Circumgalactic Mg II Emission from an Isotropic Starburst Galaxy Outflow
  Mapped by KCWI
  We present spatially-resolved spectroscopy from the Keck Cosmic Web Imager
(KCWI) of a star-forming galaxy at z=0.6942, which shows emission from the Mg
II 2796, 2803 Angstrom doublet in the circumgalactic medium (CGM) extending ~37
kpc at 3-sigma significance in individual spaxels (1-sigma detection limit 4.8
x 10^{-19} erg s^-1 cm^-2 arcsec^-2). After deconvolution with the seeing, we
obtain 5-sigma detections extending for ~31 kpc measured in 7-spaxel (1.1
arcsec^2) apertures. Spaxels covering the galaxy stellar regions show clear
P-Cygni-like emission/absorption profiles with the blueshifted absorption
extending to relative velocities of v = -800 km/s; however, the P-Cygni
profiles give way to pure emission at large radii from the central galaxy. We
have performed three-dimensional radiative transfer modeling to infer the
geometry and velocity and density profiles of the outflowing gas. Our
observations are most consistent with an isotropic outflow rather than
biconical wind models with half-opening angles phi <= 80 deg. Furthermore, our
modeling suggests that a wind velocity profile that decreases with radius is
necessary to reproduce the velocity widths and strengths of Mg II line emission
profiles at large circumgalactic radii. The extent of the Mg II emission we
measure directly is further corroborated by our modeling, where we rule out
outflow models with extent <30 kpc.
