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, an HST/ACS image, and integral field spectroscopy with Keck/KCWI, we detected Lya emission from a $1.7L_{\ast}$ galaxy at $z_{\rm gal}=2.0711$ associated with a Lyman limit system with weak MgII ($W_r(2796)=0.24$ Ang) in quasar field J143040$+$014939. The galaxy is star-forming (${\rm SFR}_{\rm FUV}=37.8$ M$_{\odot}$ yr$^{-1}$) and clumpy: either an edge-on disk ($i=85^{\circ}$) 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 ($Φ=89^{\circ}$). From photoionization modeling of the absorption system, we infer a total line-of-sight CGM metallicity of ${\rm [Si/H]}=-1.5^{+0.4}_{-0.3}$. The absorption system is roughly kinematically symmetric about $z_{\rm gal}$, with a full MgII velocity spread of $\sim210$ 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 $V_{\rm out}=109-588$ km s$^{-1}$ for half opening angles of $θ_0=14^{\circ}-75^{\circ}$. Assuming a constant $V_{\rm out}$, it would take on average $t_{\rm out}\sim111-597$ Myr for the gas to reach 66 kpc. The outflow is energetic, with a mass outflow rate of $\dot{M}_{\rm out}<52{\pm37}$ M$_{\odot}$ yr$^{-1}$ and mass loading factor of $η<1.4{\pm1.0}$. We aim to build a sample of $\sim50$ MgII absorber--galaxy pairs at this epoch to better understand gas flows when they are most actively building galaxies.