8.5. Optical Colors
There are not sufficient uniform data on the CSS sources to warrant a discussion of their colors. However, work in progress by Gelderman et al. and de Vries et al. should provide the necessary information. The r - i colors of a subset of GPS galaxies (from Stanghellini et al. 1993) have been compared with the Bruzual-Charlot stellar evolution models by O'Dea et al. (1996a). Figure 18 shows that the colors are in good agreement with those of passively evolving ellipticals with an old stellar population. Even the colors of a Sa galaxy are too blue to be consistent with the GPS colors.
The lack of evidence for substantial amounts of young stars in the r - i colors of the GPS host galaxies is consistent with the result that the GPS sources tend not to show an alignment effect. However, it is somewhat surprising given the fact that the GPS galaxies appear to be merging/interacting with companions and may have recently acquired large amounts of gas. This implies that a starburst triggered either by the merger or by the radio source should be visible. The lack of evidence for a circumnuclear starburst in the GPS galaxies may be due to the inner regions of the galaxies being very "dust enshrouded."