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

Abstract

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 z_gal = 2.0711 associated with a Lyman limit system with weak Mg II (W_r(2796) = 0.24 Å) in quasar field J143040+014939. The galaxy is star-forming (SFR_FUV = 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 z_gal, 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 V_out = 109-588 km s^-1 for half-opening angles of θ₀ = 14°-75°. Assuming a constant V_out, it would take on average t_out ∼ 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.