RADIO-CONTINUUM SOURCES IN NEARBY AND BRIGHT E/S0 GALAXIES - ACTIVE NUCLEI VERSUS STAR FORMATION

Results from a sensitive VLA search for 6 cm sources in the nuclei of 198 nearby elliptical and lenticular (E/SO) galaxies are presented. The survey improves upon and complements past efforts by going to submillijansky levels and by concentrating on galaxies of lower optical luminosities, down to M(B) approximately - 17.5 mag. Fifty-two galaxies are detected. The vast majority of these detections are unresolved (< 3 arcsec or 580 pc) and coincide closely with the optical nuclei. Some detections correspond to powers as low as approximately 2 X 10(19) W Hz-1, the same as the power of the Galactic Center region on similar linear scales. The surveyed galaxies, plus 12 additional ones with existing VLA images, comprise a volume- and optical-magnitude-limited sample. Eleven of the 12 previously imaged galaxies show a flat-spectrum radio core that is a signpost of a low-power active nucleus. Radio spectral indices are not yet available for the many new radio core detections, so it is possible that not all of these cores are flat-spectrum sources that signal active nuclei. Indeed, some of the surveyed detections are extended and exhibit linear or diffuse morphologies which resemble the radio sources seen in the star-forming nuclei of spiral galaxies (Wrobel & Heeschen 1988). Furthermore, some of the radio detections, primarily among the lenticulars, can be termed "star-formation candidates", as they conform to the Wunderlich et al. (1987) 6 cm versus far-infrared relation defined by galaxies currently forming stars. In contrast, many of the ellipticals depart from this relation, presumably due to excess radio emission from an active nucleus (Walsh et al. 1989). If the radio emission from ellipticals and lenticulars is indeed dominated by different processes, then future surveys of E/SO galaxies must involve carefully selected samples of ellipticals and lenticulars. If the active nuclei in ellipticals are driven by accretion of galactic gases by a massive central object, then what galaxian properties can enhance this activity and generate low-power (10(19-23) W Hz-1) radio cores? We examine issues related to optical luminosity and isophote shapes, and find for the 67 ellipticals in this study: (i) A weak, statistical tendency for optically luminous ellipticals to host more powerful radio sources, confirming previous studies involving more powerful radio emitters: (ii) Low-power radio cores are hosted by ellipticals with axial ratios ranging from E0 through E3, whereas other studies show that more powerful radio sources (> 10(23) W Hz-1 total) are preferentially found among E0 and E1 galaxies; and (iii) Low-power radio cores are mainly hosted by ellipticals with boxy or irregular optical isophotes, rather than disky ones, but this could arise because of finding (i) and the fact that the more optically luminous ellipticals have boxy or irregular isophotes. Issues related to availability of gas in elliptical nuclei, while of potential importance to the level of activity expected, could not be addressed because too few data concerning gas content were available. We urge that these 67 ellipticals should be targets of sensitive searches for H I, optical emission-line gas, and x-ray emitting gas.