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We present new measurements of the spatial distribution and kinematics ofneutral hydrogen in the circumgalactic and intergalactic medium surroundingstar-forming galaxies at z ~ 2. Using the spectra of ~ 3000 galaxies withredshifts <z> +/- 0.4 from the Keck Baryonic Structure Survey (KBSS), weassemble a sample of more than 200,000 distinct foreground-background pairswith projected angular separations of 3 - 500 arcsec and spectroscopicredshifts, with <$z_{fg}$> = 2.23 and <$z_{bg}$> = 2.57. The ensemble ofsightlines and foreground galaxies is used to construct a 2D map of the meanexcess Ly$\alpha$ optical depth relative to the intergalactic mean as afunction of projected galactocentric distance (20 < $D_{tran}$/pkpc < 4000) andline-of-sight velocity. We provide information on the line-of-sight kinematicsof H I gas as a function of projected distance $D_{tran}$. We compare the mapwith cosmological zoom-in simulation, finding qualitative agreement betweenthem. A simple two-component (accretion, outflow) analytical model generallyreproduces the observed line-of-sight kinematics and projected spatialdistribution of H I. The best-fitting model suggests that galaxy-scale outflowswith initial velocity $v_{out}$ ~ 600 km/s dominate the kinematics ofcircumgalactic H I out to $D_{tran}$ ~ 50 kpc, while H I at $D_{tran}$ > 100kpc is dominated by infall with characteristic $v_{in}$ < $v_c$, where $v_c$ isthe circular velocity of the host halo ($M_h$ ~ $10^{12} M_\odot$). Over theimpact parameter range 80 < $D_{tran}$/pkpc < 200, the H I line-of-sightvelocity range reaches a minimum, with a corresponding flattening in therest-frame Ly$\alpha$ equivalent width. These observations can be naturallyexplained as the transition between outflow-dominated and accretion-dominatedflows. Beyond $D_{tran}$ ~ 300 kpc, the line of sight kinematics are dominatedby Hubble expansion.

monthly notices of the royal astronomical society

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