D_n4000 as a function of stellar mass for three different over-density bins. The vertical dotted line shows the representativeness limit of our survey. We show the best fit (with errors) from Eq. (6) for each density bin as shaded regions. We find an underlying correlation between stellar masses and D_n4000: the median D_n4000 of galaxies in high-density regions is higher than in low-density regions. The difference between low- and high-density environments is larger at higher stellar masses. We compare with the results from GCLASS (Muzzin et al. 2012), from VVDS (Vergani et al. 2008), from SHARDS (Hernán-Caballero et al. 2013), and from LEGA-C (Wu et al. 2018), which also show the same trends. We also show here the median value of D_n4000 as a function of both stellar mass and local density in in the right panel. This map is a smooth interpolation of the trends found in our sample and highlights the increase of the median D_n4000 with increasing stellar mass and local density.