Résumé du preprint Irfu-09-507

The mid-infrared colour-magnitude relation of early-type galaxies in the Coma cluster as measured by Spitzer-IRS
M. S. Clemens, A. Bressan, P. Panuzzo, R. Rampazzo, L. Silva, L. Buson, G. L. Granato
We use 16 μm, 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.