Madden, S.C. , Vigroux, L. , et al.
1 CEA, Saclay, SAP, France
Early-type galaxies have traditionally been thought of as being void
of ISM. But it is now clear that they contain a rich variety of ISM,
including hot and cold gas phase media. The presence of dust in
early-type galaxies is known through observations of optical dust
patches and lanes (eg. Goodfrooij et al 1994; Dokkum and Franx 1995),
while IRAS observations have shown that 
50% of E and S0 galaxies emit
at 60 and/or 100 microns (Knapp et al. 1989, 1992; Jura et al
1987). Dust masses from IRAS observations show that on the order of
can be present in a sample of elliptical
galaxies (Jura et al 1987). While the presence of dust in early-type
galaxies, in principle, should not be surprising due to the
mass-loosing nature of the evolving red giants, the quantity of dust
is high relative to the expected life-times in the hot interstellar
medium (Draine and Salpeter 1979; Tsai and Matthews 1995).
The dust component of the ISM is a critical element in constraining the evolutionary history of early-type galaxies. These galaxies have been the subject of recent studies in deep surveys and one of the outstanding problems in the present-day models is the possible effects of dust obscurations (Franceschini et al 1998). Determining just how much dust is present in local elliptical galaxies, as well as its distribution relative to the gas and stellar components, will provide clues to the origin of the dust and provide templates for cosmological studies.
We have conducted an ISOCAM survey of a sample of early-type galaxies to address these questions. With observations of 3 broad-band filters centered at 4.5, 6.7 and 15 microns, we are able to determine the characteristics of the mid-infrared emission in early-type galaxies. The MIR emission can arise from 3 possible components: 1) the evolved stellar emission; 2) dust emission from mass-loosing AGB stars or dust accumulated from external origin and 3) the non-thermal emission associated from central radio sources. Studying the characteristics of MIR spectral energy distribution, as well as the spatial distribution of the emission, we are able to determine the nature of the MIR emission. We find that 3 galaxies of our sample of 20 show purely stellar emission in the the MIR and 3 galaxies are dominated by non-thermal emission. The rest of the galaxies contain dust emission including clear detections of the PAH-like features from 6 to 9 microns. The presence of PAH features in these galaxies will help place constraints on the excitation and nature of these species. We construct ISOCAM color-color diagrams which can serve as tools in distinguishing early-type galaxies having purely stellar emission in the MIR, from dusty galaxies and those dominated by non-thermal emission. This work can then be used to help correctly incorporate dust into galaxy evolutioary models necessary to interprete the current and future deep surveys.