2020MNRAS.491.4442L

We present spatially resolved Echelle spectroscopy of an intervening Mg II-Fe II-Mg I absorption-line system detected at zabs = 0.73379 towards the giant gravitational arc PSZ1 G311.65-18.48. The absorbing gas is associated with an inclined disc-like star-forming galaxy, whose major axis is aligned with the two arc-segments reported here. We probe in absorption the galaxy's extended disc continuously, at ≃3 kpc sampling, from its inner region out to 15x the optical radius. We detect strong (W_0^{2796}> 0.3Å) coherent absorption along 13 independent positions at impact parameters D = 0-29 kpc on one side of the galaxy, and no absorption at D = 28-57 kpc on the opposite side (all de-lensed distances at zabs). We show that (1) the gas distribution is anisotropic; (2) W_0^{2796}, W_0^{2600}, W_0^{2852}, and the ratio W₀{2600}/W_0^{2796}, all anticorrelate with D; (3) the W_0^{2796}-D relation is not cuspy and exhibits significantly less scatter than the quasar-absorber statistics; (4) the absorbing gas is co-rotating with the galaxy out to D <= 20 kpc, resembling a 'flat' rotation curve, but at D >= 20 kpc velocities decline below the expectations from a 3D disc-model extrapolated from the nebular [O II] emission. These signatures constitute unambiguous evidence for rotating extra-planar diffuse gas, possibly also undergoing enriched accretion at its edge. Arguably, we are witnessing some of the long-sought processes of the baryon cycle in a single distant galaxy expected to be representative of such phenomena.