The ALPINE-ALMA [CII] Survey: A Triple Merger at z~4.56
  We report the detection of [CII]158um emission from a system of three
closely-separated sources in the COSMOS field at z~4.56, as part of the ALMA
Large Program to INvestigate CII at Early times (ALPINE). The two dominant
sources are closely associated, both spatially (1.6"~11kpc) and in velocity
(~100km/s), while the third source is slightly more distant (2.8"~18kpc,
~300km/s). The second strongest source features a slight velocity gradient,
while no significant velocity gradient is seen in the other two sources. Using
the observed [CII] luminosities, we derive a total
log(SFR_[CII]/[Msol/year])=2.8+/-0.2, which may be split into contributions of
59%, 31%, and 10% from the central, east, and west sources, respectively.
Comparison of these [CII] detections to recent zoom-in cosmological simulations
suggests an ongoing major merger. We are thus witnessing a system in a major
phase of mass build-up by merging, including an on-going major merger and an
upcoming minor merger, which is expected to end up in a single massive galaxy
by z~2.5.
