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 Rotation Curve Velocity field from 21cm Line

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

The “Mice”

our view of the Milky Way in the 21cm line

the fuel for HI Recent Star Formation Blue galaxies young stars

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

.…. ionized…… CMB Big Bang Nucleosynthesis

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

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

Where are the missing baryons ???

??? 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

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

Emission Lines: from Photoionization Lyman alpha

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

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

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

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

Recombination competes with Re-ionization

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

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 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 = n /(n n a ) e p recomb recomb e 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

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

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

Radio spectroscopy

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

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 Log( M /M ) (Zwaan et al 1997) 65 detections HI solar 65 detections (Zwaan et al 1997)

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