Star formation histories of Coma cluster galaxies matched to simulated orbits hint at quenching around first pericenter

Abstract

Context. The star formation in galaxies in present-day clusters has almost entirely been shut down, but the exact mechanism that quenched these galaxies is still uncertain. Aims. By tracing the orbital and star formation histories of galaxies within the Coma cluster, we seek to understand the role of the high-density cluster environment in quenching these galaxies. Methods. We combine star formation histories extracted from high-signal-to-noise spectra of 11 early-type galaxies around the center of the Coma cluster with probability distributions for their orbital parameters obtained using an N-body simulation to connect their orbital and star formation histories. Results. We find that all 11 galaxies likely quenched near their first pericentric approach. Higher stellar mass galaxies (log(M·/M·) > 10) had formed a higher fraction of their stellar mass (more than ∼90%) than their lower mass counterparts (∼80-90%) by the time they fell into the cluster (when they cross 2.5rvir). We find that the expected infall occurred around z ∼ 0.6, followed by the first pericentric passage ∼4 Gyr later. Galaxies in our sample formed a significant fraction of their stellar mass, up to 15%, between infall and first pericenter, and had assembled more than ∼98% of their cumulative stellar mass by first pericenter. Conclusions. Unlike previous low-redshift studies that suggest that star formation continues until about first apocenter or later, the high percentage of stellar mass already formed by first pericenter in our sample galaxies points to star formation ceasing within a gigayear after the first pericentric passage. We consider the possible physical mechanisms driving quenching and find that our results resemble the situation in clusters at z ∼ 1, where active stripping of gas (ram-pressure or tidally driven) seems to be required to quench satellites by their first pericentric passage. However, a larger sample will be required to conclusively account for the unknown fraction of preprocessed satellites in the Coma cluster.