Aims. The paper is devoted to the study of 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. Methods. 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 (10 9 −16 × 10 9 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. Results. 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 σ c 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 ρ xyz , but neither metallicity nor α-enhancement show clear trends. However, we find that while low density environments (LDE) (ρ xyz ≤ 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 ∆ log Z/∆ 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.

We present optical VLT spectroscopy of 16 dwarf elliptical galaxies (or dEs) comparable in mass to NGC 205, and belonging to the Fornax cluster and to nearby groups of galaxies. Using full-spectrum fitting, we derive radial profiles of the SSP-equivalent ages and metallicities. We make a detailed analysis with ulyss and steckmap of the star-formation history in the core of the galaxies and in an aperture of one effective radius. We resolved the history into 1 to 4 epochs. The statistical significance of these reconstructions were carefuly tested; the two programs give remarkably consistent results.The old stellar population of the dEs, which dominates their mass, is likely coeval with that of massive ellipticals or bulges, but the star formation efficiency is lower. Important intermediate age (1-5 Gyr) populations, and frequently tails of star formation until recent times are detected. These histories are reminiscent of their lower mass dSph counterparts of the Local Group.Most galaxies (10/16) show significant metallicity gradients, with metallicity declining by 0.5 dex over one half-light radius on average. These gradients are already present in the old population. The flattened (or discy), rotating objects (6/16) have flat metallicity profiles. This may be consistent with a distinct origin for these galaxies or it may be due to their geometry. The central SSP-equivalent age varies between 1 and 6 Gyr, with the age slowly increasing with radius in the vast majority of objects. The group and cluster galaxies have similar radial gradients and star-formation histories.The strong and old metallicity gradients place important constraints on the possible formation scenarios of dEs. Numerical simulations of the formation of spherical low-mass galaxies reproduce these gradients, but they require a longer time for them to build up. A gentle depletion of the gas, by ram-pressure stripping or starvation, could drive the gas-rich, star-forming progenitors to the present dEs.

We studied the stellar populations of 40 early‐type galaxies using medium‐resolution long‐slit spectroscopy along their major axes (and along the minor axis for two of them). The sample, including elliptical and lenticular galaxies as well as dwarf galaxies, is combined with other previously published data in order to discuss the systematics of the radial gradients of age and metallicity over a large mass range, from 107 M⊙ to 1012 M⊙ (−9.2 > MB > −22.4 mag). The well‐known mass–metallicity relation is continuous throughout the whole mass range, in the sense that more massive galaxies are more metal‐rich. The age–mass relation is consistent with the idea of downsizing: smaller galaxies have more extended star formation histories than more massive ones. The transition‐type dwarfs (intermediate between dwarf irregular and dwarf elliptical galaxies) deviate from this relation having younger mean age, and the low‐mass dwarf spheroidals have older ages, marking a discontinuity in the relation, possibly due to selection effects. In all mass regimes, the mean metallicity gradients are approximately −0.2 and the mean age gradients +0.1 dex per decade of radius. The individual gradients are widely spread: −0.1 < ∇Age < 0.4 and −0.54 < ∇[Fe/H] < +0.2. We do not find evidence for a correlation between the metallicity gradient and luminosity, velocity dispersion, central age or age gradient. Likewise, we do not find a correlation between the age gradient and any other parameter in bright early‐type galaxies. In faint early‐types with MB≳−17 mag, on the other hand, we find a strong correlation between the age gradient and luminosity: the age gradient becomes more positive for fainter galaxies. Together with the observed downsizing phenomenon this indicates that, as time passes, star formation persists in dwarf galaxies and becomes more centrally concentrated. However, this prolonged central star formation is not reflected in the metallicity profiles of the dwarfs in our sample. We conclude that various physical mechanisms can lead to similar gradients and that these gradients are robust against the environmental effects. In particular, the gradients observed in dwarf galaxies certainly survived the transformation of the progenitors through tidal harassment or/and ram‐pressure stripping. The diversity of metallicity gradients amongst dwarf elliptical galaxies may reflect a plurality of progenitors’ morphologies. The dwarfs with steep metallicity gradients could have originated from blue compact dwarfs and those with flat profiles from dwarf irregulars and late‐type spirals.

Aims. A significant fraction of early-type galaxies (ETGs) exhibit emission lines in their optical spectra. We attempt to identify the producing the emission mechanism and the ionized gas in ETGs, and its connection with the host galaxy evolution. Methods. We analyzed intermediate-resolution optical spectra of 65 ETGs, mostly located in low density environments and exhibiting spectros-copic diagnostic lines of ISM from which we had previously derived stellar population properties. To extract the emission lines from the galaxy spectra, we developed a new fitting procedure that accurately subtracts the underlying stellar continuum, and accounts for the uncertainties caused by the age-metallicity degeneracy. Results. Optical emission lines are detected in 89% of the sample. The incidence and strength of emission correlate with neither the E/S0 classification, nor the fast/slow rotator classification. By means of the classical [OIII]/Hβ versus [NII]/Hα diagnostic diagram, the nuclear galaxy activity is classified such that 72% of the galaxies with emission are LINERs, 9% are Seyferts, 12% are composite/transition objects, and 7% are non-classified. Seyferts have young luminostiy-weighted ages ( 5 Gyr), and appear, on average, significantly younger than LINERs and composites. Excluding the Seyferts from our sample, we find that the spread in the ([OIII], Hα, or [NII]) emission strength increases with the galaxy central velocity dispersion σ c . Furthermore, the [NII]/Hα ratio tends to increase with σ c . The [NII]/Hα ratio decreases with increasing galactocentric distance, indicative of either a decrease in the nebular metallicity, or a progressive "softening" of the ionizing spectrum. The average nebular oxygen abundance is slightly less than solar, and a comparison with the results obtained in Paper III from Lick indices shows that it is ≈0.2 dex lower than that of stars. Conclusions. The nuclear (r < r e /16) 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 an important role of PAGB star photoionization at larger radii. For the major fraction of the sample, the nuclear emission is consistent with excitation caused by either a low-accretion rate AGN or fast shocks (200-500 km s −1 ) in a relatively gas poor environment (n 100 cm −3 ), or both. The derived [SII]6717/6731 ratios are consistent with the low gas densities required by the shock models. The derived nebular metallicities are indicative of either an external origin of the gas, or an overestimate of the oxygen yields by SN models.

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