The Qz5 Survey (I): How the HI Mass Density of the Universe Evolves With Cosmic Time

Abstract

We report that the neutral hydrogen (HI) mass density of the Universe (${\rho }_{HI}$) increases with cosmic time since $z\sim 5$, peaks at $z\sim 3$, and then decreases toward $z\sim 0$. This is the first result of Qz5, our spectroscopic survey of 63 quasars at $z\gtrsim 5$ with VLT/X-SHOOTER and Keck/ESI aimed at characterizing intervening HI gas absorbers at $z\sim 5$. The main feature of Qz5 is the high resolution ($R\sim 7000-9000$) of the spectra, which allows us to (1) accurately detect high column density HI gas absorbers in an increasingly neutral intergalactic medium at $z\sim 5$ 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 $z\gtrsim 2$. We also measure the lowest ${\rho }_{HI}$ at $z\gtrsim 2$ from our sample of DLAs and subDLAs, corresponding to ${\rho }_{HI}=0.56_{0.31}^{0.82}\times 10^8$M${}_{\odot }$Mpc${}^{-3}$ at $z\sim 5$. Taking into account our measurements at $z\sim 5$ and systematic biases in the DLA detection rate at lower spectral resolutions, we conclude that ${\rho }_{HI}$ doubles from $z\sim 5$ to $z\sim 3$. 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.

Accepted for publication in ApJ. Main text is composed of 18 pages and 8 figures

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