Nearby early-type galaxies with ionized gas - IV. Origin and powering mechanism of the ionized gas

摘要

Aims. A significant fraction of early-type galaxies (ETGs)exhibit emission lines in their optical spectra. We attempt to identifythe producing the emission mechanism and the ionized gas in ETGs, andits connection with the host galaxy evolution. Methods. We analyzed intermediate-resolution optical spectra of65ETGs, mostly located in low density environments and exhibitingspectros-copic diagnostic lines of ISM from which we had previouslyderived stellar population properties. To extract the emission linesfrom the galaxy spectra, we developed a new fitting procedure thataccurately subtracts the underlying stellar continuum, and accounts forthe uncertainties caused by the age-metallicity degeneracy. Results. Optical emission lines are detected in89% of thesample. The incidence and strength of emission correlate with neitherthe E/S0 classification, nor the fast/slow rotator classification. Bymeans of the classical [OIII]/Hversus [NII]/Hdiagnostic diagram, the nuclear galaxy activity is classified such that72% of the galaxies with emission are LINERs, 9% are Seyferts, 12% arecomposite/transition objects, and 7% are non-classified. Seyferts haveyoung luminostiy-weighted ages (5Gyr),and appear, on average, significantly younger than LINERs andcomposites. Excluding the Seyferts from our sample, we find that thespread in the ([OIII], H,or [NII]) emission strength increases with the galaxy central velocity dispersion .Furthermore, the [NII]/Hratio tends to increase with .The [NII]/Hratio decreases with increasing galactocentric distance, indicative ofeither a decrease in the nebular metallicity, or a progressive``softening'' of the ionizing spectrum. The average nebular oxygenabundance is slightly less than solar, and a comparison with theresults obtained in PaperIII from Lick indices shows that it is 0.2dex lower than that of stars. Conclusions. The nuclear ()emission can be attributed to photoionization by PAGB stars alone only for 22%of the LINER/composite sample. On the other hand, we cannot exclude animportant role of PAGB star photoionization at larger radii. For themajor fraction of the sample, the nuclear emission is consistent withexcitation caused by either a low-accretion rate AGN or fast shocks(200-500kms-1) in a relatively gas poor environment (cm-3),or both. The derived [SII]6717/6731 ratios are consistent with the lowgas densities required by the shock models. The derived nebularmetallicities are indicative of either an external origin of the gas,or an overestimate of the oxygen yields by SN models. Key words: galaxies: elliptical and lenticular, cD - galaxies: ISM - galaxies: active - galaxies: abundances