A survey of the ISM in early-type galaxies .1. The ionized gas

We present results of a CCD optical imaging survey of the ionized gas in 73 luminous elliptical and lenticular galaxies, selected from the RC3 catalog to represent a broad variety of X-ray, radio, infrared and kinematical properties. For each galaxy we have used broad-band R images and narrow-band images centered at the H alpha and [MI] emission lines to derive the luminosity and distribution of the ionized gas. We found that a large fraction of E (72%) and S0 (85%) galaxies in our sample contain ionized gas. The gas morphology appears to be rather smooth for most galaxies; however similar to 12% of the sample galaxies show a very extended filamentary structure. According to the morphology and size of the gas distribution, the galaxies have been classified into three broad groups, named small disk (SD), regular extended (RE) and filamentary structure (F). The mean diameter of the emitting region ranges between 1 and 10 kpc; the derived mass of the ionized gas ranges between 10(3) and 10(5) solar masses. A significant correlation between H alpha+[NII] and X-ray luminosities is found for those galaxies (27% of the sample) for which we have detected ionized gas and are also listed as X-ray sources. However, there are relatively strong X-ray emitting galaxies for which we have not detected H alpha+[NII] emission and objects which show emission-lines but are not listed either in the EINSTEIN or in the ROSAT databases. The distribution of datapoint and upper limits in this diagram suggests that galaxies with warm gas are also X-ray emitters, while there are X-ray emitters without measurable H alpha+[NII] emission. Similar characteristics are present in the correlation between the infrared luminosity in the 12 mu m band and L(H alpha+[NII]); correlations with other infrared wavelengths are weaker. A strong correlation was also found between the H alpha+[NII] luminosity and the luminosity in the B band inside the region occupied by the line-emitting gas. We use these correlations to discuss the possible mechanisms responsible for the gas ionization and excitation, analyzing in particular the role of the post-AGE stars and the thermal conduction from the X-ray halo in providing the necessary source of ionization.