Impact of varying R₂₁ and α_{¹²CO(1−0)} on the molecular gas depletion time The top panels show the radial trend of the depletion time for M101 (left), M51 (center), and for the combination of the sightlines from both galaxies (right). The depletion time depends on a measurement of the molecular gas mass. The grey points (with purple edge color) show the measurements using CO(2–1) and fixed R₂₁ and α_{¹²CO(1−0)} values. The points color-coded by SFR surface density are from using CO(1–0) and a radially interpolated α_{¹²CO(1−0)}. The bottom panels compare the radially binned trends of both measurements. The grey line indicates the residual between the two trend lines.