The relationship between cluster environment and molecular gas content of star-forming galaxies in the EAGLE simulation

Abstract

We employ the EAGLE hydrodynamical simulation to uncover the relationship between cluster environment and $\rm H_2$ content of star-forming galaxies at redshifts spanning $0\leq z\leq 1$. To do so, we divide the star-forming sample into those that are bound to clusters and those that are not. We find that, at any given redshift, the galaxies in clusters generally have less $\rm H_2$ than their non-cluster counterparts with the same stellar mass (corresponding to an offset of $\lesssim 0.5$ dex), but this offset varies with stellar mass and is virtually absent at $M_\star\lesssim10^{9.3}~{\rm M}_\odot$. The $\rm H_2$ deficit in star-forming cluster galaxies can be traced back to a decline in their $\rm H_2$ content that commenced after first infall into a cluster, which occurred later than a typical cluster galaxy. Evolution of the full cluster population after infall is generally consistent with `slow-then-rapid' quenching, but galaxies with $M_\star\lesssim 10^{9.5}~{\rm M}_\odot$ exhibit rapid quenching. Unlike most cluster galaxies, star-forming ones were not pre-processed in groups prior to being accreted by clusters. For both of these cluster samples, the star formation efficiency remained oblivious to the infall. We track the particles associated with star-forming cluster galaxies and attribute the drop in $\rm H_2$ mass after infall to poor replenishment, depletion due to star formation, and stripping of $\rm H_2$ in cluster environments. These results provide predictions for future surveys, along with support and theoretical insights for existing molecular gas observations that suggest there is less $\rm H_2$ in cluster galaxies.