The Galactic Squeeze: How Aggregate and Highly Dynamical Environments Shape Star Formation in the Local Universe

Abstract

We investigate how galaxy evolution varies with environment in the nearby Universe by comparing an ``average'' reference volume in the Southern Galactic Pole (SGP) dataset from \citet{VanKempen2024} to the Nexus region, a dynamically assembling superstructure centred on the Abell~4038 galaxy cluster. We quantify environmental effects using the quenched fraction ($f_{\mathrm{Q}}$) and the specific star formation rate ($\mathrm{sSFR}$) for the star-forming population, measured as functions of stellar mass and group-scale halo mass from \citet{VanKempen2026}. We decouple the stellar--halo mass dependence, demonstrating that $f_{\mathrm{Q}}$ increases with stellar mass in both field and group environments, while group galaxies show an additional dependence on halo mass. The Nexus exhibits systematic differences relative to the SGP baseline, consistent with increased heterogeneity in accretion histories and pre-processing within a forming superstructure. For star-forming galaxies, the mean $\log(\mathrm{sSFR})$ declines strongly with stellar mass and shows additional environment-linked suppression in group-scale halos. Within the Nexus, splitting the sample into three projected radial zones around Abell~4038 shows that environmental regulation is not spatially uniform, driven largely by variations in the sampled halo mass function. Finally, a projected phase-space (PPS) analysis of Abell~4038 links quenching to orbital history within the cluster, though this trend is strongly mass-dependent: low-mass galaxies ($\log(M_{\mathrm{stellar}}) < 10$) show no significant change in $f_{\mathrm{Q}}$. These results demonstrate that the drivers of galaxy evolution depend jointly on stellar mass, local group halo mass, and location within the surrounding large-scale structure, motivating future large-scale, multi-wavelength cosmic web surveys.