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 Van Kempen et al. (2024) to the Nexus region, a dynamically assembling superstructure centred on the Abell 4038 galaxy cluster. We quantify environmental effects using the quenched fraction ( f _Q ) and the mean and scatter of the specific star formation rate (sSFR) for the star-forming population, measured as functions of stellar mass and improved group-scale halo mass estimates from Van Kempen et al. (2026). One-dimensional binned trends are summarised with logistic fits for f _Q and a power-law describes the binned mean log sSFR trends. We also de-couple the stellar–halo mass dependence in both the f _Q and mean log sSFR analyses, demonstrating a joint dependence: f _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 sSFR declines strongly with stellar mass and shows additional environment-linked suppression in group-scale halos, while the scatter in log sSFR varies primarily with stellar mass and shows comparatively weaker dependence on halo mass. However, these differences are generally within the uncertainties, and larger samples of dynamically evolving Nexus-like structures are required to determine whether they reflect genuine environmental effects or cosmic variance. Within the Nexus, splitting the sample into three projected radial zones around Abell 4038 shows that both quenching and the properties of the star-forming population vary systematically with distance from the node and forming supercluster, largely driven by differences in the sampled halo mass function, indicating that environmental regulation is not spatially uniform across the structure. Finally, a projected phase-space (PPS) analysis of Abell 4038 shows higher f _Q in regions associated with earlier infall, linking quenching to orbital history within the cluster. However, when grouping infall PPS zones and splitting by stellar mass, this trend is strongly mass dependent, with low-mass galaxies (log M _stellar < 10) showing no significant change in f _Q . These results demonstrate that the drivers of galaxy evolution depend jointly on stellar mass, the halo mass of the local group environment, and location within the surrounding large-scale structure. This motivates future, larger-statistics, multi-wavelength studies that combine wide-area spectroscopy with tracers of gas, dust, and hot halos to test quenching and star formation regulation mechanisms across the cosmic web.