Distributions of the D_n4000 spectral index. The top panel illustrates the D_n4000 distributions for sources in low-density fields. The bottom panel, presents the D_n4000 distributions for sources in high-density fields. For the galaxy control sample, the distributions are shown by the black, solid lines. The distributions of AGNs at low (42 < log[L_{X,2#x2212;10 keV} (erg s⁻¹)] < 43), moderate (43 < log[L_{X,2#x2212;10 keV} (erg s⁻¹)] < 44), and high L_X (44 < log[L_{X,2#x2212;10 keV} (erg s⁻¹)] < 45) are presented by the green dashed, blue dotted, and red dashed-dotted lines, respectively.