THE GASEOUS ENVIRONMENT OF HIGH-zGALAXIES: PRECISION MEASUREMENTS OF NEUTRAL HYDROGEN IN THE CIRCUMGALACTIC MEDIUM OFz∼ 2-3 GALAXIES IN THE KECK BARYONIC STRUCTURE SURVEY

Abstract

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 (NH 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 NH 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 NH 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(NH 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 NH i remain significantly elevated to transverse distances of ∼2 physical Mpc from galaxies in our sample. Absorbers with NH i >1014.5 cm−2 are tightly correlated with the positions of galaxies, while absorbers with lower NH i are correlated with galaxy positions only on ≳Mpc 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 rvir of galaxies are larger by ≃ 50% than randomly chosen absorbers of the same NH i, suggesting higher gas temperatures and/or increased turbulence likely caused by some combination of accretion shocks and galactic winds around galaxies with Mhalo ≃ 1012 M☉ at z ∼ 2–3.