Aaron R Rowntree

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.

We study the stellar mass-gas metallicity relation (MZR) which shows a significant scatter for a fixed stellar mass. By defining global environments, nodes, filaments, and voids within the Horizon Run 5 cosmological hydrodynamical simulation, we explore when and where the enrichment of galaxies occurs, analysing key evolution parameters such as star-formation rate and changes in gas-fraction and gas-metallicity per unit time. At high redshift ($z\gt 4.5$), there are minimal deviations from the MZR due to environment, however, larger deviations emerge as redshift decreases. Low stellar mass galaxies in nodes, $M_{\star } < 10^{9.8}\, \text{M}_{\odot }$, start showing deviations at $z = 3.5$, whilst other environments do not. For, $z < 2$, filaments and voids begin to show deviations above and below the MZR, respectively. By $z = 0.625$, the last epoch of HR5, deviations exist for all stellar masses and environments, with a maximum value of 0.13 dex at $M_{\star } \approx 10^{9.35}\, \text{M}_{\odot }$, between the median gas metallicities of node and void galaxies. To explain this environmental variance we discuss gas accretion, AGN, ram-pressure-stripping and strangulation as regulators of $Z_{\mathrm{ g}}$. Concurrently, at high metallicities, for $z < 2$, while massive galaxies in nodes show increasing $Z_{\mathrm{ g}}$ and decreasing [O/Fe], void galaxies show a turnover where $Z_{\mathrm{ g}}$ falls with decreasing [O/Fe]. This directly points to the importance of cold-gas accretion in retaining lower $Z_{\mathrm{ g}}$ in massive void galaxies for $z < 2$, whilst its absence in nodes allowed $Z_{\mathrm{ g}}$ to access higher values.

Metallicity offers a unique window into the baryonic history of the cosmos, being instrumental in probing evolutionary processes in galaxies between different cosmic environments. We aim to quantify the contribution of these environments to the scatter in the mass–metallicity relation (MZR) of galaxies. By analysing the galaxy distribution within the cosmic skeleton of the Horizon Run 5 cosmological hydrodynamical simulation at redshift z = 0.625, computed using a careful calibration of the T-ReX filament finder, we identify galaxies within three main environments: nodes, filaments, and voids. We also classify galaxies based on the dynamical state of the clusters and the length of the filaments in which they reside. We find that the cosmic environment significantly contributes to the scatter in the MZR; in particular, both the gas metallicity and its average relative standard deviation increase when considering denser large-scale environments. The difference in the average metallicity between galaxies within relaxed and unrelaxed clusters is ≈0.1dex, with both populations displaying positive residuals, δZg, from the averaged MZR. Moreover, the difference in metallicity between node and void galaxies accounts for $\approx 0.14 \, \text{dex}$ in the scatter of the MZR at stellar mass $M_{\star } \approx 10^{9.35}\, \text{M}_{\odot }$. Finally, both the average [O/Fe] in the gas and the galaxy gas fraction decrease when moving to higher large-scale densities in the simulation, suggesting that the cores of cosmic environments host – on average – older and more massive galaxies, whose enrichment is affected by a larger number of Type Ia Supernova events.