Environmental quenching and hierarchical cluster assembly: Evidence from spectroscopic ages of red-sequence galaxies in Coma

TL;DR

This study investigates how stellar ages of red-sequence galaxies in the Coma cluster vary with distance from the cluster center, revealing different environmental effects on giant and dwarf galaxies and supporting hierarchical assembly models.

Contribution

It provides the first detailed analysis of stellar age gradients in Coma galaxies, highlighting the contrasting environmental influences on giants and dwarfs, and tests environmental quenching models against observations.

Findings

Giant galaxy ages are mainly mass-dependent with slight environmental influence.

Dwarf galaxy ages strongly depend on cluster-centric radius, with younger ages farther out.

Environmental quenching models can reproduce observed gradients, especially with high efficiency for dwarfs.

Abstract

We explore the variation in stellar population ages for Coma cluster galaxies as a function of projected cluster-centric distance, using a sample of 362 red-sequence galaxies with high signal-to-noise spectroscopy. The sample spans a wide range in luminosity (0.02-4 L*) and extends from the cluster core to near the virial radius. We find a clear distinction in the observed trends of the giant and dwarf galaxies. The ages of red-sequence giants are primarily determined by galaxy mass, with only weak modulation by environment, in the sense that galaxies at larger cluster-centric distance are slightly younger. For red-sequence dwarfs (with mass <10^10 Msun), the roles of mass and environment as predictors of age are reversed: there is little dependence on mass, but strong trends with projected cluster-centric radius are observed. The average age of dwarfs at the 2.5 Mpc limit of our sample is approximately half that of dwarfs near the cluster centre. The gradient in dwarf galaxy ages is a global cluster-centric trend, and is not driven by the ongoing merger of the NGC 4839 group to the south west of Coma. We interpret these results using environmental histories extracted from the Millennium Simulation for members of massive clusters. Hierarchical cluster assembly naturally leads to trends in the accretion times of galaxies as a function of projected cluster-centric radius. On average, simulated galaxies now located in cluster cores joined halos above any given mass threshold earlier than those now located in the outskirts of clusters. We test environmental quenching models, in which star formation is halted in galaxies when they enter halos of a given mass, or become satellite galaxies. The models broadly reproduce the gradients observed in Coma, but for dwarf galaxies the efficiency of environmental quenching must be very high to match the strong trends observed.