Quenching time-scales of galaxies in the EAGLE simulations

We use the EAGLE simulations to study the connection between the quenching time-scale, tau(Q), and the physical mechanisms that transform star-forming galaxies into passive galaxies. By quantifying tau(Q) in two complementary ways - as the time over which (i) galaxies traverse the green valley on the colour-mass diagram, or (ii) leave the main sequence of star formation and subsequently arrive on the passive cloud in specific star formation rate (SSFR)-mass space - we hod that the tau(Q) distribution of high-mass centrals, low-mass centrals, and satellites are divergent. In the low stellar mass regime where M-star < 10(9.6) M-circle dot, centrals exhibit systematically longer quenching time-scales than satellites (approximate to 4 Gyr compared to approximate to 2 Gyr). Satellites with low stellar mass relative to their halo mass cause this disparity, with ram pressure stripping quenching these galaxies rapidly. Low-mass centrals are quenched as a result of stellar feedback, associated with long tau(Q) greater than or similar to 3 Gyr. At intermediate stellar masses where 10(9.7) < M-star < 10(10.3) M-circle dot, tau(Q) are the longest for both centrals and satellites, particularly for galaxies with higher gas fractions. At M-star greater than or similar to 10(10.3) M-circle dot, galaxy merger counts and black hole activity increase steeply for all galaxies. Quenching time-scales for centrals and satellites decrease with stellar mass in this regime to tau(Q) less than or similar to 2 Gyr. In anticipation of new intermediate redshift observational galaxy surveys, we analyse the passive and star-forming fractions of galaxies across redshift, and find that the tau(Q) peak at intermediate stellar masses is responsible for a peak (inflection point) in the fraction of green valley central (satellite) galaxies at z approximate to 0.5-0.7.