Stellar mass surface density – SFR surface density, Σ_* − Σ_SFR of the star formation knots (left) in NGC 4809 (marked as stars) and NGC 4810 (marked as pluses), colored by stellar age. The marker sizes are scaled to the knot sizes. Histogram of the stellar ages of the star formation knots (right). The dashed lines represent the median values of log(age/yr), about 7.3 (NGC 4810) and 7.8 (NGC 4809), respectively.