NEUTRAL HYDROGEN

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NEUTRAL HYDROGEN
Frank Briggs RSAA and ATNF
z 8
z 0
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Neutral gas in nearby GALAXIES
Optical Radio
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Neutral gas in nearby GALAXIES
Radio only
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Neutral gas in nearby GALAXIES
Velocity field from 21cm Line
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Neutral gas in nearby GALAXIES
first strong evidence for Dark Matter in
galaxies
Velocity field from 21cm Line
Important Kinematic Tracer
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The Mice
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our view of the Milky Way in the 21cm line
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HI
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• HI fuel for star formation
• . galaxies
• HI kinematic tracer
• . deduce galaxy mass
  distributions

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looks like HI is an endangered species !!!
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Where are the missing baryons ???
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??? How do you find H clouds ???
??? How do you find HI clouds ???
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Energy Levels in Hydrogen
E 0
Energy
Ground State
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Energy Levels in Hydrogen
E 0
Energy
Ground State
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Energy Levels in Hydrogen
E 0
Energy
Ground State
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Energy Levels in Hydrogen
E 0
To excite optical UV lines need at least
10.2 eV
Energy
Ground State
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Energy Levels in Hydrogen
E 0
Hyperfine splitting of H ground state
Energy
21 cm line
Ground State
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Recombination competes with
Re-ionization
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Time scales for Recombination
t n /R
recomb
e
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Time scales for Recombination in IGM
t n /R
recomb
e
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Time scales for Recombination
t n /R
recomb
e
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Cartoon for Z 0
Sea of ionized baryons
F
?? Just HI ??
Stars HI gas
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Is there an invisible population of LSB but
HI rich clouds ???
How could it escape notice ???
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Radio spectroscopy
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Optical Luminosity Function
(2df Madgwick etal)
info on HI Content of galaxies
Absolute Magnitude
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HI Mass Function (dashed curve)
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HI Content according to galaxy mass
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Blind surveys for HI clouds
Arecibo Telescope
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Feed Systems
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Arecibo Telescope
very good for looking straight up !
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Frequency
Time
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Arecibo Survey results
Log( M /M )
HI
solar
(Zwaan et al 1997)
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Arecibo HI content according to galaxy mass
(Zwaan et al 1997)
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where are HI selected galaxies located ???
Follow the same Large Scale Structure as
optically selected
except in rich clusters, which are HI
deficient
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Latest (and greatest) survey HIPASS
Consortium ATNF, Melbourne, Cardiff, RSAA
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Latest (and greatest) survey HIPASS
Results detected 1000s of galaxies no clear
IG HI Clouds
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Summary of HI in nearby (z0) Universe

• good tracer of galaxies
• but, only a tiny fraction of total baryons are
  neutral
• looks like hydrogen must be confined, so that
  it can
• shield itself against the ionizing background
• if confined to sufficient density to remain
  neutral,
• the gas is unstable to collapse and star
  formation
• gt at z0, HI is inevitably associated
  with stars