Too dense to go through: the role of low-mass clusters in the pre-processing of satellite galaxies

We study the evolution of satellite galaxies in clusters of the c-eagle simulations, a suite of 30 high-resolution cosmological hydrodynamical zoom-in simulations based on the eagle code. We find that the majority of galaxies that are quenched at z = 0 (greater than or similar to 80per cent) reached this state in a dense environment (log(10)M(200)[M-circle dot] >= 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 (less than or similar to 1 Gyr). Just a small fraction of galaxies (less than or similar to 15 per cent) is capable of retaining their gas for a longer period of time, but after 4 Gyr, 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(ICM) similar to 3 x10(-5) n(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.