Effect of the large scale environment on the stellar content of early-type galaxies

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In order to study the influence of the environment on the stellar population of early-type galaxies we have analyzed the relations between the density of the environment (lrho) and the residuals to the Mg2-sigma relation, Rₐ, and between lrho and the residuals to the Fundamental Plane, Rf. Our sample of galaxies covers the range of densities, between field galaxies and poor clusters. We confirm the existence of a slight environmental bias on the mean stellar population and we show that this effect is due to a small fraction of galaxie s containing a younger stellar sub-population while the majority of galaxies, in any environment, are equally old. The galaxies which are likely to contain a young stellar sub-population (negative Rₐ and Rf) are preferentially found in the less dense environment. This segregation of the stellar population may reflect the morphology-density relation or may result from a higher rate of recent star formation activity in sparse environments. Using the rotational support to assess the presence of a disk and to parameterize the morphology, we subtracted the contribution of the morphological segregation to population segregation. It is not yet possible to rule out that the morphology-density relation is at the origin of the population-density relation.