. In this sense, the
  universe appears smooth on the largest scales.

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• The universe also appears to be isotropic (the
  same in all directions) on these scales.
  Excluding directions that are obscured by our
  Galaxy, we count roughly the same number of
  galaxies per square degree in any patch of the
  sky we choose to observe, provided we look deep
  (far) enough that local inhomogeneities dont
  distort our sample. In other words, any deep
  pencil-beam survey of the sky should count about
  the same number of galaxies, regardless of which
  patch of the sky is chosen.

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• Galaxy Energy Spectra The spectrum of the energy
  emitted from a normal galaxy differs from that of
  an active galaxy. This plot illustrates the
  general run of intensity for all galaxies of a
  particular type and does not represent any single
  galaxy.

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• Seyfert Galaxy (a) This image of Seyfert galaxy
  NGC 7742 resembles a fried egg, with a ring of
  blue-tinted star-forming regions surrounding a
  very bright yellow core that spans 1 kpc across.
  This active spiral galaxy resides about 24 Mpc
  away. (b) The Circinus galaxy, also a Seyfert
  with a bright compact core, lies some 4 Mpc
  awayit is one of the closest active
  galaxies. (NASA)

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• In 1943 Carl Seyfert, an American optical
  astronomer studying spiral galaxies from Mount
  Wilson Observatory, discovered the type of active
  galaxy that now bears his name. Seyfert galaxies
  are a class of astronomical objects whose
  properties lie between those of normal galaxies
  like the Milky Way and those of the most violent
  active galaxies known. This fact suggests to many
  astronomers that Seyferts represent an
  evolutionary link between these two extremes. The
  spectral lines of Seyfert galaxies are usually
  substantially redshifted, telling us that most
  Seyferts reside at large distances (hundreds of
  megaparsecs) from us. (Sec. 24.5) However, a few
  lie just 20 or 30 Mpc away.

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• First, maps of Seyfert energy emission show that
  nearly all the radiation stems from a small
  central region known as the galactic nucleus.
  This region lies at the center of the overexposed
  white patch in Figure 25.2(a) another is shown
  in Figure 25.2(b). Astronomers suspect that a
  Seyfert nucleus may be quite similar to the
  center of a normal galaxy such as the Milky Way
  or the Andromeda Galaxy, but with one very
  important difference The nucleus of a Seyfert is
  10,000 times brighter than the center of our
  Galaxy. Indeed, the brightest Seyfert nuclei are
  10 times more energetic than the entire Milky Way.

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• Second, Seyfert galaxies emit their radiation in
  two broad frequency ranges. The stars in the
  Seyferts galactic disk and spiral arms produce
  about the same amount of visible radiation as
  those of a normal spiral galaxy. However, most of
  the energy from the Seyferts nucleus is emitted
  in the form of invisible radio and infrared
  radiation, which cannot be explained as coming
  from starsit must be nonstellar in origin.

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• Third, Seyfert spectral lines bear little or no
  resemblance to those produced by ordinary stars,
  although they do have many similarities to the
  spectral lines observed toward the center of our
  own Galaxy. (Sec. 23.7) Seyfert spectra contain
  strong emission lines of highly ionized heavy
  elements, especially iron. The lines are very
  broad, indicating either that the galaxys gases
  are tremendously hot (more than 108 K) or that
  they are rotating very rapidly (at about 1000
  km/s) around some central object. (Sec. 4.4) The
  first possibility can be ruled out, since such a
  high temperature would cause all the gas to be
  ionized, in which case no spectral lines would be
  produced. Thus, the broadening indicates rapid
  internal motion in the nucleus.

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• Finally, extensive monitoring of Seyfert
  radiation over long periods of time has shown
  that the energy emission often varies over time.
  Figure 25.3 shows an example of luminosity
  variations for a typical Seyfert. Such radiative
  changes are unlike anything found in the Milky
  Way or in any other normal galaxy. A Seyferts
  luminosity can double or half within a fraction
  of a year.

