Power spectrum analysis of the main annuli for both targets. Left panels: NGC 4621 analysis. The upper panel shows the logarithm of the power spectrum of the residual frame (grey dots) and the best-fit P(k) curve (black solid line). In middle panel the difference between observed and fitted data is shown. The flat region of log    P₀(k_fit > k) between vertical dashed lines in the lower panel is used to evaluate the best-fit parameters P₀, and P₁ (Cantiello et al. 2005; Biscardi et al. 2008). Right panels: as left, but for NGC 4374.