Correlations between UBV colors and fine structure in E and S0 galaxies - A first attempt at dating ancient merger events

Abstract

New high-quality UBV colors of E and S0 galaxies compiled in the RC3 are analyzed, and correlations between them and the fine-structure index Sigma are explored. The color residuals Delta(U-B)e,0, Delta(B-V)e,0, and Delta(U-V)e,0 (where e refers to the effective or half-light aperture) correlate with Sigma for 35 E and 34 S0 galaxies both separately and combined. Typical coefficients are -0.4 to -0.6, and the slopes of the mean relations are significant at the 2.1 sigma-4.2 sigma level. The new correlations show that the detected systematic variations are not limited to the nuclei, but occur globally in the stellar populations of E and S0 galaxies. The color residuals and previously determined line-strength residuals correlate closely, demonstrating that it is cosmic scatter that dominates the vertical scatter in the color-magnitude and line-strength-magnitude relations. It is suggested that the deviations in color and line strength from mean relations have a common physical cause which, because of the correlations with fine structure, is probably a variation in the mean age of the stellar populations.