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 $z_{\mathrm{gal}}=2.0711$ associated with a Lyman limit system with weak MgII ($W_r(2796)=0.24$ Ang) in quasar field J143040$+$014939. The galaxy is star-forming (${\mathrm{SFR}}_{\mathrm{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 ($\Phi =89^{\circ }$). From photoionization modeling of the absorption system, we infer a total line-of-sight CGM metallicity of $[Si/H]=-1.5_{-0.3}^{+0.4}$. The absorption system is roughly kinematically symmetric about $z_{\mathrm{gal}}$, with a full MgII velocity spread of $\sim 210$ 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 $V_{\mathrm{out}}=109-588$ km s${}^{-1}$ for half opening angles of ${\theta }_0=14^{\circ }-75^{\circ }$. Assuming a constant $V_{\mathrm{out}}$, it would take on average $t_{\mathrm{out}}\sim 111-597$ Myr for the gas to reach 66 kpc. The outflow is energetic, with a mass outflow rate of ${\dot{M}}_{\mathrm{out}}<52\pm 37$ M${}_{\odot }$ yr${}^{-1}$ and mass loading factor of $\eta <1.4\pm 1.0$. We aim to build a sample of $\sim 50$ MgII absorber--galaxy pairs at this epoch to better understand gas flows when they are most actively building galaxies.

30 pages, 9 figures, 5 tables. Accepted for publication in ApJ on 26 October 2020