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

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 $\log [Z_\mathrm{ g}/\mathrm{Z}_\odot ]$ where all environments display a maximum 0.22 dex increase in the average $\log [Z_\mathrm{ g}/\mathrm{Z}_\odot ]$ compared to non-merging galaxies. The node population also displays a consistent 0.1 dex negative offset in $\mathrm{d} \log [Z_\mathrm{ g}/\mathrm{Z}_{\odot }]$, the residual $Z_\mathrm{ g}$ 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 $\mathrm{d} \log [Z_g/\mathrm{Z}_{\odot }]$ 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 $M_{\mathrm{ gas}}/{M_{\mathrm{ tot}}}$ show increased $\mathrm{d} \log [Z_\mathrm{ g}/\mathrm{Z}_{\odot }]$ across all redshifts and environments, again a 0.25 dex maximum for node galaxies. These galaxies also spike in $\mathrm{d} \log [Z_\mathrm{ g}/\mathrm{Z}_{\odot }]$ 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 $\mathrm{d} \log [Z_\mathrm{ g}/\mathrm{Z}_{\odot }]$ 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.