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 \sim 2 .
Using the spectra of \simeq 3000 galaxies with redshifts \langle z \rangle = 2.3 \pm 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 \arcsec - 500 \arcsec and spectroscopic redshifts , with \langle z _ { fg } \rangle = 2.23 and \langle z _ { bg } \rangle = 2.57 ( foreground , background redshifts , respectively . )
The ensemble of sightlines and foreground galaxies is used to construct a 2-D map of the mean excess \ion HI Ly \alpha optical depth relative to the intergalactic mean as a function of projected galactocentric distance ( 20 \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } D _ { tran } / { pkpc } \lower 2 % .15 pt \hbox { $ \buildrel < \over { \sim } $ } 4000 ) and line-of-sight velocity.Careful attention to accurate galaxy systemic redshifts , coupled with detailed knowledge of the effective spectral resolution of background-galaxy composite spectra , provides significant information on the line-of-sight kinematics of \ion 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 \ion HI .
The best-fitting model suggests that galaxy-scale outflows with initial velocity v _ { out } \simeq 600 km s ^ { -1 } dominate the kinematics of circumgalactic \ion HI out to D _ { tran } \simeq 50 kpc , while \ion HI at D _ { tran } \gtrsim 100 kpc is dominated by infall with characteristic v _ { in } \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } v _ { c } , where v _ { c } is the circular velocity of the host halo ( M _ { h } \sim 10 ^ { 12 } M _ { \odot } ) .
Over the impact parameter range 80 \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } D _ { tran } / { pkpc } \lower 2 % .15 pt \hbox { $ \buildrel < \over { \sim } $ } 200 , the \ion 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 } \simeq 300 kpc , the line of sight kinematics are dominated by Hubble expansion .