Ibar, Eduardo2021-07-222021-07-222020http://repositoriobibliotecas.uv.cl/handle/uvscl/2148We report the detection of [C II] λ158 μm emission from a system of three closely separated sources in the Cosmic Evolution Survey (COSMOS) field at z ∼ 4.56 , as part of the Atacama Large Millimeter/submillimeter Array (ALMA) Large Program to INvestigate C II at Early times (ALPINE). The two most luminous sources are closely associated, both spatially (1.6 arcsec ∼ 11 kpc) and in velocity (∼100 km s−1), while the third source is slightly more distant (2.8 arcsec ∼ 18 kpc, ∼300 km s−1). The second most luminous source features a slight velocity gradient, while no significant velocity gradient is seen in the other two sources. Using the observed [C II] luminosities, we derive a total log10(SFR[CII][M⊙yr−1])=2.8±0.2⁠, which may be split into contributions of 59, 31, and 10 per cent from the central, east, and west sources, respectively. Comparison of these [C II] 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 ongoing major merger and an upcoming minor merger, which is expected to end up in a single massive galaxy by z ∼ 2.5.enGALAXIES: EVOLUTIONGALAXIES: HIGH-REDSHIFTGALAXIES: INTERACTIONSGALAXIES: KINEMATICS AND DYNAMICSArticulohttps://doi.org/10.1093/mnrasl/slz154