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.

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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Using the third data release of the Sloan Digital Sky Survey (SDSS), we have rigorously defined a volume-limited sample of early-type galaxies in the redshift range 0.005 < z 0.1. We have defined the density of the local environment for each galaxy using a method which takes account of the redshift bias introduced by survey boundaries if traditional methods are used. At luminosities greater than our absolute r-band magnitude cut-off of −20.45, the mean density of environment shows no trend with redshift. We calculate the Lick indices for the entire sample and correct for aperture effects and velocity dispersion in a model-independent way. Although we find no dependence of redshift or luminosity on environment, we do find that the mean velocity dispersion, σ , of early-type galaxies in dense environments tends to be higher than in low-density environments. Taking account of this effect, we find that several indices show small but very significant trends with environment that are not the result of the correlation between indices and velocity dispersion. The statistical significance of the data is sufficiently high to reveal that models accounting only for α-enhancement struggle to produce a consistent picture of age and metallicity of the sample galaxies, whereas a model that also includes carbon enhancement fares much better. We find that early-type galaxies in the field are younger than those in environments typical of clusters but that neither metallicity, α-enhancement nor carbon enhancement are influenced by the environment. The youngest early-type galaxies in both field and cluster environments are those with the lowest σ . However, there is some evidence that the objects with the largest σ are slightly younger, especially in denser environments.Independent of environment both the metallicity and α-enhancement grow monotonically with σ . This suggests that the typical length of the star formation episodes which formed the stars of early-type galaxies decreases with σ . More massive galaxies were formed in faster bursts.We argue that the timing of the process of formation of early-type galaxies is determined by the environment, while the details of the process of star formation, which has built up the stellar mass, are entirely regulated by the halo mass. These results suggest that the star formation took place after the mass assembly and favours an anti-hierarchical model. In such a model, the majority of the mergers must take place before the bulk of the stars form. This can only happen if there exists an efficient feedback mechanism which inhibits the star formation in low-mass haloes and is progressively reduced as mergers increase the mass.

A candidate diffuse stellar substructure was previously reported in the halo of the nearby dwarf starburst galaxy NGC 4449 by Karachentsev et al. We map and analyze this feature using a unique combination of deep integrated-light images from the Black Bird 0.5-meter telescope, and high-resolution wide-field images from the 8-meter Subaru telescope, which resolve the nebulosity into a stream of red giant branch stars, and confirm its physical association with NGC 4449. The properties of the stream imply a massive dwarf spheroidal progenitor, which after complete disruption will deposit an amount of stellar mass that is comparable to the existing stellar halo of the main galaxy. The ratio between luminosity or stellar-mass between the two galaxies is ∼ 1:50, while the indirectly measured dynamical mass-ratio, when including dark matter, may be ∼ 1:10-1:5. This system may thus represent a "stealth" merger, where an infalling satellite galaxy is nearly undetectable by conventional means, yet has a substantial dynamical influence on its host galaxy. This singular discovery also suggests that satellite accretion can play a significant role in building up the stellar halos of low-mass galaxies, and possibly in triggering their starbursts.