The environmental dependence of the Mg2 correlations for elliptical galaxies

We present Mg₂ measurements for 51 Coma cluster ellipticals. Unlike previous studies, our sample includes many ellipticals that lie well outside the cluster core. These data have been used to investigate how the σ–Mg₂ and D–Mg₂ relationships of Coma ellipticals vary with distance from the cluster centre. We find a small environmental effect in the sense that ellipticals lying in the cluster halo, compared with those in the inner regions, have: (i) a larger intrinsic scatter about the Mg₂ relations, and (ii) their Mg₂ relations significantly offset to a lower Mg₂ for a given σ or D. For a 1° sample division we find that the halo sample, as compared to an inner sample, has an average σ–Mg₂ relation that is offset by $$0.017 \pm 0.005$$ mag and an average D-Mg₂ relation that is offset by $$0.025 \pm 0.008$$ mag. Previously we have reported that for the same sample of ellipticals the D–σ relation does not vary significantly with distance from the cluster centre. For the 1° division the measured offset in the D–σ relation is $$0.039 \pm 0.024$$ dex, in the sense that the halo ellipticals possess, on average, slightly larger D diameters at a fixed σ. We show that the D–σ, σ-Mg₂ and D-Mg₂ offsets are all consistent with the hypothesis that halo ellipticals have, on average, younger stellar populations.