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

In this paper we study the underlying stellar population of a sample of 65 nearby early-type galaxies predominantly located in low density environments. Ages, metallicities and [alpha/Fe] ratios have been derived through the comparison of Lick indices measured at different galacto-centric distances with new SSP models which account for the presence of alpha/Fe enhancement. The SSPs cover a wide range of ages, metallicities and [alpha/Fe] ratios. To derive the stellar population parameters we have devised an algorithm based on the probability density function. We derive a large spread in age ((1-15) Gyrs). Age does not show any significant trend with central velocity dispersion sigma_c but E galaxies appear on average older than S0. On the contrary, an increasing trend of metallicity and [alpha/Fe] with sigma_c is observed, testifying that the chemical enrichment was more efficient and the duration of the star formation shorter in more massive galaxies. We have also sought for possible correlations with the local galaxy density but neither metallicity nor alpha-enhancement show clear trends. However we find that while low density environments (LDE) contain very young objects (from 1 to 4 Gyr), none of the galaxies in the higher density environments (HDE) is younger than 5 Gyrs. Considering the lack of environmental effect on the [alpha/Fe] ratio and the high value of [alpha/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.