2025ApJ...994...85P

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 Lyα 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; $8\lesssim {\mathrm{log}}({M}_{* }/{M}_{\odot })\lesssim 11$) 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 Lyα 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 Lyα 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 Lyα and (especially) C IV are modulated primarily by halo mass, and that a significant fraction of the CGM gas may be unbound.