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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 report statistically significant detection of Hi21 cm emission from intermediate-redshift (z ≈ 0.2–0.6) galaxies. By leveraging multisightline galaxy survey data from the Cosmic Ultraviolet Baryon Survey and deep radio observations from the MeerKAT Absorption Line Survey, we have established a sample of ≈6000 spectroscopically identified galaxies in 11 distinct fields to constrain the neutral gas content at intermediate redshifts. The galaxies sample a broad range in stellar mass, from $\log M_{\mathrm{star}}/M_{\odot }\approx 8$to $\log M_{\mathrm{star}}/M_{\odot }\approx 11$, with a median of ${〈\log M_{\mathrm{star}}/M_{\odot }〉}_{\mathrm{med}}\approx 10$and a wide range in redshift fromz ≈ 0.24 toz ≈ 0.63 with a median of 〈z〉_med = 0.44. While no individual galaxies show detectable Hiemission, the emission line signal is detected in the stacked spectra of all subsamples at greater than 4σsignificance. The observed total Hi21 cm line flux translates to a Himass,M_{H I}≈10¹⁰M_⊙. We find a high Hi-to-stellar-mass ratio ofM_HI/M_star ≈ 6 for low-mass galaxies with $〈\log M_{\mathrm{star}}/M_{\odot }〉\approx 9.3$(>3.7σ). For galaxies with $〈\log M_{\mathrm{star}}/M_{\odot }〉\approx 10.6$, we findM_HI/M_star ≈ 0.3 (>4.7σ). In addition, the redshift evolution of Himass, 〈M_{H I}〉, in both low- and high-mass field galaxies, inferred from the stacked emission-line signal, aligns well with the expectation from the cosmic star formation history. This suggests that the overall decline in the cosmic star formation activity across the general galaxy population may be connected to a decreasing supply of neutral hydrogen. Finally, our analysis has revealed significant 21 cm signals at distances greater than 75 kpc from these intermediate-redshift galaxies, indicating a substantial reservoir of Higas in their extended surroundings. 

ABSTRACT We present the measured gas-phase metal column densities in 155 sub-damped Ly α systems (subDLAs) with the aim to investigate the contribution of subDLAs to the chemical evolution of the Universe. The sample was identified within the absorber-blind XQ-100 quasar spectroscopic survey over the redshift range 2.4 ≤ zabs ≤ 4.3. Using all available column densities of the ionic species investigated (mainly C iv, Si ii, Mg ii, Si iv, Al ii, Fe ii, C ii, and O i; in order of decreasing detection frequency), we estimate the ionization-corrected gas-phase metallicity of each system using Markov chain Monte Carlo techniques to explore a large grid of cloudy ionization models. Without accounting for ionization and dust depletion effects, we find that the H i-weighted gas-phase metallicity evolution of subDLAs is consistent with damped Ly α systems (DLAs). When ionization corrections are included, subDLAs are systematically more metal poor than DLAs (between ≈0.5σ and ≈3σ significance) by up to ≈1.0 dex over the redshift range 3 ≤ zabs ≤ 4.3. The correlation of gas phase [Si/Fe] with metallicity in subDLAs appears to be consistent with that of DLAs, suggesting that the two classes of absorbers have a similar relative dust depletion pattern. As previously seen for Lyman limit systems, the gas phase [C/O] in subDLAs remains constantly solar for all metallicities indicating that both subDLAs and Lyman limit systems could trace carbon-rich ejecta, potentially in circumgalactic environments. 

Abstract Sub-damped Lyman α systems (subDLAs; H i column densities of 19.0 ≤ logN(H i) < 20.3) are rarely included in the cosmic H i census performed at redshifts zabs ≳ 1.5, yet are expected to contribute significantly to the overall H i mass budget of the Universe. In this paper, we present a blindly selected sample of 155 subDLAs found along 100 quasar sightlines (with a redshift path-length ΔX = 475) in the XQ-100 legacy survey to investigate the contribution of subDLAs to the H i mass density of the Universe. The impact of X-Shooter’s spectral resolution on Ly α absorber identification is evaluated, and found to be sufficient for reliably finding absorbers down to a column density of logN(H i) ≥ 18.9. We compared the implications of searching for subDLAs solely using H i absorption versus the use of metal lines to confirm the identification, and found that metal-selection techniques would have missed 75 subDLAs. Using a bootstrap Monte Carlo simulation, we computed the column density distribution function (f(N, X)) and the cosmological H i mass density ($$\Omega _{\rm H\,{\small I}}$$) of subDLAs and compared with our previous work based on the XQ-100 damped Lyman α systems. We do not find any significant redshift evolution in f(N, X) or $$\Omega _{\rm H\,{\small I}}$$ for subDLAs. However, subDLAs contribute 10–20 per cent of the total $$\Omega _{\rm H\,{\small I}}$$ measured at redshifts 2 < z < 5, and thus have a small but significant contribution to the H i budget of the Universe.