Nearby early-type galaxies with ionized gas. III. Analysis of line-strength indices with new stellar population models

We study the underlying stellar population of a sample of 65 nearby early-type galaxies, predominantly located in low density environments, a large fraction of which show emission lines. Ages, metallicities, and [ α/Fe] ratios have been derived through the comparison of Lick indices measured at different galacto-centric distances (7 apertures and 4 gradients) with new simple stellar population (SSP) models that account for the presence of α/Fe-enhancement. The SSPs cover a wide range of ages (109{-}16 × 109 yr), metallicities (0.0004 ≤ Z ≤0.05), and [ α/Fe] ratios (0-0.8). To derive the stellar population parameters, we use an algorithm that provides, together with the most likely solution in the (age, Z, [ α/Fe] ) space, the probability density function along the age-metallicity degeneracy. We derive a large spread in age, with SSP-equivalent ages ranging from a few to 15 Gyr. Age does not show any significant trend with central velocity dispersion σ_c, but E galaxies appear on average older than lenticulars. On the contrary, a significant increasing trend of metallicity and [ α/Fe] with σc is observed, testifying that the chemical enrichment was more efficient and the duration of the star formation shorter in more massive galaxies. These latter two relations do not depend on galaxy morphological type. We have also sought possible correlations with the local galaxy density ρxyz, but neither metallicity nor α-enhancement show clear trends. However, we find that while low density environments (LDE) (ρxyz ≤ 0.4) contain very young objects (from 1 Gyr to 4 Gyr), none of the galaxies in the higher density environments (HDE) (40% of galaxies with a measured density) is younger than 5 Gyr. Considering the lack of environmental effect on the [ α/Fe] ratio and the high value of [ α/Fe] in some young massive objects, we argue that young galaxies in LDE are more likely due to recent rejuvenation episodes. By comparing the number of “rejuvenated” objects with the total number of galaxies in our sample, and by means of simple two-SSP component models, we estimate that, on average, the rejuvenation episodes do not involve more than 25% of the total galaxy mass. The good quality of the data also allow us to analyze the gradients of stellar populations. An average negative metallicity gradient Δ log Z/Δ log (r/r_e) ˜ -0.21 is firmly detected, while the age and α-enhancement spatial distributions within r_e/2 appear quite flat. These observations suggest that, in a given galaxy, the star formation proceeded on similar timescales all across the central r_e/2 region, but with an increasing efficiency toward the center.