Star formation, metallicity gradient and ionized gas: clues to the formation of the elliptical galaxies NGC6868 and NGC5903

The stellar population, metallicity distribution and ionized gas in the elliptical galaxies NGC 6868 and NGC 5903 are investigated in this paper by means of long-slit spectroscopy and stellar population synthesis. Lick indices in both galaxies present a negative gradient indicating an overabundance of Fe, Mg, Na and TiO in the central parts with respect to the external regions. Concerning the emitting gas conspicuously detected in NGC 6868, we test three hypotheses as ionizing source: an H II region, post-AGB stars and an Active Galactic Nucleus (AGN). Diagnostic diagrams involving the ratios $[NII]_{\lambda6584}/H\alpha$, $[OI]_{\lambda6300}/H\alpha$ and $[SII]_{\lambda6717,31}/H\alpha$, indicate that values measured in the central region of NGC 6868 are typical of LINERs. Together with the stellar population synthesis, this result suggests that the main source of gas ionization in NGC 6868 is non-thermal, produced by a low-luminosity AGN, probably with some contribution of shocks to explain ionization at distances of $\sim3.5$ kpc from the nucleus.