An environment-dependent halo mass function as a driver for the early quenching of z ≥ 1.5 cluster galaxies

Many z approximate to 1.5 galaxies with a stellar mass (M star) >= 1010M(circle dot) are already quenched in both galaxy clusters (>50 per cent) and the field (>20 per cent), with clusters having a higher quenched fraction at all stellar masses compared to the field. A puzzling issue is that these massive quenched galaxies have stellar populations of similar age in both clusters and the field. This suggests that, despite the higher quenched fraction in clusters, the dominant quenching mechanism for massive galaxies is similar in both environments. In this work, we use data from the cosmological hydrodynamic simulations Hydrangea and EAGLE to test whether the excess quenched fraction of massive galaxies in z=1.5 clusters results from fundamental differences in their halo properties compared to the field. We find that (i) at 10(10)<= M star/M-circle dot <= 10(11), quenched fractions in the redshift range 1.5<z<3.5 are consistently higher for galaxies with higher peak maximum circular velocity of the dark matter halo (vmax,peak), and (ii) the distribution of vmax,peak is strongly biased towards higher values for cluster satellites compared to the field centrals. Due to this difference in the halo properties of cluster and field galaxies, secular processes alone may account for (most of) the environmental excess of massive quenched galaxies in high-redshift (proto) clusters. Taken at face value, our results challenge a fundamental assumption of popular quenching models, namely that clusters are assembled from an unbiased subset of infalling field galaxies. If confirmed, this would imply that such models must necessarily fail at high redshift, as indicated by recent observations.