The age gradients of galaxies in EAGLE: outside-in quenching as the origin of young bulges in cluster galaxies

TL;DR

This study uses EAGLE simulations to explain why cluster galaxies often have younger bulges than their discs, attributing it to outside-in quenching caused by environmental effects like gas stripping.

Contribution

It demonstrates that outside-in quenching in clusters leads to younger bulges, explaining observed age gradients and contrasting with field galaxy evolution.

Findings

Cluster galaxies experience outside-in quenching after infall.

Field galaxies maintain inside-out growth with younger discs.

Environmental effects cause positive age gradients in cluster galaxies.

Abstract

Many disc galaxies in clusters have been found with bulges of similar age or younger than their surrounding discs, at odds with field galaxies of similar morphology and their expected inside-out formation. We use the EAGLE simulations to test potential origins for this difference in field and cluster galaxies. We find, in agreement with observations, that on average disc-dominated field galaxies in the simulations have older inner regions, while similar galaxies in groups and clusters have similarly aged or younger inner regions. This environmental difference is a result of outside-in quenching of the cluster galaxies. Prior to group/cluster infall, galaxies of a given present-day mass and morphology exhibit a similar evolution in their specific star formation rate (sSFR) profiles. Post-infall, the outer sSFRs of group and cluster galaxies significantly decrease due to interstellar medium stripping, while the central sSFR remains similar to field galaxies. Field disc galaxies instead generally retain radially increasing sSFR profiles. Thus, field galaxies continue to develop negative age gradients (younger discs), while cluster galaxies instead develop positive age gradients (younger bulges).