Too dense to go through: The importance of low-mass clusters for satellite quenching

We study the evolution of satellite galaxies in clusters of the $\text{textsc}c-eagle$ simulations, a suite of 30 high-resolution cosmological hydrodynamical zoom-in simulations based on the $\text{textsc}eagle$ code. We find that the majority of galaxies that are quenched at $z=0$ ($\gtrsim$ 80$\mathrm{\%}$) reached this state in a dense environment (log${}_{10}$M${}_{200}$[M${}_{\odot }$]$\ge$13.5). At low redshift, regardless of the final cluster mass, galaxies appear to reach their quenching state in low-mass clusters. Moreover, galaxies quenched inside the cluster that they reside in at $z=0$ are the dominant population in low-mass clusters, while galaxies quenched in a different halo dominate in the most massive clusters. When looking at clusters at $z>0.5$, their in situ quenched population dominates at all cluster masses. This suggests that galaxies are quenched inside the first cluster they fall into. After galaxies cross the cluster’s $r_{200}$ they rapidly become quenched ($\lesssim$ 1Gyr). Just a small fraction of galaxies ($\lesssim 15\mathrm{\%}$) is capable of retaining their gas for a longer period of time, but after 4Gyr, almost all galaxies are quenched. This phenomenon is related to ram pressure stripping and is produced when the density of the intracluster medium reaches a threshold of ${\rho }_{\mathrm{I}\mathrm{C}\mathrm{M}}$ $\sim 3\times {10}^{-5}$ n${}_{\mathrm{H}}$ (cm${}^{-3}$). These results suggest that galaxies start a rapid-quenching phase shortly after their first infall inside $r_{200}$ and that, by the time they reach $r_{500}$, most of them are already quenched.