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We use 16 μm,  Spitzer ‐Infrared Spectrograph, blue peak‐up 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   K   s   ‐[16] colour in these models is very sensitive to the relative population of dusty asymptotic giant branch 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   K   s   ‐[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 per cent) of early‐type galaxies in the Coma cluster. The corollary of this is that 32 per cent 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.

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