The ALPINE-ALMA [CII] Survey: CGM pollution and gas mixing by tidal
  stripping in a merging system at z~4.57
  We present ALMA observations of a merging system at z ~ 4.57, observed as a
part of the ALMA Large Program to INvestigate [CII] at Early times (ALPINE)
survey. Combining ALMA [CII] 158 micron and far-infrared continuum data with
multi-wavelength ancillary data we find that the system is composed of two
massive (Mstar >~ 10^10 Msun) star-forming galaxies experiencing a major merger
(stellar mass ratio r_mass ~ 0.9) at close spatial (~13 kpc; projected) and
velocity (delta_v < 300 km/s) 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.
ALMA reveals also 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
characterized 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.
