Abstract

We report the detection of [C II] lambda 158 mu m emission from a system of three closely separated sources in the Cosmic Evolution Survey (COSMOS) field at z similar to 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 similar to 11 kpc) and in velocity (similar to 100 km s(-1)), while the third source is slightly more distant (2.8 arcsec similar to 18 kpc, similar to 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 log(10)(SFR[C II] [M-circle dot 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 similar to 2.5.