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 λ λ2796, 2803 doublet in the circumgalactic medium (CGM) extending ∼37 kpc at 3σ significance in individual spaxels (1σ detection limit 4.8 × 10⁻¹⁹ erg s⁻¹ cm⁻² arcsec⁻²). The target galaxy, selected from a near-UV spectroscopic survey of Mg ii line profiles at 0.3 < z < 1.4, has a stellar mass log (M _*/M _⊙) = 9.9, a star formation rate of 50 M _⊙ yr⁻¹, and a morphology indicative of a merger. After deconvolution with the seeing, we obtain 5σ detections of Mg ii line emission extending for ∼31 kpc measured in 7-spaxel (1.1 arcsec²) 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⁻¹; however, the P Cygni profiles give way to pure emission at large radii from the central galaxy. We have performed 3D 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 ϕ ≤ 80°. 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.