Journal Article

The mid-infrared colour–magnitude relation of early-type galaxies in the Coma cluster as measured by <i>Spitzer</i>-IRS*

M. S. Clemens, A. Bressan, P. Panuzzo, R. Rampazzo, L. Silva, L. Buson and G. L. Granato

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 392, issue 3, pages 982-991
Published in print January 2009 | ISSN: 0035-8711
Published online January 2009 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2008.14080.x
The mid-infrared colour–magnitude relation of early-type galaxies in the Coma cluster as measured by Spitzer-IRS*

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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 Ks-[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 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 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.

Keywords: galaxies: clusters: general; galaxies: elliptical and lenticular cD; galaxies: evolution; galaxies: photometry

Journal Article.  7454 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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