The Role of Large-Scale Environment in Shaping the Stellar Mass-Gas Metallicity Relation Across Time

TL;DR

The paper demonstrates that the scatter in the stellar mass–gas metallicity relation (MZR) is strongly modulated by the large-scale environment and evolves with redshift. Using Horizon Run 5 (HR5) with a T-ReX-based skeleton to define nodes, filaments, and voids, it shows minimal environmental deviations at high redshift ($z>4.5$) but increasing deviations toward $z\approx0.5$, most pronounced for low-mass galaxies in nodes. The study links these offsets to processes such as gas accretion, ram-pressure stripping, starvation, and AGN feedback, and highlights distinct [O/Fe] evolution across environments, including a turnover in voids at late times for massive galaxies. Together, these results reveal how the cosmic web shapes chemical enrichment and the evolution of gas content across time, offering a framework to interpret upcoming spectroscopic surveys.

Abstract

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 ($z>4.5$), 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 $Z_{g}$. Concurrently, at high metallicities, for $z < 2$, while massive galaxies in nodes show increasing $Z_{g}$ and decreasing [O/Fe], void galaxies show a turnover where $Z_{g}$ falls with decreasing [O/Fe]. This directly points to the importance of cold-gas accretion in retaining lower $Z_{g}$ in massive void galaxies for $z < 2$, whilst its absence in nodes allowed $Z_{g}$ to access higher values.

Figures (12)

• Figure 1: A 20 cMpc thick slice of the galaxy distribution in HR5 with the skeletons computed using T-ReX for 4 different redshift snapshots at $z = 0.5$ (top left), $1$ (top right), $2$ (bottom left) and $4$ (bottom right), overlayed (black lines). The coloured contours represent a 2d-kernel density estimate of the galaxy distribution, with the more dense regions in red and less dense regions in blue used to qualitatively assess the skeleton.
• Figure 2: Probability density functions in filament length (left) and $d_{skel}$ (right) computed using skeletons at varying redshifts in HR5. Each 1-d histogram contains 20 equal-sized bins between $-2.5$ and $2$ log(cMpc). The lighter blues represent a lower redshift, while the darker blues represent a higher redshift, as seen in the legend.
• Figure 3: Left Panel: The MZR in HR5 for 9 snapshots from $z = 0.5$ to $4.5$ calculated as the median $\mathrm{Z_gas}$ in 15 consecutive, equal size bins in $M_{\star}$. Middle Panel: The standard deviation, $\sigma$, of the MZR in 20 consecutive, equal-size bins in stellar mass for the same 9 redshift snapshots. Right Panel: The Stellar Mass distribution, in terms of number of galaxies, for the 9 redshift snapshots. The different shades between grey and red represent different redshift snapshots, with redder being a more present snapshot. The shaded regions show the standard error on the median value in each bin.
• Figure 4: The total MZR in HR5 for 6 snapshots at redshifts, z, 0.5, 1, 1.5, 2, 3.5 and 4.5. The blue line shows the median HR5 MZR across 10 equal sized bins at the redshift specified in the bottom right corner of each subplot. The blue shaded region shows the interquartile range of the distribution in of metallicities in each bin. The dotted grey line shows the overall fit taken from the observed data points shown as the grey points from the study specified in the top left of each subplot.
• Figure 5: The MZR for node galaxies (orange), filament galaxies (blue), void galaxies (purple) and the total population (black dashed), over 10 consecutive, equal size bins in $M_{\star}$, for 6 of the chosen redshift snapshots. Bins with less than 15 galaxies have been removed. The shaded regions represent the standard error on the median value in each bin.