Log₁₀([O III]/Hβ) vs. log₁₀([N II]/Hα). Within this plot we can see the distribution of the spectroscopically selected subsample of composite galaxies that have been classified by the new diagnostic as passive-composite galaxies, i.e., composites where the ionization source originates from hot evolved stars. The circular data points are derived from the photoionization models from the work of Byler et al. (2019), and they are overlaid to indicate the locations of galaxies with aging stellar populations spanning from 2 to 14 Gyr. The circles are color-coded to denote the age of the stellar populations. The black boundary lines are the same as defined in Fig. 2. The black dots in the top plot represent the training sample of the principal classes, included for illustrative purposes.