Age and metallicity obtained from the spectral fitting for IGR J01528-0326, IGR J02524-0829, IGR J04451-0445, IGR J18244-5622, IGR J18308+0928, PBC J0041.6+2534, PBC J0759.9+2324, and PBC J0919.9+3712. The color scale refers to the mass fraction for each bin of age and metallicity. The red dot represents the mass-weighted age ⟨ t / Gyr ⟩ _M and the mass-weighted metallicity ⟨   [M/H]   ⟩ _M for each galaxy. The yellow dots represent the mass-weighted age ⟨ t / Gyr ⟩ _M and the mass-weighted metallicity ⟨   [M/H]   ⟩ _M in the case of two distinct stellar populations considered for the young and old component (see Sect. 4).