The Age of Cluster Galaxies from Continuum Colors

TL;DR

This study uses a new continuum color technique to determine the ages of early-type galaxies in clusters, revealing two populations with different ages and environmental dependencies, shedding light on galaxy formation histories.

Contribution

Introduces a novel continuum color method for galaxy age determination and uncovers the existence of two distinct galaxy populations with different formation histories.

Findings

Two galaxy populations with mean ages of 12 and 9 Gyrs.

Older galaxies follow mass-metallicity relations consistent with monolithic collapse.

Younger population properties correlate with cluster environment.

Abstract

We determine the age of 1,104 early-type galaxies in eight rich clusters ($z = 0.0046$ to $0.175$) using a new continuum color technique. We find that galaxies in clusters divide into two populations, an old population with a mean age similar to the age of the Universe (12 Gyrs) and a younger population with a mean age of 9 Gyrs. The older population follows the expected relations for mass and metallicity that imply a classic monolithic collapse origin. Although total galaxy metallicity is correlated with galaxy mass, it is uncorrelated with age. It is impossible, with the current data, to distinguish between a later epoch of star formation, longer duration of star formation or late bursts of star formation to explain the difference between the old and young populations. However, the global properties of this younger population are correlated with cluster environmental factors, which implies secondary processes, post-formation epoch, operate on the internal stellar population of a significant fraction of cluster galaxies. In addition, the mean age of the oldest galaxies in a cluster are correlated with cluster velocity dispersion implying that galaxy formation in massive clusters begins at earlier epochs than less massive clusters.