Differential population synthesis of S0 galaxies. II. Methods and results

The integrated spectra of the disk and nuclei of a sample of S0 galaxies are compared to those of normal elliptical galaxies using a differential technique to search for and analyze any possible differences between the two galaxy types. A fiducial elliptical sequence representing stellar populations of uniform age but variable metallicity is constructed. Comparison of the S0 disk spectra to this sequence at a location appropriate for their line strengths reveals the presence, in roughly half of the S0 disks studied, of hot subpopulations which are not present in normal ellipticals. These subpopulations can contribute from 5 percent to as much as 50 percent or more of the V light in a particular S0 disk. It is argued that the subpopulations can be divided into two categories: those that are similar to the solar neighborhood population and are actively forming stars; and those that are older, intermediate-age populations. This supports the view that S0 galaxies have evolved from spiral or spiral-like galaxies. 45 references