Hδ_A plotted as a function of D_n(4000) and spiral arm strength (2 + log s_arm). Panels a: the color associated with each data point encodes the average arm strength of the surrounding galaxies with |Δx|≤0.05 and |Δy|≤0.5 (the box is illustrated in the bottom-left corner). The curves show evolution tracks obtained using the GALAXEV stellar population synthesis code (Bruzual & Charlot 2003) with the provided simple stellar population model of a metallicity Z = 0.019 and a Chabrier (2003) initial mass function. Dashed and solid curves respectively show results for an instantaneous burst of star formation and continuous star formation that declines exponentially with time with a characteristic timescale of 4 Gyr. Panel b: the color encodes the different narrow ranges of D_n(4000). The typical scatter in Hδ_A at a given 2 + log s_arm and a given D_n(4000) is illustrated in the bottom-left corner.