Histograms indicating the shapes and ranges of the distributions of the SPS parameters (Table 1) used as input to the FSPS to produce our mock sample of galaxies. In the top row, from left to right, we show the distribution densities of metallicity (Z), the dust fraction exposed to a high ionization field (γ), and the ionization parameter of the gas (log₁₀U), respectively. On the bottom row on the left, we show the minimum (average) ionization field of the ISM (U_min), while on the bottom right we see the fraction of dust in the form of PAH molecules (q_PAH). Each parameter set contains these five parameters drawn randomly and independently from each other based on the probability distributions shown above.