neutral hydrogen frank briggs rsaa and atnf z = 8 z = 0
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NEUTRAL HYDROGENNEUTRAL HYDROGEN
Frank Briggs
RSAA and ATNF
z = 8 z = 0
Neutral gas in nearby GALAXIES
Optical + Radio
Neutral gas in nearby GALAXIES
Radio only
Australia TelescopeCompact Array
Neutral gas in nearby GALAXIES
Velocity field from 21cm Line
Rotation Curve
Neutral gas in nearby GALAXIES
Velocity field from 21cm Line
Rotation Curvefirst strong evidence for
Dark Matter in galaxies
Important KinematicTracer
The “Mice”
our view of the Milky Way in the 21cm line
HI Blue galaxies young stars RecentStar Formation
the fuel for
• HI = fuel for star formation …. galaxies
• HI = kinematic tracer …. deduce galaxy mass distributions
Big Bang Nucleosynthesis
.….
ion
ized
……
CMB
dampedLyman-abs-lines
21cm Line Studies (HIPASS)
M-0.5
Tot 0
1/3300
looks like HI is an endangered species !!!
Where are the missing baryons ???Where are the missing baryons ???
??? How do you find H clouds ???
??? How do you find HI clouds ???
nHI
nHI +nHII
1
If neutral fraction
Energy Levels in Hydrogen
E = 0
Ground State
En
erg
y
Lyman
BalmerPaschen
Emission Lines: from Photoionization
Lymanalpha
Energy Levels in Hydrogen
E = 0
Ground State
En
erg
yfree electron
Photo-ionization
Recombination
radiative decay
Energy Levels in Hydrogen
E = 0
Ground State
En
erg
yfree electron
Photo-ionization
Recombination
radiative decay
free electronLose Kinetic Energy!
Energy Levels in Hydrogen
E = 0
Ground State
En
erg
y To excite optical & UV lines …need at least 10.2 eV
• Hot Stars• Quasars
Energy Levels in Hydrogen
E = 0
Ground State
En
erg
y
Hyperfine splitting of H ground state
21 cm line
E = 6 x 10 eV-6
thermally excited by collisions in cold gas
Recombination competes with Re-ionization
Time scales for Recombination …
t = n /Rrecomb e
# Electrons /cm3
# Recombinations /sec /cm3= n /(n n )e e p recomb
Time scales for Recombination in IGM…
t = n /Rrecomb e = n /(n n )e e p recomb
= 1 /( n ) p recomb
= T /( n 2 x 10) sec p1/2 -11
but, n ~ n proportional ~ (1+z)3 p baryon
t proportional T /(1+z)recomb31/2
Time scales for Recombination …
t = n /Rrecomb e = n /(n n )e e p recomb
t proportional T /(1+z)recomb31/2
at z=9 t ~ 10 yrsrecomb8
at z=0 t ~ 10 yrsrecomb11
i.e.
Sea of ionized baryons
StarsStars & HI gasHI gas?? Just HI ??
Cartoon for Z = 0
Is there an invisible population of LSB but HI rich clouds ???
How could it escape notice ???
Radio spectroscopy
Absolute Magnitude
Optical Luminosity Function
+ info on HI Content of galaxies …
(2df Madgwick etal)
HI Mass Function: (dashed curve)
HI Content according to galaxy mass
““Blind” surveys for HI clouds…Blind” surveys for HI clouds… AreciboTelescope
Feed Systems
AreciboTelescope
… very good for looking straight up !
Frequ
ency
Time
Arecibo Survey results
65 detections
(Zwaan et al 1997)
Log( M /M )HI solar
Arecibo: HI content according to galaxy mass(Zwaan et al 1997)
where are HI selectedgalaxies located ???
Follow the same Large Scale Structure as optically selected…
… except in rich clusters, which are HI deficient
Latest (and greatest) survey: HIPASS
Consortium: ATNF, Melbourne, Cardiff, RSAA +
Latest (and greatest) survey: HIPASS
Results: detected 1000’s of galaxies… no clear IG HI Clouds
Summary of HI in nearby (z~0) 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 => at z=0, HI is inevitably associated with stars