Mid-infrared colour gradients and the colour-magnitude relation in Virgo early-type galaxies

Abstract

We make use of Spitzer imaging between 4 and 16 micron and near-infrared data at 2.2 micron to investigate the nature and distribution of the mid-infrared emission in a sample of early-type galaxies in the Virgo cluster. These data allow us to conclude, with some confidence, that the emission at 16 micron in passive ETGs is stellar in origin, consistent with previous work concluding that the excess mid-infrared emission comes from the dusty envelopes around evolved AGB stars. There is little evidence for the mid-infrared emission of an unresolved central component, as might arise in the presence of a dusty torus associated with a low-luminosity AGN. We nonetheless find that the 16 micron emission is more centrally peaked than the near-infrared emission, implying a radial stellar population gradient. By comparing with independent evidence from studies at optical wavelengths, we conclude that a metallicity that falls with increasing radius is the principal driver of the observed gradient. We also plot the mid-infrared colour-magnitude diagram and combine with similar work on the Coma cluster to define the colour-magnitude relation for absolute K-band magnitudes from -26 to -19. Because a correlation between mass and age would produce a relation with a gradient in the opposite sense to that observed, we conclude that the relation reflects the fact that passive ETGs of lower mass also have a lower average metallicity. The colour-magnitude relation is thus driven by metallicity effects. In contrast to what is found in Coma, we do not find any objects with anomalously bright 16 micron emission relative to the colour-magnitude relation. Although there is little overlap in the mass ranges probed in the two clusters, this may suggest that observable ``rejuvenation'' episodes are limited to intermediate mass objects.

8 pages, 4 figures

References in corpus (7)

• The star formation history of early-type galaxies as a function of mass and environment
• SPITZER IRS spectra of Virgo early type galaxies: detection of stellar silicate emission
• The mid-infrared colour-magnitude relation of early-type galaxies in the Coma cluster as measured by Spitzer-IRS
• Stellar population gradients in early-type cluster galaxies with VIMOS IFU
• A deep AAOmega survey of low-luminosity galaxies in the Shapley Supercluster: Stellar population trends
• The history of star formation and mass assembly in early-type galaxies
• The Star Formation History of the Virgo early-type galaxy NGC4435: the Spitzer Mid Infrared view

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• About the linearity of the color-magnitude relation of early-type galaxies in the Virgo cluster
• AMUSE-VIRGO. III: mid-infrared photometry of early-type galaxies and limits on obscured nuclear emission