Abstract

We present spatially resolved Echelle spectroscopy of an intervening MgII-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 15× the optical radius.We detect strong (W2796 0 > 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) W2796 0 , W2600 0 , W2852 0 , and the ratio W2600 0 /W2796 0 , all anticorrelate with D; (3) the W2796 0 -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 arewitnessing some of the long-sought processes of the baryon cycle in a single distant galaxy expected to be representative of such phenomena.