D4000_n index as a function of the age for BC03 low resolution models with exponentially declining star formation history, the Chabier IMF (when not otherwise specified), and different τ and Z. Solid magenta, solid blue, solid green, solid red, red-dashed and solid cyan lines correspond to models with τ = 0.4 Gyr and Z = 2.5 Z_⊙, Z_⊙, 0.85 Z_⊙, 0.7 Z_⊙, 0.5 Z_⊙ and 0.2 Z_⊙. Dashed and dot-dashed blue lines indicate models with Z = Z_⊙ and τ = 0.3 Gyr and 0.6 Gyr, respectively. Dotted and dashed black lines indicate models with Z = 0.2 Z_⊙, τ = 0.4 Gyr and resolution 20 Å  and 17 Å  respectively. Dot-dashed magenta line indicates the evolution of D4000_n index for models with Z = 2.5 Z_⊙, τ = 0.4 Gyr and the Salpeter IMF (Salpeter 1955). Dashed horizontal green and red lines indicate the value of D4000_n at z = 1.0 and at z = 0.5 for low- and high-Σ MPGs, respectively.