Ionized gas in elliptical and S0 galaxies. I. A survey for H-alpha and forbidden N II emission

A spectroscopic survey of a large sample of southern E and S0 galaxies in order to detect ionized gas in the nuclei is reported. The strongest line in the 6000-7000 A range was nearly always forbidden N II 6584 A, followed by H-alpha and forbidden S II 6716, 6731 A. Identical detection rates of about 50 percent were obtained for the forbidden N II line in both E and S0 galaxies. The mass of ionized gas in early-type galaxies was very small, with values typically in the range 1000-10,000 solar masses. The relative emission-line strengths in virtually every case were indistinguishable from those of LINER nuclei. The observed values of the forbidden N II 6584 A/H-alpha ratios fell mostly between one and three and seem correlated with galaxy absolute magnitude. The existence of such a correlation may be indicative of metallicity differences. 58 references