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

We present results from the Keck Baryonic Structure Survey (KBSS), a unique spectroscopic survey of the distant universe designed to explore the details of the connection between galaxies and intergalactic baryons within the same survey volumes, focusing particularly on scales from ~50 kpc to a few Mpc. The KBSS is optimized for the redshift range z ~ 2-3, combining S/N ~100 Keck/HIRES spectra of 15 of the brightest QSOs in the sky at z ~= 2.5-2.9 with very densely sampled galaxy redshift surveys within a few arcmin of each QSO sightline. In this paper, we present quantitative results on the distribution, column density, kinematics, and absorber line widths of neutral hydrogen (H I) surrounding a subset of 886 KBSS star-forming galaxies with 2.0 <~ z <~ 2.8 and with projected distances <=3 physical Mpc from a QSO sightline. Using Voigt profile decompositions of the full Lyα forest region of all 15 QSO spectra, we compiled a catalog of ~6000 individual absorbers in the redshift range of interest, with 12 <= log (N _{H I} ) <=21. These are used to measure H I absorption statistics near the redshifts of foreground galaxies as a function of projected galactocentric distance from the QSO sightline and for randomly chosen locations in the intergalactic medium (IGM) within the survey volume. We find that N _{H I} and the multiplicity of velocity-associated H I components increase rapidly with decreasing galactocentric impact parameter and as the systemic redshift of the galaxy is approached. The strongest H I absorbers within ~= 100 physical kpc of galaxies have N _{H I} ~3 orders of magnitude higher than those near random locations in the IGM. The circumgalactic zone of most significantly enhanced H I absorption is found within transverse distances of <~ 300 kpc and within ±300 km s^-1 of galaxy systemic redshifts. Taking this region as the defining bounds of the circumgalactic medium (CGM), nearly half of absorbers with log(N _{H I} ) > 15.5 are found within the CGM of galaxies meeting our photometric selection criteria, while their CGM occupy only 1.5% of the cosmic volume. The spatial covering fraction, multiplicity of absorption components, and characteristic N _{H I} remain significantly elevated to transverse distances of ~2 physical Mpc from galaxies in our sample. Absorbers with N _{H I} >10^14.5 cm^-2 are tightly correlated with the positions of galaxies, while absorbers with lower N _{H I} are correlated with galaxy positions only on gsimMpc scales. Redshift anisotropies on these larger scales indicate coherent infall toward galaxy locations, while on scales of ~100 physical kpc peculiar velocities of Δv ~= ±260 km s^-1 with respect to the galaxies are indicated. The median Doppler widths of individual absorbers within 1-3 r _vir of galaxies are larger by ~= 50% than randomly chosen absorbers of the same N _{H I} , suggesting higher gas temperatures and/or increased turbulence likely caused by some combination of accretion shocks and galactic winds around galaxies with M _halo ~= 10¹² M _⊙ at z ~ 2-3.

Based on data obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration, and was made possible by the generous financial support of the W.M. Keck Foundation.