The Keck Baryonic Structure Survey: Using foreground/background galaxy pairs to trace the structure and kinematics of circumgalactic neutral hydrogen at \(z \sim 2\)

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 ~ 2. Using the spectra of ~ 3000 galaxies with redshifts +/- 0.4 from the Keck Baryonic Structure Survey (KBSS), 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 = 2.23 and = 2.57. The ensemble of sightlines and foreground galaxies is used to construct a 2D map of the mean excess Ly\(\alpha\) optical depth relative to the intergalactic mean as a function of projected galactocentric distance (20 < \(D_{tran}\)/pkpc < 4000) and line-of-sight velocity. We provide information on the line-of-sight kinematics of H I 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 H I. The best-fitting model suggests that galaxy-scale outflows with initial velocity \(v_{out}\) ~ 600 km/s dominate the kinematics of circumgalactic H I out to \(D_{tran}\) ~ 50 kpc, while H I at \(D_{tran}\) > 100 kpc is dominated by infall with characteristic \(v_{in}\) < \(v_c\), where \(v_c\) is the circular velocity of the host halo (\(M_h\) ~ \(10^{12} M_\odot\)). Over the impact parameter range 80 < \(D_{tran}\)/pkpc < 200, the H I 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 \(D_{tran}\) ~ 300 kpc, the line of sight kinematics are dominated by Hubble expansion.