2025A&A...701A..38R

Context. Numerous studies have established that the physical properties of a galaxy are profoundly influenced by its surrounding environment. While gas inflows can supply the necessary fuel for star formation, high-density and high-temperature conditions can suppress star-forming activity through various quenching processes. Investigations into large-scale structures, such as filaments and overdense regions in the cluster outskirts at R ≥ 2R₂₀₀, have predominantly focused on the low-z Universe. To move to intermediate-z and explore galaxy pathways combined with environmental effects, it is crucial to join wide-field spectroscopy and deep photometry.
Aims. Our primary objective is to spectroscopically analyse the photometric overdensity structures previously observed in the outskirts of the massive cluster MACS J0416.1-2403 (z = 0.397), interpreted as evidence of ongoing group infall into the cluster. With this study we aim to enhance our understanding of the evolutionary processes occurring within these substructures and their role in the pre-processing scenario. Additionally, we aim to investigate the global behaviour of galaxies in the outskirts in relation to their g - r colour, K-band luminosity (a proxy for stellar mass), and local density, emphasizing the influence of the environment on galaxy evolution.
Methods. We conducted a spectroscopic analysis extending to the outskirts up to 5.5R₂₀₀ (∼10 Mpc), using the AAOmega spectrograph. The large field of view (1 deg²) and depth of the observations allowed us to explore galaxies up to the cluster's periphery and across a wide stellar mass range, reaching down to the limit of dwarf galaxies. Redshifts were obtained through independent but comparable methods: Redrock, EZ, and Redmost, ensuring consistency and accuracy in our measurements.
Results. We identified 148 new spectroscopic cluster members from a sample of 1236 objects. We found that 81 out of the 148 galaxies are located in filamentary and overdense regions, supporting the role of filamentary infall in the cluster mass assembly history. A spectral analysis revealed that galaxies in high-density regions are more massive, redder, and more passive, compared to galaxies in low-density regions that appear to be bluer, less massive, and more star-forming. These findings underscore the significance of environmental effects, particularly in overdense regions, and the role of pre-processing phenomena in shaping galaxy properties before cluster infall.