1. NEUTRAL HYDROGEN
Frank Briggs
RSAA and ATNF
z = 8
z = 0

2. Neutral gas in nearby GALAXIES
Optical +
Radio

3. Neutral gas in nearby GALAXIES
Radio
only
Australia Telescope
Compact Array

4. Neutral gas in nearby GALAXIES
Rotation Curve
Velocity field
from
21cm Line

5. Dark Matter Important Kinematic Tracer Neutral gas in nearby GALAXIES
Rotation Curve
first strong
evidence for
Dark Matter
in galaxies
Velocity field
from
21cm Line
Important
Kinematic
Tracer

9. The “Mice”

10. our view of the Milky Way in the 21cm line

11. the fuel for HI
Recent
Star Formation
Blue galaxies
young stars

12. HI = fuel for star formation
…. galaxies
HI = kinematic tracer
…. deduce galaxy mass distributions

13. .…. ionized……
CMB
Big Bang Nucleosynthesis

14. damped
Lyman-a
abs-lines
21cm
Line
Studies
(HIPASS)

15. looks like HI is an endangered species !!!W
Tot
1/3300 W M
-0.5
looks like HI is an endangered species !!!

16. Where are the missing baryons ???

17. ??? How do you find HI clouds ???
??? How do you find H clouds ???
??? How do you find HI clouds ???
nHI
nHI +nHII
~ 1
If neutral fraction

18. Paschen Balmer Lyman Energy Levels in Hydrogen E = 0 Energy
Ground State

19. Emission Lines: from Photoionization
Lyman
alpha

20. Recombination radiative decay Photo- ionization
Energy Levels in Hydrogen
free electron
Photo-
ionization
Recombination
E = 0
radiative
decay
Energy
Ground State

21. Recombination radiative decay Photo- ionization
Energy Levels in Hydrogen
free electron
Photo-
ionization
free electron
Lose Kinetic Energy!
Recombination
E = 0
radiative
decay
Energy
Ground State

22. Energy Levels in Hydrogen
To excite optical & UV lines
…need at least 10.2 eV
Energy
Hot Stars
Quasars
Ground State

23. DE = 6 x 10 eV Energy Levels in Hydrogen E = 0 Energy
Hyperfine splitting of
H ground state
21 cm line
Ground State
DE = 6 x 10 eV -6
thermally excited by collisions in cold gas

24. Recombination
competes with
Re-ionization

25. Time scales for Recombination …
t = n /R
= n /(n n a ) e p
recomb
recomb e
# Electrons /cm 3
# Recombinations /sec /cm 3

26. Time scales for Recombination in IGM…
t = n /R
= n /(n n a ) e p
recomb
recomb e
= 1 /( n a ) p
recomb
= T /( n 2 x ) sec p
1/2
-11
but, n ~ n proportional ~ (1+z) 3 p
baryon
t proportional T /(1+z)
recomb 3
1/2

27. Time scales for Recombination …
t = n /R
= n /(n n a ) e p
recomb
recomb e
t proportional T /(1+z)
recomb 3
1/2
at z=9 t ~ yrs
recomb 8
at z=0 t ~ yrs
recomb 11

28. F Stars & HI gas Cartoon for Z = 0 Sea of ionized baryons
?? Just HI ??
Stars & HI gas
i.e.

29. Is there an invisible population of LSB but HI rich clouds ???
How could it escape notice ???

30. Radio spectroscopy

31. + info on HI Content of galaxies …
Optical Luminosity Function
(2df Madgwick etal)
+ info on HI Content of galaxies …
Absolute Magnitude

32. HI Mass Function: (dashed curve)

33. HI Content according to
galaxy mass

34. “Blind” surveys for HI clouds…
Arecibo
Telescope

35. Feed Systems

36. Arecibo
Telescope
… very good for looking straight up !

37. Frequency
Time

39. Arecibo Survey results Log( M /M ) (Zwaan et al 1997) 65 detections HI
solar
65 detections
(Zwaan et al 1997)

40. Arecibo: HI content according to galaxy mass
(Zwaan et al 1997)

41. where are HI selected
galaxies located ???
Follow the same
Large Scale Structure
as optically selected…
… except in rich clusters,
which are HI deficient

42. Latest (and greatest) survey: HIPASS
Consortium: ATNF, Melbourne, Cardiff, RSAA +

43. Latest (and greatest) survey: HIPASS
Results: detected 1000’s of galaxies… no clear IG HI Clouds

45. 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