The CGM at Cosmic Noon with KCWI: Outflows from a Star-forming Galaxy at z = 2.071

We present the first results from our "CGM at Cosmic Noon with KCWI" program to study gas flows in the circumgalactic medium (CGM) at z = 2–3. Combining the power of a high-resolution VLT/UVES quasar spectrum, a Hubble Space Telescope/Advanced Camera for Surveys image, and integral field spectroscopy with Keck/KCWI (Keck Cosmic Web Imager), we detected Lyα emission from a 1.7L* galaxy at zgal = 2.0711 associated with a Lyman limit system with weak Mg ii (Wr(2796) = 0.24 Å) in quasar field J143040+014939. The galaxy is star-forming (SFRFUV = 37.8 M⊙ yr−1) and clumpy: either an edge-on disk (i = 85°) or, less likely, a major merger. The background quasar probes the galaxy at an impact parameter of D = 66 kpc along the projected galaxy minor axis (Φ = 89°). From photoionization modeling of the absorption system, we infer a total line-of-sight CGM metallicity of $[\mathrm{Si}/{\rm{H}}]=-{1.5}_{-0.3}^{+0.4}$. The absorption system is roughly kinematically symmetric about zgal, with a full Mg ii velocity spread of ∼210 km s−1. Given the galaxy–quasar orientation, CGM metallicity, and gas kinematics, we interpret this gas as an outflow that has likely swept up additional material. By modeling the absorption as a polar outflow cone, we find the gas is decelerating with average radial velocity Vout = 109–588 km s−1 for half-opening angles of θ0 = 14°–75°. Assuming a constant Vout, it would take on average tout ∼ 111–597 Myr for the gas to reach 66 kpc. The outflow is energetic, with a mass outflow rate of ${\dot{M}}_{\mathrm{out}}\lt 52\pm 37$ M⊙ yr−1 and mass loading factor of η < 1.4 ± 1.0. We aim to build a sample of ∼50 Mg ii absorber–galaxy pairs at this epoch to better understand gas flows when they are most actively building galaxies.