We use a semi-analytical model to study the impact of reionization , and the associated radiative feedback , on galaxy formation .
Two feedback models have been considered : ( i ) a standard prescription , according to which star formation is totally suppressed in galaxies with circular velocity below a critical threshold ( model CF06 ) and ( ii ) a characterization based on the filtering scale ( model G00 ) , allowing for a gradual reduction of the gas available for star formation in low-mass galaxies .
In model CF06 reionization starts at z \lower 2.0 pt \hbox { $ { < \atop \hbox { \raise 4.0 pt \hbox { $ \sim$ } } } $ } 15 - 20 , is 85 % complete by z \sim 10 ; at the same z , the ionized fraction is 16 \% in model G00 .
The models match SDSS constraints on the evolution of the neutral hydrogen fraction at z < 7 , but predict different Thomson optical depths , \tau _ { \mathrm { e } } = 0.1017 ( CF06 ) , and 0.0631 ( G00 ) ; such values are within 1 \sigma of the WMAP 3-yr determination .
Both models are in remarkable good agreement with additional existing data ( evolution of Lyman-limit systems , cosmic star formation history , high- z galaxy counts , IGM thermal history ) , which therefore can not be used to discriminate among different feedback models .
Deviations among radiative feedback prescriptions emerge when considering the expected HI 21 cm background signal , where a \sim 15 mK absorption feature in the range 75-100 MHz is present in model G00 and a global shift of the emission feature preceding reionization towards larger frequencies occurs in the same model .
Single dish observations with existing or forthcoming low-frequency radio telescopes can achieve mK sensitivity , allowing the identification of these features provided that foregrounds can be accurately subtracted .