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 ???
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