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• Seyfert Time Variability The irregular variations
  of a particular Seyfert galaxys luminosity over
  a period of two decades. Because this Seyfert,
  called 3C 84, emits most strongly in the radio
  part of the electromagnetic spectrum, these
  observations were made with large radio
  telescopes. The optical and X-ray luminosities
  vary as well. (NRAO)

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• Centaurus A Radio Lobes Lobe-radio galaxies, such
  as Centaurus A shown here optically in (a), have
  giant radio-emitting regions (b) extending a
  million parsecs or more beyond the central
  galaxy. The lobes cannot be imaged in visible
  light and are observable only with radio
  telescopes. The lobes are shown here in false
  color, with decreasing intensity from red to
  yellow to green to blue. (ESO NRAO)

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• Centaurus A, Close Up The main image (b) shows an
  optical photograph of Centaurus A, one of the
  most massive and peculiar galaxies known, thought
  to be the result of a collision between two
  galaxies that took place 500 million years ago.
  The pastel false colors mark the radio emission
  shown in Figure the data in this case were more
  recently acquired and have higher resolution. (a)
  Although the radio lobes emit no visible light,
  they do emit X-rays, as shown in this Chandra
  image. (c) Increasingly high-resolution optical
  views of the galaxys core region, taken by the
  Hubble Space Telescope. (NASA SAO J. Burns)

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• Cygnus A (a) Cygnus A also appears to be two
  galaxies in collision. (b) On a much larger
  scale, it displays radio-emitting lobes on either
  side of the optical image. The optical galaxy in
  (a) is about the size of the small dot at the
  center of (b). Note the thin line of
  radio-emitting material joining the right lobe to
  the central galaxy. The distance from one lobe to
  the other is approximately a million
  light-years. (NOAO NRAO)

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NGC 1265
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• HeadTail Radio Galaxy (a) Radiograph, in false
  color, of the active headtail galaxy NGC 1265.
  (b) The same radio data, in contour form,
  superposed on the optical image of the galaxy.
  Astronomers reason that this object is moving
  rapidly through space, trailing a tail behind
  as it goesa little like a comet, but on a vastly
  larger scale. (NRAO Palomar/Caltech)

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M86
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• CoreHalo Radio Radio contour map of a typical
  corehalo radio galaxy, the one near the center
  called M86. The radio emission from such a galaxy
  comes from a bright central nucleus, or core,
  surrounded by an extended, less intense halo. The
  radio map is superimposed on an optical image of
  the galaxy and some of its neighbors, a
  wider-field version of which was shown previously
  in Figure (Credit Harvard-Smithsonian Center for
  Astrophysics)

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• M87 Jet The giant elliptical galaxy M87 (also
  called Virgo A) is displayed here at several
  different wavelengths. (a) A long optical
  exposure of its halo and embedded central region.
  (b) A short optical exposure of its core and an
  intriguing jet of matter, on a smaller scale than
  (a). (c) A radio image of its jet, on a somewhat
  expanded scale compared with (b). The red dot at
  left marks the bright nucleus of the galaxy the
  red and yellow blob near the center of the image
  corresponds to the bright knot visible in the
  jet in (b). (d) A near-infrared image of the jet,
  at roughly the same scale as (c). (NOAO NRAO
  NASA)

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• Radio Galaxy A central energy source produces
  high-speed jets of matter that interact with
  intergalactic gas to form radio lobes. The system
  may appear to us as either a lobe or a corehalo
  radio galaxy, depending on our location with
  respect to the jets and lobes.Jets such as this
  are a very common feature of active galaxies. As
  we will see, they play a vital role in our
  understanding of these energetic objects. The M87
  jet also suggests a simplifying connection
  between the two types of radio galaxies just
  discussed. It is likely that the differences
  between corehalo and lobe-radio galaxies are
  largely a matter of perspective (Figure 25.10).
  If we view the jets and lobes from the side, we
  see a lobe-radio galaxy, but if we view the jet
  almost head-onin other words, looking through
  the lobewe see a core-halo system. More
  Precisely 25-1 discusses another curious
  characteristic of some active galaxies and
  quasars that also supports this view.
• Both Seyferts and radio galaxies emit comparably
  large amounts of energy, and as we have seen,
  there is good evidence that the energy source in
  each is a compact region at the center of an
  otherwise relatively normal-looking galaxy. In
  lobe-radio galaxies, that energy is fired out
  from the nucleus in the form of narrow,
  high-speed jets of matter that travel into the
  intergalactic medium and become extended lobes
  far from the center of the galaxy. As a result,
  the energy from a lobe-radio galaxy is ultimately
  emitted (in the form of radio radiation) from a
  region well outside the visible galaxy. However,
  in all cases studied so far, the central compact
  nucleus is the place where the energy is actually
  produced.

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