The ALPINE-ALMA [CII] survey:Circumgalactic medium pollution and gas mixing by tidal stripping in a merging system at z similar to 4.57

We present ALMA observations of a merging system at z similar to 4.57, observed as a part of the ALMA Large Program to INvestigate [CII] at Early times (ALPINE) survey. Combining ALMA [CII]158 mu m and far-infrared continuum data with multi-wavelength ancillary data, we find that the system is composed of two massive (M-star greater than or similar to 10(10) M-circle dot) star-forming galaxies experiencing a major merger (stellar mass ratio r(mass)greater than or similar to 0.9) at close spatial (similar to 13 kpc; projected) and velocity (Delta v<300 km s(-1)) separations, and two additional faint narrow [CII]-emitting satellites. The overall system belongs to a larger scale protocluster environment and is coincident to one of its overdensity peaks. Additionally, ALMA reveals the presence of [CII] emission arising from a circumgalactic gas structure, extending up to a diameter-scale of 30 kpc. Our morpho-spectral decomposition analysis shows that about 50% of the total flux resides between the individual galaxy components, in a metal-enriched gaseous envelope characterised by a disturbed morphology and complex kinematics. Similarly to observations of shock-excited [CII] emitted from tidal tails in local groups, our results can be interpreted as a possible signature of interstellar gas stripped by strong gravitational interactions, with a possible contribution from material ejected by galactic outflows and emission triggered by star formation in small faint satellites. Our findings suggest that mergers could be an efficient mechanism of gas mixing in the circumgalactic medium around high-z galaxies, and thus play a key role in the galaxy baryon cycle at early epochs.