The ALPINE-ALMA [CII] survey. The contribution of major mergers to the galaxy mass assembly at z ~ 5

Galaxy mergers are thought to be one of the main mechanisms of the mass-assembly of galaxies in the Universe, but there is still little direct observational evidence of how frequent they are at z>4. Recently, many works have suggested a possible increase in the fraction of major mergers in the early Universe, reviving the debate on which processes (e.g., cold accretion, star formation, mergers) most contribute to the mass build-up of galaxies through cosmic time. We discuss about the importance of major mergers in this context. In our analysis we used, for the first time, the morpho-kinematic information provided by the [CII] emission, along with archival multi-wavelength data, to identify major mergers at z~5. In particular, we find a merger fraction of ~0.44 (0.34) at z~4.5 (5.5) from ALPINE. By combining our results with those at lower redshifts, we computed the cosmic evolution of the merger fraction which is described by a rapid increase from the local Universe to higher redshifts, a peak at z~3, and a slow decrease toward earlier epochs. Depending on the timescale prescription used, this fraction translates into a merger rate per galaxy ranging between ~0.1 and ~4.0 Gyr^-1 at z~5, which in turn corresponds to an average number of major mergers per galaxy between 1 and 8 in ~12.5 Gyr (from z=6 to the local Universe). When convolved with the galaxy number density at different epochs, the merger rate provides the merger rate density which becomes approximately constant over time at 14 if we assume a merger timescale that quickly decreases with increasing redshift. Our new constraints on the merger fraction from the ALPINE survey at z~5 reveal the presence of a significant merging activity in the early Universe. Whether this population of mergers can provide a relevant contribution to the galaxy mass-assembly at these redshifts and through the cosmic epochs is strongly dependent on the assumption of the merger timescale. However, our results show that a merger timescale evolving as (1+z)^-2 agrees well with state-of-the-art cosmological simulations, suggesting a considerable role of mergers in the build-up of galaxies at early times