INTERSTELLAR MATTER IN SHAPLEY-AMES ELLIPTIC GALAXIES .2. THE DISTRIBUTION OF DUST AND IONIZED-GAS

We present results of deep optical CCD imaging for a complete, optical magnitude-limited sample of 56 elliptical galaxies from the RSA catal og. For each galaxy we have obtained broad-band images (in B, V, and I ) and narrow-band images using interference filters isolating the Halp ha+[NII] emission lines to derive the amount and morphology of dust an d ionized gas. Detailed consideration of systematic errors due to effe cts of sky background subtraction and removal of stellar continuum lig ht from the narrow-band images is described. The flux calibration of t he narrow-band images is performed by deconvolving actually measured s pectral energy distributions with the filter transmission curves. We a lso present optical long-slit spectroscopy to determine the [NII]/Halp ha intensity ratio of the ionized gas. Dust lanes and/or patches have been detected in 23 galaxies (41%) from this sample using both colour- index images and division by purely elliptical model images. We achiev ed a detection linlit for dust absorption of A(B) approximately 0.02. Accounting for selection effects, the true fraction of elliptical gala xies containing dust is estimated to be of order 80%. This detection r ate is comparable to that of the IRAS satellite, and significantly lar ger than results of previous optical studies. Ionized gas has been det ected in 32 galaxies (57%). The spectroscopic data confirm the presenc e and distribution of ionized gas as seen in the direct imaging. All e lliptical galaxies in our sample in which a number of emission lines i s detected show very similar emission-line intensity ratios, which are typical of LINER nuclei. The amounts of detectable dust and ionized g as are generally small-of order 10(4) - 10(5) M. of dust and 10(3) - 1 0(4) M. of ionized gas. The dust and ionized gas show a wide variety o f distributions-extended along either the apparent major axis, or the minor axis, or a skewed axis, indicating that triaxiality is in genera l required as a galaxy figure. In some cases (NGC 1275, NGC 2325, NGC 3136, NGC 3962, NGC 4696, NGC 5018, NGC 5044, NGC 5813, IC 1459) the i nterstellar matter has a patchy or filamentary distribution, suggestiv e of a recent interaction event. The distributions of dust and ionized gas are consistent with being physically associated with each other.