2020MNRAS.499.1721C

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 <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 H I Lyαoptical depth relative to the intergalactic mean as a function of projected galactocentric distance (20 <= D_tran/pkpc <= 4000) and line-of-sight velocity. We obtain accurate galaxy systemic redshifts, providing significant 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^–1 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 <= circular velocity. 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 Lyαequivalent width. These observations can be naturally explained as the transition between outflow-dominated and accretion-dominated flows. Beyond D_tran ≃ 300 pkpc (∼1 cMpc), the line-of-sight kinematics are dominated by Hubble expansion.