The Keck Baryonic Structure Survey: using foreground/background galaxy pairs to trace the structure and kinematics of circumgalactic neutral hydrogen at z ∼ 2

We present new measurements of the spatial distribution and kinematics of neutral hydrogen in the circumgalactic and intergalactic medium surrounding star-forming galaxies at z similar to 2. Using the spectra of similar or equal to 3000 galaxies with redshifts < z > = 2.3 +/- 0.4 from the Keck Baryonic Structure Survey, we assemble a sample of more than 200 000 distinct foreground-background pairs with projected angular separations of 3-500 arcsec and spectroscopic redshifts, with < z(fg)> = 2.23 and < z(bg)> = 2.57 (foreground, background redshifts, respectively.) The ensemble of sightlines and foreground galaxies is used to construct a 2D map of the mean excess HI Ly alpha optical depth relative to the intergalactic mean as a function of projected galactocentric distance (20 less than or similar to D-tran/pkpc less than or similar to 4000) and line-of-sight velocity. We obtain accurate galaxy systemic redshifts, providing significant information on the line-of-sight kinematics of HI gas as a function of projected distance D-tran. We compare the map with cosmological zoom-in simulation, finding qualitative agreement between them. A simple two-component (accretion, outflow) analytical model generally reproduces the observed line-of-sight kinematics and projected spatial distribution of HI. The best-fitting model suggests that galaxy-scale outflows with initial velocity v(out) similar or equal to 600 km s(-1) dominate the kinematics of circumgalactic HI out to D-tran similar or equal to 50 kpc, while HI at D-tran greater than or similar to 100 kpc is dominated by infall with characteristic v(in) less than or similar to circular velocity. Over the impact parameter range 80 less than or similar to D-tran/pkpc less than or similar to 200, the HI line-of-sight velocity range reaches a minimum, with a corresponding flattening in the rest-frame Ly alpha equivalent width. These observations can be naturally explained as the transition between outflow-dominated and accretion-dominated flows. Beyond D-tran similar or equal to 300 pkpc (similar to 1 cMpc), the line-of-sight kinematics are dominated by Hubble expansion.