The Neutral Hydrogen Cosmological Mass Density at z= 5

摘要

We present the largest homogeneous survey of redshift> 4.4 damped Lya systems (DLAs)using the spectra of 163 quasars that comprise the Giant Gemini GMOS(GGG)survey. With this survey we make the most precise high-redshift measurement of the cosmological mass density of neutral hydrogen, Ω_(HI). After correcting for systematic effects using a combination of mock and higher-resolution spectra, we find ΩHI ={formula} at(z)=4.9, assuming a 20% contribution from lower column density systems below the DLA threshold. By comparing to literature measurements at lower redshifts, we show that ΩHI can be described by the functional form ΩHI(z)oc(1 + z)~(0.4). This gradual decrease from z= 5 to 0 suggests that in the galaxies which dominate the cosmic star formation rate, Hi is a transitory gas phase fuelling star formation which must be continually replenished by more highly-ionized gas from the intergalactic medium, and from recycled galactic winds.