The Qz5 Survey. I. How the HI Mass Density of the Universe Evolves with Cosmic Time

We report that the neutral hydrogen (HI) mass density of the Universe (rho HI) increases with cosmic time since z similar to 5, peaks at z similar to 3, and then decreases toward z similar to 0. This is the first result of Qz5, our spectroscopic survey of 63 quasars at z greater than or similar to 5 with VLT/X-SHOOTER and Keck/ESI aimed at characterizing intervening HI gas absorbers at z similar to 5. The main feature of Qz5 is the high resolution (R similar to 7000-9000) of the spectra, which allows us to (1) accurately detect high column density HI gas absorbers in an increasingly neutral intergalactic medium at z similar to 5 and (2) determine the reliability of previous rho HI measurements derived with lower resolution spectroscopy. We find 5 intervening Damped Ly alpha absorbers (DLAs) at z>4.5, which corresponds to the lowest DLA incidence rate (0.034(0.02)(0.05)) at z greater than or similar to 2. We also measure the lowest rho(HI) at z greater than or similar to 2 from our sample of DLAs and subDLAs, corresponding to rho(HI)=0.56(0.31)(0.82)x10(8 )M(circle dot) Mpc(-3) at z similar to 5. Taking into account our measurements at z similar to 5 and systematic biases in the DLA detection rate at lower spectral resolutions, we conclude that rho HI doubles from z similar to 5 to z similar to 3. From these results emerges a qualitative agreement between how the cosmic densities of HI gas mass, molecular gas mass, and star-formation rate build up with cosmic time.