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 lambda lambda 2796, 2803 doublet in the circumgalactic medium (CGM) extending similar to 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)). 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 5 sigma detections of Mg ii line emission extending for similar to 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(-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 <= 80 degrees. 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.

Automated summary

The study utilized the Keck Cosmic Web Imager (KCWI) to conduct spatially resolved spectroscopy of a star-forming galaxy at z = 0.6942, revealing emission from the Mg ii doublet. Through 3D radiative transfer modeling, the geometry, velocity, and density profiles of the outflowing gas were inferred.