The mid-infrared colour-magnitude relation of early-type galaxies in the Coma cluster as measured by Spitzer-IRS

TL;DR

This study uses mid-infrared photometry to analyze early-type galaxies in the Coma cluster, revealing a colour-magnitude relation sensitive to age and metallicity, and identifying a subset with recent rejuvenation episodes.

Contribution

First to use 16 micron Spitzer-IRS data to establish the mid-infrared colour-magnitude relation for early-type galaxies in the Coma cluster, linking it to stellar population properties.

Findings

Most galaxies (~68%) are passively evolving with ~10 Gyr age.

The Ks-[16] colour is highly sensitive to stellar age.

Approximately 32% show signs of recent star formation or rejuvenation.

Abstract

We use 16 micron, Spitzer-IRS, blue peakup photometry of 50 early-type galaxies in the Coma cluster to define the mid-infrared colour-magnitude relation. We compare with recent simple stellar population models that include the mid-infrared emission from the extended, dusty envelopes of evolved stars. The Ks-[16] colour in these models is very sensitive to the relative population of dusty Asymptotic Giant Branch (AGB) stars. We find that the passively evolving early-type galaxies define a sequence of approximately constant age (~10 Gyr) with varying metallicity. Several galaxies that lie on the optical/near-infrared colour-magnitude relation do not lie on the mid-infrared relation. This illustrates the sensitivity of the Ks-[16] colour to age. The fact that a colour-magnitude relation is seen in the mid-infrared underlines the extremely passive nature of the majority (68%) of early-type galaxies in the Coma cluster. The corollary of this is that 32% of the early-type galaxies in our sample are not `passive', insofar as they are either significantly younger than 10 Gyr or they have had some rejuvenation episode within the last few Gyr.