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116 lines
4.5 KiB
116 lines
4.5 KiB
15 years ago
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<sect1 id="ai-ellipgal">
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<sect1info>
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<author>
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<firstname>Jasem</firstname>
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<surname>Mutlaq</surname>
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<affiliation><address>
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</address></affiliation>
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</author>
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</sect1info>
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<title>Elliptical Galaxies</title>
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<indexterm><primary>Elliptical Galaxies</primary>
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</indexterm>
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<para> Elliptical galaxies are spheroidal concentrations of billions
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of stars that resemble Globular Clusters on a grand scale. They have
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very little internal structure; the density of stars declines smoothly
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from the concentrated center to the diffuse edge, and they can have a
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broad range of ellipticities (or aspect ratios). They typically
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contain very little interstellar gas and dust, and no young stellar
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populations (although there are exceptions to these rules). Edwin
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Hubble referred to Elliptical galaxies as <quote>early-type</quote>
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galaxies, because he thought that they evolved to become Spiral
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Galaxies (which he called <quote>late-type</quote> galaxies).
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Astronomers actually now believe the opposite is the case (&ie;, that
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Spiral galaxies can turn into Elliptical galaxies), but Hubble's
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early- and late-type labels are still used. </para>
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<para>
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Once thought to be a simple galaxy type, ellipticals are now known to
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be quite complex objects. Part of this complexity is due
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to their amazing history: ellipticals are thought to be the end
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product of the merger of two Spiral galaxies. You can
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view a computer simulation MPEG movie of such a merger at <ulink
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url="http://oposite.stsci.edu/pubinfo/pr/2002/11/vid/v0211d3.mpg">
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this NASA HST webpage</ulink> (warning: the file is 3.4 MB).
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</para>
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<para>
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Elliptical galaxies span a very wide range of sizes and
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luminosities, from giant Ellipticals hundreds of thousands of light
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years across and nearly a trillion times brighter than the sun, to
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dwarf Ellipticals just a bit brighter than the average globular
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cluster. They are divided to several morphological classes:
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</para>
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<variablelist>
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<varlistentry>
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<term>cD galaxies:</term>
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<listitem><para>
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Immense and bright objects that can
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measure nearly 1 Megaparsec (3 million light years) across. These
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titans are only found near the centers of large, dense clusters of
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galaxies, and are likely the result of many galaxy
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mergers.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term>Normal Elliptical galaxies</term>
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<listitem><para>Condensed Object with
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relatively high central surface brightness. They include the giant
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ellipticals (gE'e), intermediate-luminosity ellipticals (E's), and
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compact ellipticals.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term>Dwarf elliptical galaxies (dE's)</term>
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<listitem><para> This class of
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galaxies is fundamentally different from normal ellipticals. Their
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diameters on the order of 1 to 10 kiloparsec with surface brightness
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that is much lower than normal ellipticals, giving them a much more
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diffuse appearance. They display the same characteristic gradual
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decline of star density from a relatively dense core out to a diffuse
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periphery.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term>Dwarf spheroidal galaxies (dSph's)</term>
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<listitem><para>Extreme low-luminosity, low
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surface-brightness and have only been observed in the vicinity of the
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Milky Way, and possibly other very nearby galaxy groups, such as the
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Leo group. Their absolute magnitudes are only -8 to -15 mag.
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The Draco dwarf spheroidal galaxy has an absolute magnitude of -8.6,
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making it fainter than the average globular cluster in the Milky Way!
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</para></listitem>
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</varlistentry>
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<varlistentry>
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<term>Blue compact dwarf galaxies (BCD's)</term>
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<listitem>
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<para> Small galaxies that are unusually
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blue. Thehave photometric colors of B-V = 0.0 to 0.30 mag, which is
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typical for relatively young stars of <firstterm>spectral type</firstterm> A.
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This suggests that BCDs
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are currently actively forming stars. These systems also have
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abundant interstellar gas (unlike other Elliptical galaxies).
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</para></listitem>
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</varlistentry>
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</variablelist>
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<tip>
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<para>
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You can see examples of Elliptical galaxies in &kstars;, using the Find
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Object window
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(<keycombo action="simul">&Ctrl;<keycap>F</keycap></keycombo>).
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Search for NGC 4881, which is the Giant cD galaxy in the Coma
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cluster of galaxies. M 86 is a normal Elliptical galaxy in the Virgo
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cluster of galaxies. M 32 is a dwarf Elliptical that is a satellite
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of our neighbor, the Andromeda galaxy (M 31). M 110 is another
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satellite of M 31 that is a borderline dwarf spheroidal galaxy
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(<quote>borderline</quote> because it is somewhat brighter than most other
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dwarf spheroidals).
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</para>
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</tip>
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</sect1>
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