Study of stellar mass and concentration impact on age of stellar population for void paired galaxies. (a) Central panel: Scatter plot of D_n(4000) as a function of log(M_*/M_sun) for pair galaxies in the void interiors (blue), in void walls (green), and in the field (black). Diamonds and solid lines indicate the median. The vertical dotted line represents log(M_*/M_sun)=9.5. Left panel: D_n(4000) normalized distribution for log(M_*/M_sun)< 9.5 paired galaxies in voids and the field. Right panel: Same as in the left panel but for log(M_*/M_sun)> 9.5. (b) Central panel: Scatter plot of D_n(4000) as a function of concentration index for pair galaxies in the void interiors (blue), in void walls (green), and in the field (black). Diamonds and solid lines indicate the median. The vertical dotted line represents C = 2.5. Left panel: D_n(4000) normalized distribution for C <  2.5 paired galaxies in voids and field. Right panel: Same as in the left panel but for C >  2.5.