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 similar to 50 kpc to a few Mpc. The KBSS is optimized for the redshift range z similar to 2-3, combining S/N similar to 100 Keck/HIRES spectra of 15 of the brightest QSOs in the sky at z similar or equal to 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 less than or similar to z less than or similar to 2.8 and with projected distances <= 3 physical Mpc from a QSO sightline. Using Voigt profile decompositions of the full Ly alpha forest region of all 15 QSO spectra, we compiled a catalog of similar to 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 similar or equal to 100 physical kpc of galaxies have N-H I similar to 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 less than or similar to 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 similar to 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 greater than or similar to Mpc scales. Redshift anisotropies on these larger scales indicate coherent infall toward galaxy locations, while on scales of similar to 100 physical kpc peculiar velocities of Delta v similar or equal to +/- 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 similar or equal to 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 similar or equal to 10(12) M-circle dot at z similar to 2-3.