Mid-infrared colour gradients and the colour–magnitude relation in Virgo early-type galaxies
Mid-infrared colour gradients and the colour-magnitude relation in Virgo early-type galaxies
M. S. Clemens 2
P. Panuzzo 1
R. Rampazzo 2
O. Vega 0
A. Bressan 0 2 3
0 INAOE , Luis Enrique Erro 1, 72840 Tonantzintla, Puebla , Mexico
1 CEA, Laboratoire AIM, Irfu/SAp, Orme des Merisiers , F-91191 Gif-sur-Yvette , France
2 INAF-Osservatorio Astronomico di Padova , Vicolo dell'Osservatorio, 5, 35122 Padova , Italy
3 SISSA-ISAS, International School for Advanced Studies , ia Beirut 4, 34014 Trieste , Italy
A B S T R A C T We make use of Spitzer imaging between 4 and 16 µm and near-infrared data at 2.2 µm to investigate the nature and distribution of the mid-infrared emission in a sample of early-type galaxies (ETGs) in the Virgo cluster. These data allow us to conclude, with some confidence, that the emission at 16 µm 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 asymptotic giant branch (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 active galactic nucleus. We none the less find that the 16-µm 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-µm 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.
galaxies; clusters; general - galaxies; elliptical and lenticular cD - galaxies; evolution - galaxies; photometry - infrared; galaxies
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The mid-infrared emission of even the most passive early-type
galaxies (ETGs) shows an excess of emission longwards of ∼8 µm
over that which is expected from purely photospheric emission. This
excess emission was detected by Impey, Wynn-Williams & Becklin
(1986) using ground-based observations and was subsequently
studied using Infrared Space Observatory (ISO; e.g. Bressan et al. 2001).
The excess has now been observed in the central regions of bright
ETGs in the Virgo cluster (Bressan et al. 2006) using Spitzer-IRS.
Although some ETGs show a mid-infrared excess that is evidently
caused by warm dust in a star-forming interstellar medium (Panuzzo
et al. 2007), the excess in passive objects can be explained as the
integrated emission from the hot dust in the envelopes of evolved
asymptotic giant branch (AGB) stars. However, hot dust emission
could also arise from the dusty torus around a central active galactic
nucleus (AGN) of low luminosity.
Although there is evidence that the emission at 16 µm is extended
in Coma cluster galaxies (Clemens et al. 2009a), the weight of
evidence for this is far from conclusive. In this paper, we discuss
Spitzer-IRS peak-up imaging at 16 µm of ETGs in the Virgo
cluster that permits the investigation of the spatial distribution of the
excess mid-infrared emission. In order to isolate the effects of the
mid-infrared excess caused by AGB stars, we also use infrared array
camera (IRAC) images at 4.5 and 8.0 µm because these are
relatively unaffected by the excess and sample the purely photospheric
component at the longest possible wavelength. We will argue that
the excess is stellar in origin, as has been previously suggested
(Bressan, Granato & Silva 1998; Athey et al. 2002; Xilouris et al.
2004).
The stellar origin of the excess mid-infrared emission makes it
a particularly useful age/metallicity diagnostic because the
contribution of the mid-infrared excess to the integrated spectrum varies
with age and metallicity in a different way to optical diagnostics
(Bressan et al. 1998). As a stellar population gets younger and/or
the metallicity increases, the mid-infrared excess increases.
However, the optical Hβ index becomes larger (optical colours bluer) as
the age decreases and/or the metallicity decreases. The opposite
behaviour of optical indices (colours, narrow-band indices and spectral
shape) and mid-IR excess with respect to age and metallicity
variations means that the mid-infrared spect (...truncated)