7 Conclusions

 We have shown the results of a CCD optical imaging survey of the dust content in 22 elliptical and S0 luminous galaxies. A large fraction (75%) of the observed galaxies have dust contents in the form of small disks, regular extended and filamentary structures. The morphology and size of the dust distribution of the observed galaxies follows very closely that of the ionized gas. For each galaxy, we have used broad band V and R filters imaging to build colour maps and derive the (V-R) colour map, A_V and A_R extinction maps. We have found typical A_V values of 0.026, A_R of 0.023. The A_V values together with simple assumption about the dust grain size and composition enables us to estimate the dust masses. We derived values in the range 10³-10⁵ $M_{\hbox{$\odot$}}$ We have studied the correlations between the dust mass and the H$\alpha$, IR and $L_{\mathrm{B}}$ luminosities.

We found that the correlation between the IRAS luminosity and the dust mass is enhanced from $25\,\mu$m to $100 \,\mu$m. Since the correlation improves towards $100 \,\mu$m it suggests that the peak of the dust emission might be beyond $100 \,\mu$m, thus the dust temperature is below T=23 K, assuming a black body law.

We also found that post-AGB stars provide sufficient UV radiation to heat the dust, as well as to ionize the gas, to account for the observed IR luminosity, which in turn explains the correlations between the dust mass, the blue luminosity and the infrared luminosity for the galaxy sample.

Acknowledgements

These observations were obtained at the ESO telescopes in La Silla, within the ESO Key-Programme 1-004-43K. We wish to Thank to Dr. C. Bonatto for fruitful discussions. We also thank the anonymous referee for a number of useful suggestions. M. Pastoriza and F. Ferrari acknowledge CNPq and PRONEX/FINEP 76.97.1003.00 for financial support.

  

Figure 5: NGC 533. V: 18, 19, 20, 21, 22; (V-R): 0.55, 0.56, 0.57, 0.58; $\mathrm{H}\alpha+[\mathrm{NII}]$: 0.2; NGC 636. V: 18, 19, 20, 21; (V-R): 0.56, 0.59, 0.62, 0.64; $\mathrm{H}\alpha+[\mathrm{NII}]$: 0.7; NGC 1600. V: 18, 19, 20, 21; (V-R): 0.625, 0.63, 0.635, 0.64; $\mathrm{H}\alpha+[\mathrm{NII}]$: 0.5

  

Figure 5: NGC 2865. V: 18, 19, 20, 21, 22; (V-R): 0.61, 0.62, 0.63, 0.64; NGC 3268. V: 18, 19, 20; (V-R): 0.68, 0.7, 0.72, 0.74, 0.76, 0.78; $\mathrm{H}\alpha+[\mathrm{NII}$: 0.1; NGC 3311. V: 19.3, 19.5, 20, 20.5; (V-R): 0.67, 0.69, 0.71; $\mathrm{H}\alpha+[\mathrm{NII}]$: 0.6

  

Figure 5: NGC 3379. V: 16, 17, 18; (V-R): 0.6, 0.61, 0.63, 0.66; $\mathrm{H}\alpha+[\mathrm{NII}]$: 0.6; NGC 3489. V: 17, 18, 19, 20, 21; (V-R): 0.48, 0.50, 0.52, 0.54; $\mathrm{H}\alpha+[\mathrm{NII}]$: 1.7; NGC 3607. V: 17, 18, 19, 20; (V-R): 0.59, 0.6, 0.61, 0.62, 0.63, 0.64; $\mathrm{H}\alpha+[\mathrm{NII}]$: 0.3

  

Figure 5: NGC 4472. V: 17, 17.5, 18, 18.5; (V-R): 0.605, 0.61, 0.615, 0.62; $\mathrm{H}\alpha+[\mathrm{NII}]$: 1.5; NGC 4473. V 17, 18, 19, 20; (V-R): 0.68, 0.7, 0.72, 0.74; $\mathrm{H}\alpha+[\mathrm{NII}]$: 1.5; NGC 4552. V 16, 17, 18; (V-R): 0.59, 0.595, 0.61, 0.63, 0.6; $\mathrm{H}\alpha+[\mathrm{NII}]$: 1.5

  

Figure 5: NGC 5044. V: 18, 19, 20, 21; (V-R): 0.66, 0.68, 0.7, 0.72; $\mathrm{H}\alpha+[\mathrm{NII}]$: 0.1; NGC 5812. V: 18, 19, 20, 21; (V-R): 0.68, 0.69, 0.7; $\mathrm{H}\alpha+[\mathrm{NII}]$: 1.2; NGC 5813. V: 18, 19, 20; (V-R): 0.67, 0.68, 0.69; $\mathrm{H}\alpha+[\mathrm{NII}]$: 1.2

  

Figure 5: NGC 5903. V: 18, 19, 20, 21; (V-R): 0.65, 0.68, 0.7; $\mathrm{H}\alpha+[\mathrm{NII}]$: 0.5; NGC 6483. V: 17, 18, 19, 20, 21, 22; (V-R): 0.6, 0.62, 0.64; NGC 6758. V: 18, 19, 20, 21, 22; (V-R): 0.6, 0.61, 0.62, 0.64, 0.66; $\mathrm{H}\alpha+[\mathrm{NII}]$: 0.2

  

Figure 5: NGC 6909. V: 17, 18, 19, 20, 21; (V-R): 0.51, 0.52, 0.53, 0.54; IC 4797. V: 18, 19, 20; (V-R): 0.58, 0.6, 0.63, 0.66; IC 4889. V: 18, 19, 20, 21; (V-R): 0.616, 0.625, 0.63; $\mathrm{H}\alpha+[\mathrm{NII}]$: 0.15

  

Figure 5: IC 5105. V 18, 19, 20, 21; (V-R): 0.61, 0.63, 0.65, 0.67; $\mathrm{H}\alpha+[\mathrm{NII}]$: 0.4