Column density distribution and cosmological mass density of neutral gas: Sloan Digital Sky Survey-III Data Release 9

Abstract: We present the first results from an ongoing survey for Damped Lyman-alpha systems (DLAs) in the spectra of z>2 quasars observed in the course of the Baryon Oscillation Spectroscopic Survey (BOSS), which is part of the Sloan Digital Sky Survey (SDSS) III. Our full (non-statistical) sample, based on Data Release 9, comprises 12,081 systems with log N(HI)>=20, out of which 6,839 have log N(HI)>=20.3. This is the largest DLA sample ever compiled, superseding that from SDSS-II by a factor of seven. Using a statistical sub-sample and estimating systematics from realistic mock data, we probe the N(HI) distribution at <z> = 2.5. Contrary to what is generally believed, the distribution extends beyond 10^22 cm^-2 with a moderate slope of index\approx-3.5. This result matches surprisingly well the opacity-corrected distribution observed at z = 0. The cosmological mass density of neutral gas in DLAs is found to be Omega_g_DLA~10^-3, evolving only mildly over the past 12 billion years.

• The paper analyzes 12,081 DLA systems, including 6,839 with log N(H)≥20.3, setting a benchmark sample seven times larger than previous surveys.
• It examines the column density distribution at z≈2.5 and finds a moderate slope of about -3.5 that extends beyond 10^22 cm⁻².
• The study finds a nearly constant cosmological mass density of ~10⁻³ for DLAs over 2<z<3.5, indicating a continuous gas supply for galaxy formation.

Column Density Distribution and Cosmological Mass Density of Neutral Gas from SDSS-III DR9

This paper focuses on the column density distribution and cosmological mass density of neutral gas as observed from the Sloan Digital Sky Survey-III (SDSS-III) Data Release 9. It specifically targets Damped Lyman-$\alpha$ systems (DLAs), a major reservoir of neutral hydrogen at high redshifts. The study is part of the Baryon Oscillation Spectroscopic Survey (BOSS), which provides an extensive dataset for investigating cosmological phenomena.

Summary of Findings

1. Sample Size and Scope: The dataset comprises 12,081 DLA systems with $\log N(H) \ge 20$, out of which 6,839 systems have $\log N(H) \ge 20.3$. This represents the largest DLA sample to date, surpassing the previous SDSS-II catalogue by a factor of seven.
2. Column Density Distribution: Analysis of a statistical sub-sample reveals that the $N(H)$ distribution at $z = 2.5$ extends beyond $10^{22}$ cm$^{-2}$. Contrary to earlier beliefs that the distribution steepens significantly at higher densities, this study finds a moderate slope with an index of approximately -3.5.
3. Cosmological Mass Density: The study finds that the cosmological mass density of neutral gas in DLAs remains approximately at $10^{-3}$ over redshifts $2 < z < 3.5$. This indicates only a mild evolution over the past 12 billion years, challenging earlier claims of a significant decrease in the density of neutral gas from $z \sim 3.5$ to $z = 2.2$.
4. Systematic Corrections: The researchers employ realistic mock simulations to estimate and correct systematic biases in the data, achieving a reported completeness and purity above 95% for systems with $\log N(H) \ge 20.3$. Through systematic corrections, they validate their measurements of the column density distribution.

Implications and Speculations

• Galactic Evolution Models: The findings have significant implications for models of galaxy formation and evolution. The observed constancy in the neutral gas mass density suggests a steady supply of material for star formation, which must be factored into simulations of galactic mass build-up over time.
• High Redshift Universe: The extension of the column density distribution beyond $10^{22}$ cm$^{-2}$ emphasizes the need to understand physical processes in high-density environments, including the conversion of atomic to molecular hydrogen, possible turbulence effects, and feedback from stellar processes.
• Future Surveys: As BOSS continues to collect data, the sample size is expected to more than double, which will further refine the understanding of DLAs and their role in the cosmic structure. Improvements in spectrograph technology and deeper surveys could reveal even larger column density systems, enhancing the current model constraints.

This paper provides a robust foundation for further exploration into the distribution of neutral gas across cosmic time and how it governs the evolution of the universe’s structure. It underscores the complexity and continuity of neutral gas dynamics and their integral role in astrophysical and cosmological research.