SIMBAD references

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 log(ρ)and the residuals to the Mg₂- σ₀relation, R_a, and between log(ρ)and the residuals to the Fundamental Plane, R_f. 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 galaxies 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 R_a and R_f) 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, V_max/σ₀, 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.