From Cluster Cores to the Low-density Field: Strong Environmental Quenching of Galaxy Star Formation at Low Redshift

Abstract We investigate how galaxy star formation activity depends on environment using a sample of 81,647 SDSS galaxies selected over 0.03 ≤ z ≤ 0.075 and 9.7 ≤ log 10 ( M ⋆ / h − 2 M ⊙ ) ≤ 11.0 , including 18,426 members from 572 clusters in the GalWCat19 catalog. We characterize environment using three complementary metrics: (1) projected nearest-neighbor density for the full sample, (2) projected clustercentric radius, and (3) host halo mass for GalWCat19 cluster members. The sSFR distribution remains bimodal across all environments, with distinct quenched and star-forming components. As local density increases, the quenched component becomes more prominent, while the characteristic sSFR of the star-forming component decreases by approximately 0.29–0.35 dex from the lowest- to highest-density classes. Within clusters, the quenched fraction decreases with increasing projected clustercentric radius, while the star-forming peak shifts by approximately 0.42 dex toward lower sSFR from the outskirts to the inner cluster region. This extends the picture from previous studies, in which environmental trends are primarily associated with changes in the quenched fraction, by showing that galaxies remaining in the star-forming population also exhibit systematically suppressed sSFR in denser environments. The dependence on host halo mass is weaker and is most apparent among lower-stellar-mass galaxies in the inner cluster regions. By measuring the environmental quenching efficiency at fixed stellar mass, we find excess quenching in cluster environments beyond that expected from stellar-mass quenching alone. These results show that environment is associated with both an increased probability of quenching and suppressed star formation among galaxies that remain star-forming, with local density and clustercentric radius exhibiting the strongest associations.