Observational Evidence of the Prevalence of Bipolar Galactic Outflows out to 10 kpc at z ≈ 1 for Massive Galaxies

Abstract Galactic outflows are believed to play a critical role in the evolution of galaxies by regulating their mass build-up and star formation [1]. These outflows tend to form bipolar shapes above and below the galactic planes and extend well into the circumgalactic medium (CGM), up to tens of kpc perpendicular to the galaxy. They have been directly observed in the local Universe, e.g., around the Milky Way and M82 [2, 3]. At higher redshifts, cosmological simulations of galaxy formation predict an increase in the frequency and efficiency of galactic outflows due to the increasing star formation activity [4]. Outflows are responsible for removing potential fuel for star formation from the galaxy, while at the same enriching the CGM and the intergalactic medium. These feedback processes, although incorporated as key elements of cosmological simulations, are still poorly constrained on CGM scales. Here we present an ultra-deep 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, while face-on galaxies display 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 the signal is not dominated by a few outliers. After dividing the galaxy sample in subsamples by mass, the bipolar emission is only detected in galaxies with stellar mass M * >∼ 10 9.5 M ⊙ .