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

Nikole M. Nielsen, Glenn G. Kacprzak, Stephanie K. Pointon, Michael T. Murphy, Christopher W. Churchill, Romeel Davé

Published 2020-02-20Version 1

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 $3.7L_{\ast}$ 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$+$014939. The galaxy is best modeled as a star-forming (SFR$_{\rm FUV}=37.8$ M$_{\odot}$ yr$^{-1}$) clumpy edge-on disk ($i=85^{\circ}$). 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, which includes Lya, MgII, SiII, SiIII, SiIV, and CIV, 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 the galaxy redshift, 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 outflowing from the host galaxy. By modeling the absorption as a polar outflow cone, we find the gas is decelerating with radial velocity $V_{\rm out}=100-350$ km s$^{-1}$ for half opening angles of $\theta_0=20^{\circ}-80^{\circ}$. Assuming a constant $V_{\rm out}$, it would take $\sim190-650$ Myr for the gas to reach 66 kpc. The outflow is energetic, with a mass outflow rate of $\dot{M}_{\rm out}=45-51$ M$_{\odot}$ yr$^{-1}$ and mass loading factor of $\eta\sim1.3$. 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.