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

TL;DR

This study uses high-resolution spectra from the Keck Baryonic Structure Survey to analyze neutral hydrogen around high-redshift galaxies, revealing dense, dynamic circumgalactic environments within 300 kpc of galaxies at z ~ 2-3.

Contribution

It provides the first detailed measurements of HI distribution, kinematics, and absorption properties in the circumgalactic medium of galaxies at z ~ 2-3, highlighting the extent and complexity of the CGM.

Findings

HI absorption is significantly enhanced near galaxies within 300 kpc.

Nearly half of strong HI absorbers are located within the CGM of galaxies.

Absorber widths suggest higher temperatures or turbulence near galaxies.

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.