SN Ia parameters as a function of log (Σ_Hα), the surface brightness in the 1 kpc radius around the SNe Ia. Upper panel: the log (Σ_Hα) distribution colored with respect to Σ_Hα subgroups: Iaϵ and Iaα are shown as empty-black and filled-blue histograms, respectively. Main panels, from top to bottom: SALT2 stretch x₁, color c, and the Hubble residuals corrected for stretch and color (with a standardization done over the whole sample). Symbol shapes and colors are the same as in Fig. 5. In the middle panel, filled-red diamonds show the mean SALT2 color measured in bins of log (Σ_Hα): x-axis error bars indicate the log (Σ_Hα) dispersion of the SNe of the considered bin. In the panel, the M₂ SNe (Iaϵ with < −0.1, see text) are shown as filled-gray circles. Right panels, from top to bottom: marginal distributions of x₁, c, and by subgroups. These bi-histograms follow the same color code as the log (Σ_Hα) histogram. The contribution of the M₂ SNe to the Iaϵ distribution is shown in gray in the lower right panel.