Mapping the z  ∼  2 Circumgalactic Medium with KBSS Galaxy Pairs *

We present new results on the spatial structure and kinematics of the circumgalactic medium (CGM) of z ∼ 2 star-forming galaxies drawn from the Keck Baryonic Structure Survey, using Ly α and metallic ion absorption recorded in spectra of background galaxies whose sight lines are projected within θ ∼ 30″ (physical distances D tran ≲ 250 kpc) of the foreground galaxy. The sample of 1033 foreground galaxies (〈 z fg 〉 = 2.03 ± 0.36; 8 ≲ log ( M * / M ⊙ ) ≲ 11 ) is probed by the spectra of 736 background galaxies obtained using Keck/KCWI ( R ∼ 1800) and Keck/LRIS ( R ∼ 1200). Using the galaxy pair ensemble, we present measurements of absorption strength ( W λ ) and line-of-sight velocity dispersion ( σ ) as a function of D tran , which together with “down-the-barrel” spectra of the foreground galaxies provide unprecedented maps of neutral hydrogen and ionized metals throughout the CGM of star-forming galaxies at “cosmic noon.” A 2D map of Ly α absorption depth (versus line-of-sight velocity and D tran ) for the full ensemble shows a distinct transition near D tran ∼ 80 kpc (close to the virial radius of the average foreground galaxy) where the line-of-sight velocity dispersion reaches a minimum, and beyond which the Ly α absorption depth flattens. Splitting the sample into subsets based on properties of the foreground galaxies, we find a strong and monotonic correlation of both W λ (C iv ) and σ (C iv ) with stellar mass ( M * ) at all D tran including DTB (i.e., D tran ∼ 0–250 kpc). Taken together, our results suggest that the kinematics, covering fraction, and spatial extent of CGM gas traced by Ly α and (especially) C iv are modulated primarily by halo mass, and that a significant fraction of the CGM gas may be unbound.