Galaxy Formation and
Dark Galaxies
John Schreiber
Outline
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Background info (formation/evolution of
galaxies)
Implications of theories
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More dwarf galaxies should be observed
These may exist as difficult-to-detect “dark galaxies”
VIRGOHI21
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Rotating H cloud in the Virgo cluster
Possible dark galaxy
Discrepancies
Background info
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What is a galaxy?
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Basic unit of
cosmology
Can contain stars,
gas, dust, dark matter
Formation of Galaxies
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Before
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As universe expands, density
enhancements either grew or
dispersed.
Smaller, more dense density
enhancements first to
collapse.
These first “lumps” that broke
free contained some H and
He, but mostly dark matter.
Protogalaxies
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When this “lump” collapses
under its own gravity
Appear abt. 14 mill. years b.p.
Should be detectable via their
heat signature
Evolution of Galaxies
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Star Formation
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Clouds of gas orbit in the
protogalaxy
Density of some of these
clouds increases as gas
falls towards the core of the
protogalaxy
Clouds collide
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Primeval galaxy
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Collision creates a high
density region where a star
cluster forms
Remaining gas heated by
stars (dissipates)
Initial rate of star formation
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Slow—cons. ang.
momentum forces a spiral
disk
Fast---elliptoid
Implications of Current Theory
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More dwarf galaxies
should exist than
currently observed
May exist as dark
galaxies
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Starless clouds of H
and dark matter
Observed via radio
waves (w/ wavelength
of ~ 21 cm)
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No dark galaxies
have currently been
discovered
Other possibilities
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Galactic cannibalism
Unaccounted for
environmental factor
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Little empirical info
about “young galaxies”
VIRGOHI21-Dark Galaxy?
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Possible dark galaxy
Rotating cloud in the
Virgo cluster
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Contains enough H to
form 100 mill. Sunsized stars
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Recently located by a
team led by Robert
Minchin
Used the radiotelescope at the
University of
Manchester
After examining it w/
optical
telescope“confident” it
is a dark galaxy
VIRGOHI21-Other Possibilities?
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May not be a dwarf
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May be two passing H clouds
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Mass from dark matter prevents rotation from tearing
galaxies apart
Measured speed of rotation implies ~1/10 dark matter
of Milky Way
This implies 100x more H should be present
Would give illusion of rotation
Counter-arguments
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Few known H clouds
If H gas ionized it would be “invisible to radio
telescopes”
Sources
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http://www.newscientist.com/article.ns?id=dn7056
http://abyss.uoregon.edu/~js/ast123/lectures/lec25.html
http://pinot.phys.uvic.ca/~jfn/mywebpage/gxform.html
http://www.oarval.org/galaxfrm.htm
http://hermes.physics.ox.ac.uk/users/Astrophysics/guide
s/universe/galformation.shtml
http://www.absoluteastronomy.com/encyclopedia/G/Ga/
Galaxy_formation_and_evolution.htm
http://www.astro.cf.ac.uk/computing/parallel/gal2/Sbc.ht
ml