NEUTRAL 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 Telescope Compact Array

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

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

The “Mice”

our view of the Milky Way in the 21cm line

HI Blue galaxies young stars Recent Star Formation the fuel for

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

Big Bang Nucleosynthesis.…. ionized…… CMB

damped Lyman-  abs-lines 21cm Line Studies (HIPASS)

 M  Tot 0 1/3300 looks like HI is an endangered species !!!

Where are the missing baryons ???

??? How do you find H clouds ??? ??? How do you find HI clouds ??? n HI n HI +n HII  1 1 If neutral fraction

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

Emission Lines: from Photoionization Lyman alpha

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

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

Energy Levels in Hydrogen E = 0 Ground State Energy To excite optical & UV lines …need at least 10.2 eV Hot Stars Quasars

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

Recombination competes with Re-ionization

Time scales for Recombination … t = n /R recomb e # Electrons /cm 3 # Recombinations /sec /cm 3 = n /(n n  ) eep recomb

Time scales for Recombination in IGM… t = n /R recomb e = n /(n n  ) eep recomb = 1 /( n  ) p recomb = T /( n 2 x 10  ) sec p 1/2 -11 but, n ~ n proportional ~ (1+z) 3 p baryon t proportional T /(1+z) recomb 3 1/2

Time scales for Recombination … t = n /R recomb e = n /(n n  ) eep recomb t proportional T /(1+z) recomb 3 1/2 at z=9 t ~ 10 yrs recomb 8 at z=0 t ~ 10 yrs recomb 11

i.e.  Sea of ionized baryons StarsHI gas Stars & HI 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… Arecibo Telescope

Feed Systems

Arecibo Telescope … very good for looking straight up !

Frequency 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 selected galaxies 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