The influence of galaxy mergers, black hole growth, and gas processes on the evolution of the stellar mass-gas metallicity relation of galaxies in different cosmic environments

Abstract We study the impact of supermassive black hole (SMBH) growth, $\\langle \\dot{M}_\\mathrm{SMBH}\\rangle$, major and minor galaxy mergers, and gas processes, on the average gas metallicity of galaxies within the Horizon Run 5 simulation, with the aim to uncover which of these processes drive the scatter in the gas metallicity-stellar mass relation (MZR) at different redshifts in nodes, filaments and voids. At z = 5, minor mergers produce the largest differential in log [Zg/Z⊙] where all environments display a maximum 0.22 dex increase in the average log [Zg/Z⊙] compared to non-merging galaxies. The node population also displays a consistent 0.1 dex negative offset in dlog [Zg/Z⊙], the residual Zg from the total MZR of all galaxies, across all redshifts, whilst filament and void galaxies show a smaller offset. Major mergers show little influence on these same properties. This suggests minor mergers regulate metallicity and contribute to galaxy mass growth concurrently, accelerating chemical evolution post merger. Between z = 1 − 3, a high $\\langle \\dot{M}_\\mathrm{SMBH}\\rangle$ leads to a larger negative offset in dlog [Zg/Z⊙] for all environments. Here, node galaxies show the largest negative offset of approximately 0.25 dex, suggesting that AGN-driven gas removal may contribute to the MZR scatter at intermediate times. Finally, galaxies with low Mgas/Mtot show increased dlog [Zg/Z⊙] across all redshifts and environments, again a 0.25 dex maximum for node galaxies. These galaxies also spike in dlog [Zg/Z⊙] at late times, below z = 1. At this time, galaxies in the nodes show negative $\\langle \\dot{M}_\\mathrm{gas} \\rangle$ whilst also showing the largest dlog [Zg/Z⊙] values we observe of 0.2 dex, suggesting the importance of the balance between gas accretion and starvation in driving MZR scatter at low redshifts.