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 1<4, including values from 10^-4 to 10^-3 Gyr^-1 Mpc^-3, depending on the assumed merger timescale. We finally compare the specific star formation and SFRD with the analogous quantities from major mergers, finding a good agreement at z>4 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.