Dust masses derived from the IRAS flux densities are found to be roughly an order of magnitude higher than those derived from the optical extinction values. This is in strong contrast from the situation in spiral galaxies. To account for this difference it was argued that most of the dust in elliptical galaxies exists as a diffuse component undetectable at optical wavelengths. This diffuse dust component produces a radial colour gradient that adds to the gradient produced by metallicity and age variations, as shown by Goudfrooij et al. (1995).
Several elliptical galaxies which show evidence for dust have isophotal properties and kinematics consistent with the current accepted merger model for these galaxies (Forbes 1991). Furthermore, it has been shown that the apparent rotation axis of the dust (as derived from the orientation of the dust lane) does not generally coincide with that of the stars. This suggests an external origin for the dust (Van Dokkun & Franx 1995).
We have previously studied the ionized gas for a large sample of 73 early-type galaxies (Macchetto et al. 1996, hereafter referred to as Paper I) and found it to be present in most elliptical (72%) and lenticular (85%) galaxies. The gas distribution appears to range from small disks (SD) to regular extended (RE) and filamentary (F) structures. The gas mass ranged from 103 to 105 . Correlations between the warm and hot gas components and the total Hand X luminosities were investigated: we found that galaxies rich in H are also X-ray emitters, while there are X-ray bright objects lacking ionized gas.
In this paper, we present results of CCD observations of 15 elliptical and 7 lenticular galaxies from the sample in Paper I, for which we have obtained deep images in both V and R bands,which are necessary for this study. We will carry out observations of the other galaxies in Paper I in the future. We have explored the correlations between the masses of the dust and ionized gas and the IR and blue luminosities.
This paper is structured as follows: in Sect. 2 we present the galaxy sample, in Sect. 3 we discuss the observations and data reduction. The description of individual galaxies is given in Sect. 3.3. The dust mass determination is presented in Sect. 4. The correlations between the dust masses and the , luminosities are presented in Sect. 5. In Sect. 6 we discuss the dust heating mechanism, while concluding remarks are given in Sect. 7.
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