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Abstract We report that the neutral hydrogen (Hi) mass density of the Universe (ρ_Hi) increases with cosmic time sincez ∼ 5, peaks atz ∼ 3, and then decreases towardz ∼ 0. This is the first result of Qz5, our spectroscopic survey of 63 quasars atz ≳ 5 with VLT/X-SHOOTER and Keck/ESI aimed at characterizing intervening Higas absorbers atz ∼ 5. The main feature of Qz5 is the high resolution (R ∼ 7000–9000) of the spectra, which allows us to (1) accurately detect high column density Higas absorbers in an increasingly neutral intergalactic medium atz ∼ 5 and (2) determine the reliability of previousρ_Himeasurements derived with lower resolution spectroscopy. We find five intervening damped Lyαabsorbers (DLAs) atz > 4.5, which corresponds to the lowest DLA incidence rate ( $0.034_{0.02}^{0.05}$) atz ≳ 2. We also measure the lowestρ_Hiatz ≳ 2 from our sample of DLAs and subDLAs, corresponding toρ_Hi $=0.56_{0.31}^{0.82}\times 10^8{M}_{\odot }$Mpc⁻³atz ∼ 5. Taking into account our measurements atz ∼ 5 and systematic biases in the DLA detection rate at lower spectral resolutions, we conclude thatρ_Hidoubles fromz ∼ 5 toz ∼ 3. From these results emerges a qualitative agreement between how the cosmic densities of Higas mass, molecular gas mass, and star formation rate build up with cosmic time. 

Abstract We present Keck Cosmic Web Imager Lyαintegral field spectroscopy of the fields surrounding 14 damped Lyαabsorbers (DLAs) atz≈ 2. Of these 14 DLAs, nine have high metallicities ([M/H] > − 0.3), and four of those nine feature a CO-emitting galaxy at an impact parameter ≲30 kpc. Our search reaches median Lyαline flux sensitivities of ∼2 × 10⁻¹⁷erg s⁻¹cm⁻²over apertures of ∼6 kpc and out to impact parameters of ∼50 kpc. We recover the Lyαflux of three known Lyα-emitting Hi-selected galaxies in our sample. In addition, we find two Lyαemitters at impact parameters of ≈50–70 kpc from the high-metallicity DLA atz≈ 1.96 toward QSO B0551-366. This field also contains a massive CO-emitting galaxy at an impact parameter of ≈15 kpc. Apart from the field with QSO B0551-366, we do not detect significant Lyαemission in any of the remaining eight high-metallicity DLA fields. Considering the depth of our observations and our ability to recover previously known Lyαemitters, we conclude that Hi-selected galaxies associated with high-metallicity DLAs atz≈ 2 are dusty and therefore might feature low Lyαescape fractions. Our results indicate that complementary approaches—using Lyα, CO, Hα, and [Cii] 158μm emission—are necessary to identify the wide range of galaxy types associated withz≈ 2 DLAs.