The Gaseous Environment of High-z Galaxies: Precision Measurements of Neutral Hydrogen in the Circumgalactic Medium of z ~ 2-3 Galaxies in the Keck Baryonic Structure Survey

Abstract: We present results from the Keck Baryonic Structure Survey (KBSS), a unique spectroscopic survey designed to explore the connection between galaxies and intergalactic baryons. The KBSS is optimized for the redshift range z ~ 2-3, combining S/N ~ 100 Keck/HIRES spectra of 15 hyperluminous QSOs with densely sampled galaxy redshift surveys surrounding each QSO sightline. We perform Voigt profile decomposition of all 6000 HI absorbers within the full Lya forest in the QSO spectra. Here we present the distribution, column density, kinematics, and absorber line widths of HI surrounding 886 star-forming galaxies with 2.0 < z < 2.8 and within 3 Mpc of a QSO sightline. We find that N_HI and the multiplicity of HI components increase rapidly near galaxies. The strongest HI absorbers within ~ 100 physical kpc of galaxies have N_HI ~ 3 dex higher than those near random locations in the IGM. The circumgalactic zone of most enhanced HI absorption (CGM) is found within 300 kpc and 300 km/s of galaxies. Nearly half of absorbers with log(N_HI) > 15.5 are found within the CGM of galaxies meeting our photometric selection, while their CGM occupy only 1.5% of the cosmic volume. The spatial covering fraction, multiplicity of absorption components, and characteristic N_HI remain elevated to transverse distances of 2 physical Mpc. Absorbers with log(N_HI) > 14.5 are tightly correlated with the positions of galaxies, while absorbers with lower N_HI are correlated only on Mpc scales. Redshift anisotropies on Mpc scales indicate coherent infall toward galaxies, while on scales of ~100 physical kpc peculiar velocities of 260 km/s are indicated. The median Doppler widths of absorbers within 1-3 virial radii of galaxies are ~50% larger than randomly chosen absorbers of the same N_HI, suggesting higher gas temperatures and/or increased turbulence likely caused by accretion shocks and/or galactic winds.

• The paper demonstrates a strong association between high-column density H I absorbers and galaxy systemic velocities, with a distinct peak at ±300 km/s.
• The paper finds that H I column density declines with increasing galactocentric distance, showing excess absorption within 300 kpc of galaxies.
• The paper constrains the CGM to 300 kpc and ±300 km/s, emphasizing the impact of galactic inflows and outflows on gas dynamics at high redshift.

The Gaseous Environment of High-$z$ Galaxies: Precision Measurements of Neutral Hydrogen

This study examines the circumgalactic medium (CGM) of star-forming galaxies at redshifts $z\sim2-3$, utilizing data from the Keck Baryonic Structure Survey (KBSS). The focus is on analyzing neutral hydrogen (\ion{H}{1}) absorption surrounding these high-redshift galaxies, providing insights into the distribution, column density, and kinematics of the gas on scales from $\sim 50$ kpc to a few Mpc. The study leverages high-resolution, high signal-to-noise ratio spectra from 15 quasars to explore these dynamics.

Key Findings

1. Velocity Distribution of Absorbers: The study highlights a correlation between \ion{H}{1} absorbers and galaxy systemic velocities, notably within $\pm700$ km s$^{-1}$. A pronounced peak is observed at $\pm300$ km s$^{-1}$, indicating that higher column density absorbers are more closely associated with galaxy redshifts.
2. Transverse Spatial Distribution: The column density of \ion{H}{1} absorbers decreases with increasing galactocentric distance, with notable excess absorption detected within 300 kpc of galaxies. Beyond this, up to 2 pMpc, a plateau of elevated column densities exists compared to random locations in the intergalactic medium (IGM).
3. High vs. Low Column Density Absorbers: Absorbers with \ > 10$^{14.5}$ cm$^{-2}$ are tightly clustered near galaxies, while those with lower column densities show less correlation, suggesting different origin or physics between high and low-density gas.
4. Strong Physical and Peculiar Velocities: Maps of absorber distributions reveal significant peculiar velocities hinting at galactic outflows and inflows, with indications of coherent inflow on larger scales and outflows extending to $\sim$ 100 kpc.
5. Broader Doppler Widths Near Galaxies: The Doppler parameters ({b_{) of absorbers close to galaxies exceed those in the general IGM, implying either higher gas temperatures or increased turbulence due to phenomena like galactic winds or accretion shocks.

Implications and Future Work

• CGM Definition and Gas Dynamics: The paper proposes defining the CGM as extending to 300 kpc and $\pm300$ km s$^{-1}$, capturing nearly half of the absorbers with \ > 10$^{15.5}$ cm$^{-2}$, albeit occupying only 1.5% of the cosmic volume.
• Comparison with Low-z Data: By contrasting with lower-redshift studies, it supports the view that circumgalactic gas is denser and more dynamically significant around high-$z$ galaxies, reflecting differences in gas accretion and galaxy formation processes.
• Theoretical Challenges: The findings emphasize the need for simulations that accurately portray the multi-scale, multi-phase dynamics of the CGM, accounting for feedback processes, in order to enhance predictions of galaxy evolution and cosmic gas distribution.

This work enhances our understanding of the gas dynamics around high-redshift galaxies and sets a foundation for future studies that combine metal line diagnostics and better simulations to unravel the complex interplay between galaxies and their gaseous environments in the early universe.