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.
@article{arxiv.1908.07777,
title = {The ALPINE-ALMA [CII] Survey: A Triple Merger at z~4.56},
author = {G. C. Jones and M. Bethermin and Y. Fudamoto and M. Ginolfi and P. Capak and P. Cassata and A. Faisst and O. Le Fevre and D. Schaerer and J. Silverman and L. Yan and S. Bardelli and M. Boquien and A. Cimatti and M. Dessauges-Zavadsky and M. Giavalisco and C. Gruppioni and E. Ibar and Y. Khusanova and A. M. Koekemoer and B. C. Lemaux and F. Loiacono and R. Maiolino and P. A. Oesch and F. Pozzi and D. Riechers and G. Rodighiero and M. Talia and L. Vallini and D. Vergani and G. Zamorani and E. Zucca},
journal= {arXiv preprint arXiv:1908.07777},
year = {2019}
}