Bipolar outflows out to 10 kpc for massive galaxies at redshift z ≈ 1

Galactic outflows are believed to play a critical role in the evolution of galaxies by regulating their mass build-up and star formation¹. Theoretical models assume bipolar shapes for the outflows that extend well into the circumgalactic medium (CGM), up to tens of kiloparsecs (kpc) perpendicular to the galaxies. They have been directly observed in the local Universe in several individual galaxies, for example, around the Milky Way and M82 (refs. ^2,3). At higher redshifts, cosmological simulations of galaxy formation predict an increase in the frequency and efficiency of galactic outflows owing to the increasing star-formation activity⁴. Galactic outflows are usually of low gas density and low surface brightness and therefore difficult to observe in emission towards high redshifts. Here we present an ultra-deep Multi-Unit Spectroscopic Explorer (MUSE) image of the mean Mg II emission surrounding a sample of galaxies at z ≈ 1 that strongly suggests the presence of outflowing gas on physical scales of more than 10 kpc. We find a strong dependence of the detected signal on the inclination of the central galaxy, with edge-on galaxies clearly showing enhanced Mg II emission along the minor axis, whereas face-on galaxies show much weaker and more isotropic emission. We interpret these findings as supporting the idea that outflows typically have a bipolar cone geometry perpendicular to the galactic disk. We demonstrate that this CGM-scale outflow is prevalent among galaxies with stellar mass M_* ≳ 10^9.5M_⊙.