Quenching timescales of galaxies in the EAGLE simulations

Abstract

We use the \eagle\ simulations to study the connection between the quenching timescale, $\ddot{I}{„}_{\mathrm{Q}}$, and the physical mechanisms that transform star-forming galaxies into passive galaxies. By quantifying $\ddot{I}{„}_{\mathrm{Q}}$ 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 find that the $\ddot{I}{„}_{\mathrm{Q}}$ distribution of high-mass centrals, low-mass centrals and satellites are divergent. In the low stellar mass regime where $M_{\star }<10^{9.6}{M}_{\odot }$, centrals exhibit systematically longer quenching timescales than satellites ($\approx 4$~Gyr compared to $\approx 2$~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 $\ddot{I}{„}_{\mathrm{Q}}\gtrsim 3$~Gyr. At intermediate stellar masses where $10^{9.7}{\mathrm{M}}_{\odot }<{\mathrm{M}}_{\star }<10^{10.3}{\mathrm{M}}_{\odot }$, $\ddot{I}{„}_{\mathrm{Q}}$ are the longest for both centrals and satellites, particularly for galaxies with higher gas fractions. At $M_{\star }\gtrsim 10^{10.3}{\mathrm{M}}_{\odot }$, galaxy merger counts and black hole activity increase steeply for all galaxies. Quenching timescales for centrals and satellites decrease with stellar mass in this regime to $\ddot{I}{„}_{\mathrm{Q}}\lesssim 2$~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 $\ddot{I}{„}_{\mathrm{Q}}$ peak at intermediate stellar masses is responsible for a peak (inflection point) in the fraction of green valley central (satellite) galaxies at $z\approx 0.5-0.7$.

Final version accepted to MNRAS. 18 pages, 16 figures