Star formation quenching in simulated group and cluster galaxies: when, how, and why?

Star formation is observed to be suppressed in group and cluster galaxies compared to the field. To gain insight into the quenching process, we have analysed similar to 2000 galaxies formed in the GIMIC suite of cosmological hydrodynamical simulations. The time of quenching varies from similar to 2Gyr before accretion (first crossing of r(200, c)) to> 4 Gyr after, depending on satellite and host mass. Once begun, quenching is rapid (less than or similar to 500 Myr) in low-mass galaxies (M-star < 10(10)M(circle dot)), but significantly more protracted for more massive satellites. The simulations predict a substantial role of outflows driven by ram pressure-but not tidal forces - in removing the star-forming interstellar matter (ISM) from satellite galaxies, especially dwarfs (M-star < 10(9) M-circle dot) where they account for nearly two thirds of ISM loss in both groups and clusters. Immediately before quenching is complete, this fraction rises to similar to 80 per cent even for Milky Way analogues (M-star approximate to 10(10.5)M(circle dot)) in groups (M-host approximate to 10(13.5)M(circle dot)). We show that (i) ISM stripping was significantly more effective at early times than at z = 0; (ii) approximately half the gas is stripped from 'galactic fountains' and half directly from the star-forming disc; (iii) galaxies undergoing stripping experience ram pressure up to similar to 100 times the average at a given group-/clustercentric radius, because they are preferentially located in overdense regions of the intra-cluster medium. Remarkably, stripping causes at most half the loss of the extended gas haloes surrounding our simulated satellites. These results contrast sharply with the current picture of strangulation-removal of the ISM through star formation after stripping of the hot halo - being the dominant mechanism quenching group and cluster satellites.